Глава 13. Storage Engines

Содержание

13.1. Setting the Storage Engine
13.2. Overview of MySQL Storage Engine Architecture
13.2.1. Pluggable Storage Engine Architecture
13.2.2. The Common Database Server Layer
13.3. The InnoDB Storage Engine
13.3.1. InnoDB as the Default MySQL Storage Engine
13.3.2. Configuring InnoDB
13.3.3. Using Per-Table Tablespaces
13.3.4. InnoDB Startup Options and System Variables
13.3.5. Creating and Using InnoDB Tables
13.3.6. Adding, Removing, or Resizing InnoDB Data and Log Files
13.3.7. Backing Up and Recovering an InnoDB Database
13.3.8. Moving an InnoDB Database to Another Machine
13.3.9. The InnoDB Transaction Model and Locking
13.3.10. InnoDB Multi-Versioning
13.3.11. InnoDB Table and Index Structures
13.3.12. InnoDB Disk I/O and File Space Management
13.3.13. InnoDB Error Handling
13.3.14. InnoDB Performance Tuning and Troubleshooting
13.3.15. Limits on InnoDB Tables
13.4. New Features of InnoDB 1.1
13.4.1. Introduction to InnoDB 1.1
13.4.2. Fast Index Creation in the InnoDB Storage Engine
13.4.3. InnoDB Data Compression
13.4.4. InnoDB File-Format Management
13.4.5. How InnoDB Stores Variable-Length Columns
13.4.6. InnoDB INFORMATION_SCHEMA tables
13.4.7. InnoDB Performance and Scalability Enhancements
13.4.8. Changes for Flexibility, Ease of Use and Reliability
13.4.9. Installing the InnoDB Storage Engine
13.4.10. Upgrading the InnoDB Storage Engine
13.4.11. Downgrading the InnoDB Storage Engine
13.4.12. InnoDB Storage Engine Change History
13.4.13. Third-Party Software
13.4.14. List of Parameters Changed in InnoDB 1.1 and InnoDB Plugin 1.0
13.5. The MyISAM Storage Engine
13.5.1. MyISAM Startup Options
13.5.2. Space Needed for Keys
13.5.3. MyISAM Table Storage Formats
13.5.4. MyISAM Table Problems
13.6. The MEMORY Storage Engine
13.7. The CSV Storage Engine
13.7.1. Repairing and Checking CSV Tables
13.7.2. CSV Limitations
13.8. The ARCHIVE Storage Engine
13.9. The BLACKHOLE Storage Engine
13.10. The MERGE Storage Engine
13.10.1. MERGE Table Advantages and Disadvantages
13.10.2. MERGE Table Problems
13.11. The FEDERATED Storage Engine
13.11.1. FEDERATED Storage Engine Overview
13.11.2. How to Create FEDERATED Tables
13.11.3. FEDERATED Storage Engine Notes and Tips
13.11.4. FEDERATED Storage Engine Resources
13.12. The EXAMPLE Storage Engine
13.13. Other Storage Engines

MySQL supports several storage engines that act as handlers for different table types. MySQL storage engines include both those that handle transaction-safe tables and those that handle nontransaction-safe tables.

MySQL Server uses a pluggable storage engine architecture that enables storage engines to be loaded into and unloaded from a running MySQL server.

To determine which storage engines your server supports by using the SHOW ENGINES statement. The value in the Support column indicates whether an engine can be used. A value of YES, NO, or DEFAULT indicates that an engine is available, not available, or available and currently set as the default storage engine.

mysql> SHOW ENGINES\G
*************************** 1. row ***************************
      Engine: FEDERATED
     Support: NO
     Comment: Federated MySQL storage engine
Transactions: NULL
          XA: NULL
  Savepoints: NULL
*************************** 2. row ***************************
      Engine: MRG_MYISAM
     Support: YES
     Comment: Collection of identical MyISAM tables
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 3. row ***************************
      Engine: MyISAM
     Support: DEFAULT
     Comment: Default engine as of MySQL 3.23 with great performance
Transactions: NO
          XA: NO
  Savepoints: NO
...

This chapter describes each of the MySQL storage engines except for NDBCLUSTER, which is covered in MySQL Cluster NDB 6.X/7.X. It also contains a description of the pluggable storage engine architecture (see Section 13.2, “Overview of MySQL Storage Engine Architecture”).

For information about storage engine support offered in commercial MySQL Server binaries, see MySQL Enterprise Server 5.1, on the MySQL Web site. The storage engines available might depend on which edition of Enterprise Server you are using.

For answers to some commonly asked questions about MySQL storage engines, see Section B.2, “MySQL 5.5 FAQ: Storage Engines”.

MySQL 5.5 supported storage engines

  • InnoDB: A transaction-safe (ACID compliant) storage engine for MySQL that has commit, rollback, and crash-recovery capabilities to protect user data. InnoDB row-level locking (without escalation to coarser granularity locks) and Oracle-style consistent nonlocking reads increase multi-user concurrency and performance. InnoDB stores user data in clustered indexes to reduce I/O for common queries based on primary keys. To maintain data integrity, InnoDB also supports FOREIGN KEY referential-integrity constraints. InnoDB is the default storage engine as of MySQL 5.5.5.

  • MyISAM: The MySQL storage engine that is used the most in Web, data warehousing, and other application environments. MyISAM is supported in all MySQL configurations, and is the default storage engine prior to MySQL 5.5.5.

  • Memory: Stores all data in RAM for extremely fast access in environments that require quick lookups of reference and other like data. This engine was formerly known as the HEAP engine.

  • Merge: Enables a MySQL DBA or developer to logically group a series of identical MyISAM tables and reference them as one object. Good for VLDB environments such as data warehousing.

  • Archive: Provides the perfect solution for storing and retrieving large amounts of seldom-referenced historical, archived, or security audit information.

  • Federated: Offers the ability to link separate MySQL servers to create one logical database from many physical servers. Very good for distributed or data mart environments.

  • CSV: The CSV storage engine stores data in text files using comma-separated values format. You can use the CSV engine to easily exchange data between other software and applications that can import and export in CSV format.

  • Blackhole: The Blackhole storage engine accepts but does not store data and retrievals always return an empty set. The functionality can be used in distributed database design where data is automatically replicated, but not stored locally.

  • Пример: The Пример storage engine is “stub” engine that does nothing. You can create tables with this engine, but no data can be stored in them or retrieved from them. The purpose of this engine is to serve as an example in the MySQL source code that illustrates how to begin writing new storage engines. As such, it is primarily of interest to developers.

It is important to remember that you are not restricted to using the same storage engine for an entire server or schema: you can use a different storage engine for each table in your schema.

Choosing a Storage Engine

The various storage engines provided with MySQL are designed with different use cases in mind. To use the pluggable storage architecture effectively, it is good to have an idea of the advantages and disadvantages of the various storage engines. The following table provides an overview of some storage engines provided with MySQL:

Table 13.1. Storage Engines Feature Summary

FeatureMyISAMMemoryInnoDBArchiveNDB
Storage limits256TBRAM64TBNone384EB
TransactionsNoNoYesNoYes
Locking granularityTableTableRowTableRow
MVCCNoNoYesNoNo
Geospatial data type supportYesNoYesYesYes
Geospatial indexing supportYesNoNoNoNo
B-tree indexesYesYesYesNoYes
Hash indexesNoYesNo[a]NoYes
Full-text search indexesYesNoNoNoNo
Clustered indexesNoNoYesNoNo
Data cachesNoN/AYesNoYes
Index cachesYesN/AYesNoYes
Compressed dataYes[b]NoYes[c]YesNo
Encrypted data[d]YesYesYesYesYes
Cluster database supportNoNoNoNoYes
Replication support[e]YesYesYesYesYes
Foreign key supportNoNoYesNoNo
Backup / point-in-time recovery[f]YesYesYesYesYes
Query cache supportYesYesYesYesYes
Update statistics for data dictionaryYesYesYesYesYes

[a] InnoDB utilizes hash indexes internally for its Adaptive Hash Index feature.

[b] Compressed MyISAM tables are supported only when using the compressed row format. Tables using the compressed row format with MyISAM are read only.

[c] Compressed InnoDB tables require the InnoDB Barracuda file format.

[d] Implemented in the server (via encryption functions), rather than in the storage engine.

[e] Implemented in the server, rather than in the storage product.

[f] Implemented in the server, rather than in the storage product.

13.1. Setting the Storage Engine

When you create a new table, you can specify which storage engine to use by adding an ENGINE table option to the CREATE TABLE statement:

CREATE TABLE t (i INT) ENGINE = INNODB;

If you omit the ENGINE option, the default storage engine is used. The default engine is InnoDB as of MySQL 5.5.5 (MyISAM before 5.5.5). You can specify the default engine by using the --default-storage-engine server startup option, or by setting the default-storage-engine option in the my.cnf configuration file.

You can set the default storage engine to be used during the current session by setting the default_storage_engine variable:

SET default_storage_engine=MYISAM;

When MySQL is installed on Windows using the MySQL Configuration Wizard, the InnoDB or MyISAM storage engine can be selected as the default. See Section 2.3.5.5, “The Database Usage Dialog”.

To convert a table from one storage engine to another, use an ALTER TABLE statement that indicates the new engine:

ALTER TABLE t ENGINE = MYISAM;

See Section 12.1.17, “CREATE TABLE Синтаксис”, and Section 12.1.7, “ALTER TABLE Синтаксис”.

If you try to use a storage engine that is not compiled in or that is compiled in but deactivated, MySQL instead creates a table using the default storage engine. This behavior is convenient when you want to copy tables between MySQL servers that support different storage engines. (For example, in a replication setup, perhaps your master server supports transactional storage engines for increased safety, but the slave servers use only nontransactional storage engines for greater speed.)

This automatic substitution of the default storage engine for unavailable engines can be confusing for new MySQL users. A warning is generated whenever a storage engine is automatically changed. To prevent this from happening if the desired engine is unavailable, enable the NO_ENGINE_SUBSTITUTION SQL mode. In this case, an error occurs instead of a warning and the table is not created or altered if the desired engine is unavailable. See Section 5.1.6, “Server SQL Modes”.

For new tables, MySQL always creates an .frm file to hold the table and column definitions. The table's index and data may be stored in one or more other files, depending on the storage engine. The server creates the .frm file above the storage engine level. Individual storage engines create any additional files required for the tables that they manage. If a table name contains special characters, the names for the table files contain encoded versions of those characters as described in Section 8.2.3, “Mapping of Identifiers to File Names”.

A database may contain tables of different types. That is, tables need not all be created with the same storage engine.

13.2. Overview of MySQL Storage Engine Architecture

The MySQL pluggable storage engine architecture enables a database professional to select a specialized storage engine for a particular application need while being completely shielded from the need to manage any specific application coding requirements. The MySQL server architecture isolates the application programmer and DBA from all of the low-level implementation details at the storage level, providing a consistent and easy application model and API. Thus, although there are different capabilities across different storage engines, the application is shielded from these differences.

The MySQL pluggable storage engine architecture is shown in Figure 13.1, “MySQL Architecture with Pluggable Storage Engines”.

Figure 13.1. MySQL Architecture with Pluggable Storage Engines

MySQL Architecture with Pluggable Storage
          Engines

The pluggable storage engine architecture provides a standard set of management and support services that are common among all underlying storage engines. The storage engines themselves are the components of the database server that actually perform actions on the underlying data that is maintained at the physical server level.

This efficient and modular architecture provides huge benefits for those wishing to specifically target a particular application need—such as data warehousing, transaction processing, or high availability situations—while enjoying the advantage of utilizing a set of interfaces and services that are independent of any one storage engine.

The application programmer and DBA interact with the MySQL database through Connector APIs and service layers that are above the storage engines. If application changes bring about requirements that demand the underlying storage engine change, or that one or more storage engines be added to support new needs, no significant coding or process changes are required to make things work. The MySQL server architecture shields the application from the underlying complexity of the storage engine by presenting a consistent and easy-to-use API that applies across storage engines.

13.2.1. Pluggable Storage Engine Architecture

MySQL Server uses a pluggable storage engine architecture that enables storage engines to be loaded into and unloaded from a running MySQL server.

Plugging in a Storage Engine

Before a storage engine can be used, the storage engine plugin shared library must be loaded into MySQL using the INSTALL PLUGIN statement. For example, if the EXAMPLE engine plugin is named example and the shared library is named ha_example.so, you load it with the following statement:

mysql> INSTALL PLUGIN example SONAME 'ha_example.so';

To install a pluggable storage engine, the plugin file must be located in the MySQL plugin directory, and the user issuing the INSTALL PLUGIN statement must have INSERT privilege for the mysql.plugin table.

The shared library must be located in the MySQL server plugin directory, the location of which is given by the plugin_dir system variable.

Unplugging a Storage Engine

To unplug a storage engine, use the UNINSTALL PLUGIN statement:

mysql> UNINSTALL PLUGIN example;

If you unplug a storage engine that is needed by existing tables, those tables become inaccessible, but will still be present on disk (where applicable). Ensure that there are no tables using a storage engine before you unplug the storage engine.

13.2.2. The Common Database Server Layer

A MySQL pluggable storage engine is the component in the MySQL database server that is responsible for performing the actual data I/O operations for a database as well as enabling and enforcing certain feature sets that target a specific application need. A major benefit of using specific storage engines is that you are only delivered the features needed for a particular application, and therefore you have less system overhead in the database, with the end result being more efficient and higher database performance. This is one of the reasons that MySQL has always been known to have such high performance, matching or beating proprietary monolithic databases in industry standard benchmarks.

From a technical perspective, what are some of the unique supporting infrastructure components that are in a storage engine? Some of the key feature differentiations include:

  • Concurrency: Some applications have more granular lock requirements (such as row-level locks) than others. Choosing the right locking strategy can reduce overhead and therefore improve overall performance. This area also includes support for capabilities such as multi-version concurrency control or “snapshot” read.

  • Transaction Support: Not every application needs transactions, but for those that do, there are very well defined requirements such as ACID compliance and more.

  • Referential Integrity: The need to have the server enforce relational database referential integrity through DDL defined foreign keys.

  • Physical Storage: This involves everything from the overall page size for tables and indexes as well as the format used for storing data to physical disk.

  • Index Support: Different application scenarios tend to benefit from different index strategies. Each storage engine generally has its own indexing methods, although some (such as B-tree indexes) are common to nearly all engines.

  • Memory Caches: Different applications respond better to some memory caching strategies than others, so although some memory caches are common to all storage engines (such as those used for user connections or MySQL's high-speed Query Cache), others are uniquely defined only when a particular storage engine is put in play.

  • Performance Aids: This includes multiple I/O threads for parallel operations, thread concurrency, database checkpointing, bulk insert handling, and more.

  • Miscellaneous Target Features: This may include support for geospatial operations, security restrictions for certain data manipulation operations, and other similar features.

Each set of the pluggable storage engine infrastructure components are designed to offer a selective set of benefits for a particular application. Conversely, avoiding a set of component features helps reduce unnecessary overhead. It stands to reason that understanding a particular application's set of requirements and selecting the proper MySQL storage engine can have a dramatic impact on overall system efficiency and performance.

13.3. The InnoDB Storage Engine

InnoDB is a high-reliability and high-performance storage engine for MySQL. Starting with MySQL 5.5, it is the default MySQL storage engine. Key advantages of InnoDB include:

  • Its design follows the ACID model, with transactions featuring commit, rollback, and crash-recovery capabilities to protect user data.

  • Row-level locking and Oracle-style consistent reads increase multi-user concurrency and performance.

  • InnoDB tables arrange your data on disk to optimize common queries based on primary keys. Each InnoDB table has a primary key index called the clustered index that organizes the data to minimize I/O for primary key lookups.

  • To maintain data integrity, InnoDB also supports FOREIGN KEY referential-integrity constraints.

  • You can freely mix InnoDB tables with tables from other MySQL storage engines, even within the same statement. For example, you can use a join operation to combine data from InnoDB and MEMORY tables in a single query.

To determine whether your server supports InnoDB use the SHOW ENGINES statement. See Section 12.7.5.17, “SHOW ENGINES Синтаксис”.

Table 13.2. InnoDB Storage Engine Features

Storage limits64TBTransactionsYesLocking granularityRow
MVCCYesGeospatial data type supportYesGeospatial indexing supportNo
B-tree indexesYesHash indexesNo[a]Full-text search indexesNo
Clustered indexesYesData cachesYesIndex cachesYes
Compressed dataYes[b]Encrypted data[c]YesCluster database supportNo
Replication support[d]YesForeign key supportYesBackup / point-in-time recovery[e]Yes
Query cache supportYesUpdate statistics for data dictionaryYes  

[a] InnoDB utilizes hash indexes internally for its Adaptive Hash Index feature.

[b] Compressed InnoDB tables require the InnoDB Barracuda file format.

[c] Implemented in the server (via encryption functions), rather than in the storage engine.

[d] Implemented in the server, rather than in the storage product.

[e] Implemented in the server, rather than in the storage product.

InnoDB has been designed for maximum performance when processing large data volumes. Its CPU efficiency is probably not matched by any other disk-based relational database engine.

The InnoDB storage engine maintains its own buffer pool for caching data and indexes in main memory. InnoDB stores its tables and indexes in a tablespace, which may consist of several files (or raw disk partitions). This is different from, for example, MyISAM tables where each table is stored using separate files. InnoDB tables can be very large even on operating systems where file size is limited to 2GB.

InnoDB is published under the same GNU GPL License Version 2 (of June 1991) as MySQL. For more information on MySQL licensing, see http://www.mysql.com/company/legal/licensing/.

Additional Resources

  • A forum dedicated to the InnoDB storage engine is available at http://forums.mysql.com/list.php?22.

  • The InnoDB storage engine in MySQL 5.5 releases includes a number performance improvements that in MySQL 5.1 were only available by installing the InnoDB Plugin. This latest InnoDB (now known as InnoDB 1.1) offers new features, improved performance and scalability, enhanced reliability and new capabilities for flexibility and ease of use. Among the top features are Fast Index Creation, table and index compression, file format management, new INFORMATION_SCHEMA tables, capacity tuning, multiple background I/O threads, multiple buffer pools, and group commit.

    For information about these features, see Section 13.4, “New Features of InnoDB 1.1”.

  • The MySQL Enterprise Backup product lets you back up a running MySQL database, including InnoDB and MyISAM tables, with minimal disruption to operations while producing a consistent snapshot of the database. When MySQL Enterprise Backup is copying InnoDB tables, reads and writes to both InnoDB and MyISAM tables can continue. During the copying of MyISAM and other non-InnoDB tables, reads (but not writes) to those tables are permitted. In addition, MySQL Enterprise Backup can create compressed backup files, and back up subsets of InnoDB tables. In conjunction with the MySQL binary log, you can perform point-in-time recovery. MySQL Enterprise Backup is included as part of the MySQL Enterprise subscription.

    For a more complete description of MySQL Enterprise Backup, see MySQL Enterprise Backup User's Guide (Version 3.5.4).

13.3.1. InnoDB as the Default MySQL Storage Engine

MySQL has a well-earned reputation for being easy-to-use and delivering performance and scalability. In previous versions of MySQL, MyISAM was the default storage engine. In our experience, most users never changed the default settings. With MySQL 5.5, InnoDB becomes the default storage engine. Again, we expect most users will not change the default settings. But, because of InnoDB, the default settings deliver the benefits users expect from their RDBMS: ACID Transactions, Referential Integrity, and Crash Recovery. Let's explore how using InnoDB tables improves your life as a MySQL user, DBA, or developer.

Trends in Storage Engine Usage

In the first years of MySQL growth, early web-based applications didn't push the limits of concurrency and availability. In 2010, hard drive and memory capacity and the performance/price ratio have all gone through the roof. Users pushing the performance boundaries of MySQL care a lot about reliability and crash recovery. MySQL databases are big, busy, robust, distributed, and important.

InnoDB hits the sweet spot of these top user priorities. The trend of storage engine usage has shifted in favor of the more efficient InnoDB. With MySQL 5.5, the time is right to make InnoDB the default storage engine.

Consequences of InnoDB as Default MySQL Storage Engine

Starting from MySQL 5.5.5, the default storage engine for new tables is InnoDB. This change applies to newly created tables that don't specify a storage engine with a clause such as ENGINE=MyISAM. (Given this change of default behavior, MySQL 5.5 might be a logical point to evaluate whether your tables that do use MyISAM could benefit from switching to InnoDB.)

The mysql and information_schema databases, that implement some of the MySQL internals, still use MyISAM. In particular, you cannot switch the grant tables to use InnoDB.

Benefits of InnoDB Tables

If you use MyISAM tables but aren't tied to them for technical reasons, you'll find many things more convenient when you use InnoDB tables in MySQL 5.5:

  • If your server crashes because of a hardware or software issue, regardless of what was happening in the database at the time, you don't need to do anything special after restarting the database. InnoDB automatically finalizes any changes that were committed before the time of the crash, and undoes any changes that were in process but not committed. Just restart and continue where you left off.

  • The InnoDB buffer pool caches table and index data as the data is accessed. Frequently used data is processed directly from memory. This cache applies to so many types of information, and speeds up processing so much, that dedicated database servers assign up to 80% of their physical memory to the InnoDB buffer pool.

  • If you split up related data into different tables, you can set up foreign keys that enforce referential integrity. Update or delete data, and the related data in other tables is updated or deleted automatically. Try to insert data into a secondary table without corresponding data in the primary table, and it gets kicked out automatically.

  • If data becomes corrupted on disk or in memory, a checksum mechanism alerts you to the bogus data before you use it.

  • When you design your database with appropriate primary key columns for each table, operations involving those columns are automatically optimized. It is very fast to reference the primary key columns in WHERE clauses, ORDER BY clauses, GROUP BY clauses, and join operations.

  • Inserts, updates, deletes are optimized by an automatic mechanism called change buffering. InnoDB not only allows concurrent read and write access to the same table, it caches changed data to streamline disk I/O.

  • Performance benefits are not limited to giant tables with long-running queries. When the same rows are accessed over and over from a table, a feature called the Adaptive Hash Index takes over to make these lookups even faster, as if they came out of a hash table.

Best Practices for InnoDB Tables

If you have been using InnoDB for a long time, you already know about features like transactions and foreign keys. If not, read about them throughout this chapter. To make a long story short:

  • Specify a primary key for every table using the most frequently queried column or columns, or an auto-increment value if there isn't an obvious primary key.

  • Embrace the idea of joins, where data is pulled from multiple tables based on identical ID values from those tables. For fast join performance, define foreign keys on the join columns, and declare those columns with the same datatype in each table. The foreign keys also propagate deletes or updates to all affected tables, and prevent insertion of data in a child table if the corresponding IDs are not present in the parent table.

  • Turn off autocommit. Committing hundreds of times a second puts a cap on performance (limited by the write speed of your storage device).

  • Bracket sets of related changes, logical units of work, with START TRANSACTION and COMMIT statements. While you don't want to commit too often, you also don't want to issue huge batches of INSERT, UPDATE, or DELETE statements that run for hours without committing.

  • Stop using LOCK TABLE statements. InnoDB can handle multiple sessions all reading and writing to the same table at once, without sacrificing reliability or high performance. To get exclusive write access to a set of rows, use the SELECT ... FOR UPDATE syntax to lock just the rows you intend to update.

  • Enable the innodb_file_per_table option to put the data and indexes for individual tables into separate files, instead of in a single giant system tablespace. (This setting is required to use some of the other features, such as table compression and fast truncation.)

  • Evaluate whether your data and access patterns benefit from the new InnoDB table compression feature (ROW_FORMAT=COMPRESSED on the CREATE TABLE statement. You can compress InnoDB tables without sacrificing read/write capability.

  • Run your server with the option --sql_mode=NO_ENGINE_SUBSTITUTION to prevent tables being created with a different storage engine if there is an issue with the one specified in the ENGINE= clause of CREATE TABLE.

Recent Improvements for InnoDB Tables (from the Plugin Era)

If you have experience with InnoDB, but not the recent incarnation known as the InnoDB Plugin, read about the latest enhancements in Section 13.4, “New Features of InnoDB 1.1”. To make a long story short:

  • You can compress tables and associated indexes.

  • You can create and drop indexes with much less performance or availability impact than before.

  • Truncating a table is very fast, and can free up disk space for the operating system to reuse, rather than freeing up space within the system tablespace that only InnoDB could reuse.

  • The storage layout for table data is more efficient for BLOBs and long text fields.

  • You can monitor the internal workings of the storage engine by querying INFORMATION_SCHEMA tables.

  • You can monitor the performance details of the storage engine by querying performance_schema tables.

  • There are many many performance improvements. In particular, crash recovery, the automatic process that makes all data consistent when the database is restarted, is fast and reliable. (Now much much faster than long-time InnoDB users are used to.) The bigger the database, the more dramatic the speedup.

    Most new performance features are automatic, or at most require setting a value for a configuration option. For details, see Section 13.4.7, “InnoDB Performance and Scalability Enhancements”. For InnoDB-specific tuning techniques you can apply in your application code, see Section 7.5, “Optimizing for InnoDB Tables”. Advanced users can review Section 13.3.4, “InnoDB Startup Options and System Variables”.

Testing and Benchmarking with InnoDB as Default Storage Engine

Even before completing your upgrade to MySQL 5.5, you can preview whether your database server or application works correctly with InnoDB as the default storage engine. To set up InnoDB as the default storage engine with an earlier MySQL release, either specify on the command line --default-storage-engine=InnoDB, or add to your my.cnf file default-storage-engine=innodb in the [mysqld] section, then restart the server.

Since changing the default storage engine only affects new tables as they are created, run all your application installation and setup steps to confirm that everything installs properly. Then exercise all the application features to make sure all the data loading, editing, and querying features work. If a table relies on some MyISAM-specific feature, you'll receive an error; add the ENGINE=MyISAM clause to the CREATE TABLE statement to avoid the error. (For example, tables that rely on full-text search must be MyISAM tables rather than InnoDB ones.)

If you didn't make a deliberate decision about the storage engine, and you just want to preview how certain tables work when they're created under InnoDB, issue the command ALTER TABLE table_name ENGINE=InnoDB; for each table. Or, to run test queries and other statements without disturbing the original table, make a copy like so:

CREATE TABLE InnoDB_Table (...) ENGINE=InnoDB AS SELECT * FROM MyISAM_Table;

Since there are so many performance enhancements in the InnoDB that is part of MySQL 5.5, to get a true idea of the performance with a full application under a realistic workload, install the real MySQL 5.5 and run benchmarks.

Test the full application lifecycle, from installation, through heavy usage, and server restart. Kill the server process while the database is busy to simulate a power failure, and verify that the data is recovered successfully when you restart the server.

Test any replication configurations, especially if you use different MySQL versions and options on the master and the slaves.

Verifying that InnoDB is the Default Storage Engine

To know what the status of InnoDB is, whether you're doing what-if testing with an older MySQL or comprehensive testing with MySQL 5.5:

  • Issue the command SHOW VARIABLES LIKE 'have_innodb'; to confirm that InnoDB is available at all. If the result is NO, you have a mysqld binary that was compiled without InnoDB support and you need to get a different one. If the result is DISABLED, go back through your startup options and configuration file and get rid of any skip-innodb option.

  • Issue the command SHOW ENGINES; to see all the different MySQL storage engines. Look for DEFAULT in the InnoDB line.

13.3.2. Configuring InnoDB

The first decisions to make about InnoDB configuration involve how to lay out InnoDB data files, and how much memory to allocate for the InnoDB storage engine. You record these choices either by recording them in a configuration file that MySQL reads at startup, or by specifying them as command-line options in a startup script.

Overview of InnoDB Tablespace and Log Files

Two important disk-based resources managed by the InnoDB storage engine are its tablespace data files and its log files. If you specify no InnoDB configuration options, MySQL creates an auto-extending 10MB data file named ibdata1 and two 5MB log files named ib_logfile0 and ib_logfile1 in the MySQL data directory. To get good performance, explicitly provide InnoDB parameters as discussed in the following examples. Naturally, edit the settings to suit your hardware and requirements.

The examples shown here are representative. See Section 13.3.4, “InnoDB Startup Options and System Variables” for additional information about InnoDB-related configuration parameters.

Considerations for Storage Devices

In some cases, database performance improves if the data is not all placed on the same physical disk. Putting log files on a different disk from data is very often beneficial for performance. The example illustrates how to do this. It places the two data files on different disks and places the log files on the third disk. InnoDB fills the tablespace beginning with the first data file. You can also use raw disk partitions (raw devices) as InnoDB data files, which may speed up I/O. See Section 13.3.3.1, “Using Raw Devices for the Shared Tablespace”.

Caution

InnoDB is a transaction-safe (ACID compliant) storage engine for MySQL that has commit, rollback, and crash-recovery capabilities to protect user data. However, it cannot do so if the underlying operating system or hardware does not work as advertised. Many operating systems or disk subsystems may delay or reorder write operations to improve performance. On some operating systems, the very fsync() system call that should wait until all unwritten data for a file has been flushed might actually return before the data has been flushed to stable storage. Because of this, an operating system crash or a power outage may destroy recently committed data, or in the worst case, even corrupt the database because of write operations having been reordered. If data integrity is important to you, perform some “pull-the-plug” tests before using anything in production. On Mac OS X 10.3 and up, InnoDB uses a special fcntl() file flush method. Under Linux, it is advisable to disable the write-back cache.

On ATA/SATA disk drives, a command such hdparm -W0 /dev/hda may work to disable the write-back cache. Beware that some drives or disk controllers may be unable to disable the write-back cache.

Caution

If reliability is a consideration for your data, do not configure InnoDB to use data files or log files on NFS volumes. Potential problems vary according to OS and version of NFS, and include such issues as lack of protection from conflicting writes, and limitations on maximum file sizes.

Specifying the Location and Size for InnoDB Tablespace Files

To set up the InnoDB tablespace files, use the innodb_data_file_path option in the [mysqld] section of the my.cnf option file. On Windows, you can use my.ini instead. The value of innodb_data_file_path should be a list of one or more data file specifications. If you name more than one data file, separate them by semicolon (“;”) characters:

innodb_data_file_path=datafile_spec1[;datafile_spec2]...

For example, the following setting explicitly creates a tablespace having the same characteristics as the default:

[mysqld]
innodb_data_file_path=ibdata1:10M:autoextend

This setting configures a single 10MB data file named ibdata1 that is auto-extending. No location for the file is given, so by default, InnoDB creates it in the MySQL data directory.

Sizes are specified using K, M, or G suffix letters to indicate units of KB, MB, or GB.

A tablespace containing a fixed-size 50MB data file named ibdata1 and a 50MB auto-extending file named ibdata2 in the data directory can be configured like this:

[mysqld]
innodb_data_file_path=ibdata1:50M;ibdata2:50M:autoextend

The full syntax for a data file specification includes the file name, its size, and several optional attributes:

file_name:file_size[:autoextend[:max:max_file_size]]

The autoextend and max attributes can be used only for the last data file in the innodb_data_file_path line.

If you specify the autoextend option for the last data file, InnoDB extends the data file if it runs out of free space in the tablespace. The increment is 8MB at a time by default. To modify the increment, change the innodb_autoextend_increment system variable.

If the disk becomes full, you might want to add another data file on another disk. For tablespace reconfiguration instructions, see Section 13.3.6, “Adding, Removing, or Resizing InnoDB Data and Log Files”.

InnoDB is not aware of the file system maximum file size, so be cautious on file systems where the maximum file size is a small value such as 2GB. To specify a maximum size for an auto-extending data file, use the max attribute following the autoextend attribute. Use the max attribute only in cases where constraining disk usage is of critical importance, because exceeding the maximum size causes a fatal error, possibly including a crash. The following configuration permits ibdata1 to grow up to a limit of 500MB:

[mysqld]
innodb_data_file_path=ibdata1:10M:autoextend:max:500M

InnoDB creates tablespace files in the MySQL data directory by default. To specify a location explicitly, use the innodb_data_home_dir option. For example, to use two files named ibdata1 and ibdata2 but create them in the /ibdata directory, configure InnoDB like this:

[mysqld]
innodb_data_home_dir = /ibdata
innodb_data_file_path=ibdata1:50M;ibdata2:50M:autoextend
Замечание

InnoDB does not create directories, so make sure that the /ibdata directory exists before you start the server. This is also true of any log file directories that you configure. Use the Unix or DOS mkdir command to create any necessary directories.

Make sure that the MySQL server has the proper access rights to create files in the data directory. More generally, the server must have access rights in any directory where it needs to create data files or log files.

InnoDB forms the directory path for each data file by textually concatenating the value of innodb_data_home_dir to the data file name, adding a path name separator (slash or backslash) between values if necessary. If the innodb_data_home_dir option is not specified in my.cnf at all, the default value is the “dot” directory ./, which means the MySQL data directory. (The MySQL server changes its current working directory to its data directory when it begins executing.)

If you specify innodb_data_home_dir as an empty string, you can specify absolute paths for the data files listed in the innodb_data_file_path value. The following example is equivalent to the preceding one:

[mysqld]
innodb_data_home_dir =
innodb_data_file_path=/ibdata/ibdata1:50M;/ibdata/ibdata2:50M:autoextend

Specifying InnoDB Configuration Options

Sample my.cnf file for small systems. Suppose that you have a computer with 512MB RAM and one hard disk. The following example shows possible configuration parameters in my.cnf or my.ini for InnoDB, including the autoextend attribute. The example suits most users, both on Unix and Windows, who do not want to distribute InnoDB data files and log files onto several disks. It creates an auto-extending data file ibdata1 and two InnoDB log files ib_logfile0 and ib_logfile1 in the MySQL data directory.

[mysqld]
# You can write your other MySQL server options here
# ...
# Data files must be able to hold your data and indexes.
# Make sure that you have enough free disk space.
innodb_data_file_path = ibdata1:10M:autoextend
#
# Set buffer pool size to 50-80% of your computer's memory
innodb_buffer_pool_size=256M
innodb_additional_mem_pool_size=20M
#
# Set the log file size to about 25% of the buffer pool size
innodb_log_file_size=64M
innodb_log_buffer_size=8M
#
innodb_flush_log_at_trx_commit=1

Note that data files must be less than 2GB in some file systems. The combined size of the log files must be less than 4GB. The combined size of data files must be at least 10MB.

Setting Up the InnoDB System Tablespace

When you create an InnoDB system tablespace for the first time, it is best that you start the MySQL server from the command prompt. InnoDB then prints the information about the database creation to the screen, so you can see what is happening. For example, on Windows, if mysqld is located in C:\Program Files\MySQL\MySQL Server 5.5\bin, you can start it like this:

C:\> "C:\Program Files\MySQL\MySQL Server 5.5\bin\mysqld" --console

If you do not send server output to the screen, check the server's error log to see what InnoDB prints during the startup process.

For an example of what the information displayed by InnoDB should look like, see Section 13.3.3.2, “Creating the InnoDB Tablespace”.

Editing the MySQL Configuration File

You can place InnoDB options in the [mysqld] group of any option file that your server reads when it starts. The locations for option files are described in Section 4.2.3.3, “Using Option Files”.

If you installed MySQL on Windows using the installation and configuration wizards, the option file will be the my.ini file located in your MySQL installation directory. See Section 2.3.5.1, “Starting the MySQL Server Instance Configuration Wizard”.

If your PC uses a boot loader where the C: drive is not the boot drive, your only option is to use the my.ini file in your Windows directory (typically C:\WINDOWS). You can use the SET command at the command prompt in a console window to print the value of WINDIR:

C:\> SET WINDIR
windir=C:\WINDOWS

To make sure that mysqld reads options only from a specific file, use the --defaults-file option as the first option on the command line when starting the server:

mysqld --defaults-file=your_path_to_my_cnf

Sample my.cnf file for large systems. Suppose that you have a Linux computer with 2GB RAM and three 60GB hard disks at directory paths /, /dr2 and /dr3. The following example shows possible configuration parameters in my.cnf for InnoDB.

[mysqld]
# You can write your other MySQL server options here
# ...
innodb_data_home_dir =
#
# Data files must be able to hold your data and indexes
innodb_data_file_path = /db/ibdata1:2000M;/dr2/db/ibdata2:2000M:autoextend
#
# Set buffer pool size to 50-80% of your computer's memory,
# but make sure on Linux x86 total memory usage is < 2GB
innodb_buffer_pool_size=1G
innodb_additional_mem_pool_size=20M
innodb_log_group_home_dir = /dr3/iblogs
#
# Set the log file size to about 25% of the buffer pool size
innodb_log_file_size=250M
innodb_log_buffer_size=8M
#
innodb_flush_log_at_trx_commit=1
innodb_lock_wait_timeout=50
#
# Uncomment the next line if you want to use it
#innodb_thread_concurrency=5

Determining the Maximum Memory Allocation for InnoDB

Warning

On 32-bit GNU/Linux x86, be careful not to set memory usage too high. glibc may permit the process heap to grow over thread stacks, which crashes your server. It is a risk if the value of the following expression is close to or exceeds 2GB:

innodb_buffer_pool_size
+ key_buffer_size
+ max_connections*(sort_buffer_size+read_buffer_size+binlog_cache_size)
+ max_connections*2MB

Each thread uses a stack (often 2MB, but only 256KB in MySQL binaries provided by Oracle Corporation.) and in the worst case also uses sort_buffer_size + read_buffer_size additional memory.

Tuning other mysqld server parameters. The following values are typical and suit most users:

[mysqld]
skip-external-locking
max_connections=200
read_buffer_size=1M
sort_buffer_size=1M
#
# Set key_buffer to 5 - 50% of your RAM depending on how much
# you use MyISAM tables, but keep key_buffer_size + InnoDB
# buffer pool size < 80% of your RAM
key_buffer_size=value

On Linux, if the kernel is enabled for large page support, InnoDB can use large pages to allocate memory for its buffer pool and additional memory pool. See Section 7.11.4.2, “Enabling Large Page Support”.

Turning Off InnoDB

Oracle recommends InnoDB as the preferred storage engine for typical database applications, from single-user wikis and blogs running on a local system, to high-end applications pushing the limits of performance. In MySQL 5.5, InnoDB is is the default storage engine for new tables.

If you do not want to use InnoDB tables, start the server with the --innodb=OFF or --skip-innodb option to disable the InnoDB storage engine. In this case, because the default storage engine is InnoDB, the server will not start unless you also use --default-storage-engine to set the default to some other engine.

13.3.3. Using Per-Table Tablespaces

By default, all InnoDB tables and indexes are stored in the system tablespace. As an alternative, you can store each InnoDB table and its indexes in its own file. This feature is called “multiple tablespaces” because each table that is created when this setting is in effect has its own tablespace.

Using multiple tablespaces is useful in a number of situations:

  • You can back up or restore a single table quickly without interrupting the use of other InnoDB tables, using the MySQL Enterprise Backup product. See Restoring a Single .ibd File for the procedure and restrictions.

  • Storing specific tables on separate physical disks, for I/O optimization or backup purposes.

  • Restoring backups of single tables quickly without interrupting the use of other InnoDB tables.

  • Using compressed row format to compress table data.

  • Reclaiming disk space when truncating a table.

Enabling and Disabling Multiple Tablespaces

To enable multiple tablespaces, start the server with the --innodb_file_per_table option. For example, add a line to the [mysqld] section of my.cnf:

[mysqld]
innodb_file_per_table

With multiple tablespaces enabled, InnoDB stores each newly created table in its own tbl_name.ibd file in the appropriate database directory. Unlike the MyISAM storage engine, with its separate tbl_name.MYD and tbl_name.MYI files for indexes and data, InnoDB stores the data and the indexes together in a single .ibd file. The tbl_name.frm file is still created as usual.

If you remove the innodb_file_per_table line from my.cnf and restart the server, InnoDB creates any new tables inside the shared tablespace files.

You can always access both tables in the system tablespace and tables in their own tablespaces, regardless of the file-per-table setting. To move a table from the system tablespace to its own tablespace, or vice versa, you can change the innodb_file_per_table setting and issue the command:

-- Move table from system tablespace to its own tablespace.
SET GLOBAL innodb_file_per_table=1;
ALTER TABLE table_name ENGINE=InnoDB;
-- Move table from its own tablespace to system tablespace.
SET GLOBAL innodb_file_per_table=0;
ALTER TABLE table_name ENGINE=InnoDB;
Замечание

InnoDB always needs the shared tablespace because it puts its internal data dictionary and undo logs there. The .ibd files are not sufficient for InnoDB to operate.

When a table is moved out of the system tablespace into its own .ibd file, the data files that make up the system tablespace remain the same size. The space formerly occupied by the table can be reused for new InnoDB data, but is not reclaimed for use by the operating system. When moving large InnoDB tables out of the system tablespace, where disk space is limited, you might prefer to turn on innodb_file_per_table and then recreate the entire instance using the mysqldump command.

Portability Considerations for .ibd Files

You cannot freely move .ibd files between database directories as you can with MyISAM table files. The table definition stored in the InnoDB shared tablespace includes the database name. The transaction IDs and log sequence numbers stored in the tablespace files also differ between databases.

To move an .ibd file and the associated table from one database to another, use a RENAME TABLE statement:

RENAME TABLE db1.tbl_name TO db2.tbl_name;

If you have a “clean” backup of an .ibd file, you can restore it to the MySQL installation from which it originated as follows:

  1. The table must not have been dropped or truncated since you copied the .ibd file, because doing so changes the table ID stored inside the tablespace.

  2. Issue this ALTER TABLE statement to delete the current .ibd file:

    ALTER TABLE tbl_name DISCARD TABLESPACE;
    
  3. Copy the backup .ibd file to the proper database directory.

  4. Issue this ALTER TABLE statement to tell InnoDB to use the new .ibd file for the table:

    ALTER TABLE tbl_name IMPORT TABLESPACE;
    

In this context, a “clean.ibd file backup is one for which the following requirements are satisfied:

  • There are no uncommitted modifications by transactions in the .ibd file.

  • There are no unmerged insert buffer entries in the .ibd file.

  • Purge has removed all delete-marked index records from the .ibd file.

  • mysqld has flushed all modified pages of the .ibd file from the buffer pool to the file.

You can make a clean backup .ibd file using the following method:

  1. Stop all activity from the mysqld server and commit all transactions.

  2. Wait until SHOW ENGINE INNODB STATUS shows that there are no active transactions in the database, and the main thread status of InnoDB is Waiting for server activity. Then you can make a copy of the .ibd file.

Another method for making a clean copy of an .ibd file is to use the MySQL Enterprise Backup product:

  1. Use MySQL Enterprise Backup to back up the InnoDB installation.

  2. Start a second mysqld server on the backup and let it clean up the .ibd files in the backup.

13.3.3.1. Using Raw Devices for the Shared Tablespace

You can use raw disk partitions as data files in the system tablespace. Using a raw disk, you can perform nonbuffered I/O on Windows and on some Unix systems without file system overhead. Perform tests with and without raw partitions to verify whether this change actually improves performance on your system.

When you create a new data file, put the keyword newraw immediately after the data file size in innodb_data_file_path. The partition must be at least as large as the size that you specify. Note that 1MB in InnoDB is 1024 × 1024 bytes, whereas 1MB in disk specifications usually means 1,000,000 bytes.

[mysqld]
innodb_data_home_dir=
innodb_data_file_path=/dev/hdd1:3Gnewraw;/dev/hdd2:2Gnewraw

The next time you start the server, InnoDB notices the newraw keyword and initializes the new partition. However, do not create or change any InnoDB tables yet. Otherwise, when you next restart the server, InnoDB reinitializes the partition and your changes are lost. (As a safety measure InnoDB prevents users from modifying data when any partition with newraw is specified.)

After InnoDB has initialized the new partition, stop the server, change newraw in the data file specification to raw:

[mysqld]
innodb_data_home_dir=
innodb_data_file_path=/dev/hdd1:3Graw;/dev/hdd2:2Graw

Then restart the server and InnoDB permits changes to be made.

On Windows, you can allocate a disk partition as a data file like this:

[mysqld]
innodb_data_home_dir=
innodb_data_file_path=//./D::10Gnewraw

The //./ corresponds to the Windows syntax of \\.\ for accessing physical drives.

When you use a raw disk partition, be sure that it has permissions that enable read and write access by the account used for running the MySQL server. For example, if you run the server as the mysql user, the partition must permit read and write access to mysql. If you run the server with the --memlock option, the server must be run as root, so the partition must permit access to root.

13.3.3.2. Creating the InnoDB Tablespace

Suppose that you have installed MySQL and have edited your option file so that it contains the necessary InnoDB configuration parameters. Before starting MySQL, verify that the directories you have specified for InnoDB data files and log files exist and that the MySQL server has access rights to those directories. InnoDB does not create directories, only files. Check also that you have enough disk space for the data and log files.

It is best to run the MySQL server mysqld from the command prompt when you first start the server with InnoDB enabled, not from mysqld_safe or as a Windows service. When you run from a command prompt you see what mysqld prints and what is happening. On Unix, just invoke mysqld. On Windows, start mysqld with the --console option to direct the output to the console window.

When you start the MySQL server after initially configuring InnoDB in your option file, InnoDB creates your data files and log files, and prints something like this:

InnoDB: The first specified datafile /home/heikki/data/ibdata1
did not exist:
InnoDB: a new database to be created!
InnoDB: Setting file /home/heikki/data/ibdata1 size to 134217728
InnoDB: Database physically writes the file full: wait...
InnoDB: datafile /home/heikki/data/ibdata2 did not exist:
new to be created
InnoDB: Setting file /home/heikki/data/ibdata2 size to 262144000
InnoDB: Database physically writes the file full: wait...
InnoDB: Log file /home/heikki/data/logs/ib_logfile0 did not exist:
new to be created
InnoDB: Setting log file /home/heikki/data/logs/ib_logfile0 size
to 5242880
InnoDB: Log file /home/heikki/data/logs/ib_logfile1 did not exist:
new to be created
InnoDB: Setting log file /home/heikki/data/logs/ib_logfile1 size
to 5242880
InnoDB: Doublewrite buffer not found: creating new
InnoDB: Doublewrite buffer created
InnoDB: Creating foreign key constraint system tables
InnoDB: Foreign key constraint system tables created
InnoDB: Started
mysqld: ready for connections

At this point InnoDB has initialized its tablespace and log files. You can connect to the MySQL server with the usual MySQL client programs like mysql. When you shut down the MySQL server with mysqladmin shutdown, the output is like this:

010321 18:33:34  mysqld: Normal shutdown
010321 18:33:34  mysqld: Shutdown Complete
InnoDB: Starting shutdown...
InnoDB: Shutdown completed

You can look at the data file and log directories and you see the files created there. When MySQL is started again, the data files and log files have been created already, so the output is much briefer:

InnoDB: Started
mysqld: ready for connections

If you add the innodb_file_per_table option to my.cnf, InnoDB stores each table in its own .ibd file in the same MySQL database directory where the .frm file is created. See Section 13.3.3, “Using Per-Table Tablespaces”.

13.3.3.3. Troubleshooting InnoDB I/O Problems

The troubleshooting steps for InnoDB I/O problems depend on when the problem occurs: during startup of the MySQL server, or during normal operations when a DML or DDL statement fails due to problems at the file system level.

Initialization Problems

If something goes wrong when InnoDB attempts to initialize its tablespace or its log files, delete all files created by InnoDB: all ibdata files and all ib_logfile files. If you already created some InnoDB tables, also delete the corresponding .frm files for these tables, and any .ibd files if you are using multiple tablespaces, from the MySQL database directories. Then try the InnoDB database creation again. For easiest troubleshooting, start the MySQL server from a command prompt so that you see what is happening.

Runtime Problems

If InnoDB prints an operating system error during a file operation, usually the problem has one of the following solutions:

  • Make sure the InnoDB data file directory and the InnoDB log directory exist.

  • Make sure mysqld has access rights to create files in those directories.

  • Make sure mysqld can read the proper my.cnf or my.ini option file, so that it starts with the options that you specified.

  • Make sure the disk is not full and you are not exceeding any disk quota.

  • Make sure that the names you specify for subdirectories and data files do not clash.

  • Doublecheck the syntax of the innodb_data_home_dir and innodb_data_file_path values. In particular, any MAX value in the innodb_data_file_path option is a hard limit, and exceeding that limit causes a fatal error.

13.3.4. InnoDB Startup Options and System Variables

This section describes the InnoDB-related command options and system variables. System variables that are true or false can be enabled at server startup by naming them, or disabled by using a --skip- prefix. For example, to enable or disable InnoDB checksums, you can use --innodb_checksums or --skip-innodb_checksums on the command line, or innodb_checksums or skip-innodb_checksums in an option file. System variables that take a numeric value can be specified as --var_name=value on the command line or as var_name=value in option files. For more information on specifying options and system variables, see Section 4.2.3, “Specifying Program Options”. Many of the system variables can be changed at runtime (see Section 5.1.4.2, “Dynamic System Variables”).

Certain options control the locations and layout of the InnoDB data files. Section 13.3.2, “Configuring InnoDB explains how to use these options. Many other options, that you might not use initially, help to tune InnoDB performance characteristics based on machine capacity and your database workload. The performance-related options are explained in Section 13.3.14, “InnoDB Performance Tuning and Troubleshooting” and Section 13.4.7, “InnoDB Performance and Scalability Enhancements”.

Table 13.3. InnoDB Option/Variable Reference

NameCmd-LineOption fileSystem VarStatus VarVar ScopeDynamic
foreign_key_checks  Yes BothYes
have_innodb  Yes GlobalNo
ignore-builtin-innodbYesYes  GlobalNo
- Variable: ignore_builtin_innodb  Yes GlobalNo
innodbYesYes    
innodb_adaptive_flushingYesYesYes GlobalYes
innodb_adaptive_hash_indexYesYesYes GlobalYes
innodb_additional_mem_pool_sizeYesYesYes GlobalNo
innodb_autoextend_incrementYesYesYes GlobalYes
innodb_autoinc_lock_modeYesYesYes GlobalNo
innodb_buffer_pool_instancesYesYesYes GlobalNo
Innodb_buffer_pool_pages_data   YesGlobalNo
Innodb_buffer_pool_pages_dirty   YesGlobalNo
Innodb_buffer_pool_pages_flushed   YesGlobalNo
Innodb_buffer_pool_pages_free   YesGlobalNo
Innodb_buffer_pool_pages_latched   YesGlobalNo
Innodb_buffer_pool_pages_misc   YesGlobalNo
Innodb_buffer_pool_pages_total   YesGlobalNo
Innodb_buffer_pool_read_ahead   YesGlobalNo
Innodb_buffer_pool_read_ahead_evicted   YesGlobalNo
Innodb_buffer_pool_read_requests   YesGlobalNo
Innodb_buffer_pool_reads   YesGlobalNo
innodb_buffer_pool_sizeYesYesYes GlobalNo
Innodb_buffer_pool_wait_free   YesGlobalNo
Innodb_buffer_pool_write_requests   YesGlobalNo
innodb_change_bufferingYesYesYes GlobalYes
innodb_checksumsYesYesYes GlobalNo
innodb_commit_concurrencyYesYesYes GlobalYes
innodb_concurrency_ticketsYesYesYes GlobalYes
innodb_data_file_pathYesYesYes GlobalNo
Innodb_data_fsyncs   YesGlobalNo
innodb_data_home_dirYesYesYes GlobalNo
Innodb_data_pending_fsyncs   YesGlobalNo
Innodb_data_pending_reads   YesGlobalNo
Innodb_data_pending_writes   YesGlobalNo
Innodb_data_read   YesGlobalNo
Innodb_data_reads   YesGlobalNo
Innodb_data_writes   YesGlobalNo
Innodb_data_written   YesGlobalNo
Innodb_dblwr_pages_written   YesGlobalNo
Innodb_dblwr_writes   YesGlobalNo
innodb_doublewriteYesYesYes GlobalNo
innodb_fast_shutdownYesYesYes GlobalYes
innodb_file_formatYesYesYes GlobalYes
innodb_file_format_checkYesYesYes GlobalNo
innodb_file_format_maxYesYesYes GlobalYes
innodb_file_per_tableYesYesYes GlobalYes
innodb_flush_log_at_trx_commitYesYesYes GlobalYes
innodb_flush_methodYesYesYes GlobalNo
innodb_force_recoveryYesYesYes GlobalNo
Innodb_have_atomic_builtins   YesGlobalNo
innodb_io_capacityYesYesYes GlobalYes
innodb_large_prefixYesYesYes GlobalYes
innodb_lock_wait_timeoutYesYesYes BothYes
innodb_locks_unsafe_for_binlogYesYesYes GlobalNo
innodb_log_buffer_sizeYesYesYes GlobalNo
innodb_log_file_sizeYesYesYes GlobalNo
innodb_log_files_in_groupYesYesYes GlobalNo
innodb_log_group_home_dirYesYesYes GlobalNo
Innodb_log_waits   YesGlobalNo
Innodb_log_write_requests   YesGlobalNo
Innodb_log_writes   YesGlobalNo
innodb_max_dirty_pages_pctYesYesYes GlobalYes
innodb_max_purge_lagYesYesYes GlobalYes
innodb_mirrored_log_groupsYesYesYes GlobalNo
innodb_old_blocks_pctYesYesYes GlobalYes
innodb_old_blocks_timeYesYesYes GlobalYes
innodb_open_filesYesYesYes GlobalNo
Innodb_os_log_fsyncs   YesGlobalNo
Innodb_os_log_pending_fsyncs   YesGlobalNo
Innodb_os_log_pending_writes   YesGlobalNo
Innodb_os_log_written   YesGlobalNo
Innodb_page_size   YesGlobalNo
Innodb_pages_created   YesGlobalNo
Innodb_pages_read   YesGlobalNo
Innodb_pages_written   YesGlobalNo
innodb_purge_batch_sizeYesYesYes GlobalNo
innodb_purge_threadsYesYesYes GlobalNo
innodb_read_ahead_thresholdYesYesYes GlobalYes
innodb_read_io_threadsYesYesYes GlobalNo
innodb_replication_delayYesYesYes GlobalYes
innodb_rollback_on_timeoutYesYesYes GlobalNo
innodb_rollback_segmentsYesYesYes GlobalYes
Innodb_row_lock_current_waits   YesGlobalNo
Innodb_row_lock_time   YesGlobalNo
Innodb_row_lock_time_avg   YesGlobalNo
Innodb_row_lock_time_max   YesGlobalNo
Innodb_row_lock_waits   YesGlobalNo
Innodb_rows_deleted   YesGlobalNo
Innodb_rows_inserted   YesGlobalNo
Innodb_rows_read   YesGlobalNo
Innodb_rows_updated   YesGlobalNo
innodb_spin_wait_delayYesYesYes GlobalYes
innodb_stats_methodYesYesYes BothYes
innodb_stats_on_metadataYesYesYes GlobalYes
innodb_stats_sample_pagesYesYesYes GlobalYes
innodb-status-fileYesYesYes GlobalNo
innodb_strict_modeYesYesYes BothYes
innodb_support_xaYesYesYes BothYes
innodb_sync_spin_loopsYesYesYes GlobalYes
innodb_table_locksYesYesYes BothYes
innodb_thread_concurrencyYesYesYes GlobalYes
innodb_thread_sleep_delayYesYesYes GlobalYes
Innodb_truncated_status_writes   YesGlobalNo
innodb_use_native_aioYesYesYes GlobalNo
innodb_use_sys_mallocYesYesYes GlobalNo
innodb_version  Yes GlobalNo
innodb_write_io_threadsYesYesYes GlobalNo
timed_mutexesYesYesYes GlobalYes
unique_checks  Yes BothYes

InnoDB Command Options

  • --ignore-builtin-innodb

    Command-Line Format--ignore-builtin-innodb
    Option-File Formatignore-builtin-innodb
    Option Sets VariableYes, ignore_builtin_innodb
    Variable Nameignore-builtin-innodb
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typeboolean

    This option causes the server to behave as if the built-in InnoDB is not present. It has these effects:

  • --innodb[=value]

    Controls loading of the InnoDB storage engine, if the server was compiled with InnoDB support. This option has a tristate format, with possible values of OFF, ON, or FORCE. See Section 5.1.7.1, “Installing and Uninstalling Plugins”.

    To disable InnoDB, use --innodb=OFF or --skip-innodb. In this case, because the default storage engine is InnoDB, the server will not start unless you also use --default-storage-engine to set the default to some other engine.

  • --innodb-status-file

    Command-Line Format--innodb-status-file
    Option-File Formatinnodb-status-file
    Variable Nameinnodb-status-file
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typeboolean
    DefaultOFF

    Controls whether InnoDB creates a file named innodb_status.<pid> in the MySQL data directory. If enabled, InnoDB periodically writes the output of SHOW ENGINE INNODB STATUS to this file.

    By default, the file is not created. To create it, start mysqld with the --innodb-status-file=1 option. The file is deleted during normal shutdown.

  • --skip-innodb

    Disable the InnoDB storage engine. See the description of --innodb.

InnoDB System Variables

  • ignore_builtin_innodb

    Whether the server was started with the --ignore-builtin-innodb option, which causes the server to behave as if the built-in InnoDB is not present. For more information, see the description of --ignore-builtin-innodb under “InnoDB Command Options” earlier in this section.

  • innodb_adaptive_flushing

    Command-Line Format--innodb_adaptive_flushing=#
    Option-File Formatinnodb_adaptive_flushing
    Option Sets VariableYes, innodb_adaptive_flushing
    Variable Nameinnodb_adaptive_flushing
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typeboolean
    DefaultON

    Specifies whether to dynamically adjust the rate of flushing dirty pages in the InnoDB buffer pool based on the workload. Adjusting the flush rate dynamically is intended to avoid bursts of I/O activity. This setting is enabled by default.

  • innodb_adaptive_hash_index

    Command-Line Format--innodb_adaptive_hash_index=#
    Option-File Formatinnodb_adaptive_hash_index
    Option Sets VariableYes, innodb_adaptive_hash_index
    Variable Nameinnodb_adaptive_hash_index
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typeboolean
    DefaultON

    Whether InnoDB adaptive hash indexes are enabled or disabled (see Section 13.3.11.4, “Adaptive Hash Indexes”). This variable is enabled by default. Use --skip-innodb_adaptive_hash_index at server startup to disable it.

  • innodb_additional_mem_pool_size

    Command-Line Format--innodb_additional_mem_pool_size=#
    Option-File Formatinnodb_additional_mem_pool_size
    Option Sets VariableYes, innodb_additional_mem_pool_size
    Variable Nameinnodb_additional_mem_pool_size
    Variable ScopeGlobal
    Dynamic VariableNo
    Deprecated5.6.3
     Permitted Values
    Typenumeric
    Default8388608
    Range2097152 .. 4294967295

    The size in bytes of a memory pool InnoDB uses to store data dictionary information and other internal data structures. The more tables you have in your application, the more memory you need to allocate here. If InnoDB runs out of memory in this pool, it starts to allocate memory from the operating system and writes warning messages to the MySQL error log. The default value is 8MB.

  • innodb_autoextend_increment

    Command-Line Format--innodb_autoextend_increment=#
    Option-File Formatinnodb_autoextend_increment
    Option Sets VariableYes, innodb_autoextend_increment
    Variable Nameinnodb_autoextend_increment
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default8
    Range1 .. 1000

    The increment size (in MB) for extending the size of an auto-extending shared tablespace file when it becomes full. The default value is 8. This variable does not affect the per-table tablespace files that are created if you use innodb_file_per_table=1. Those files are auto-extending regardless of the value of innodb_autoextend_increment. The initial extensions are by small amounts, after which extensions occur in increments of 4MB.

  • innodb_autoinc_lock_mode

    Command-Line Format--innodb_autoinc_lock_mode=#
    Option-File Formatinnodb_autoinc_lock_mode
    Option Sets VariableYes, innodb_autoinc_lock_mode
    Variable Nameinnodb_autoinc_lock_mode
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typenumeric
    Default1
    Valid Values

    0

    1

    2

    The locking mode to use for generating auto-increment values. The permissible values are 0, 1, or 2, for “traditional”, “consecutive”, or “interleaved” lock mode, respectively. Section 13.3.5.3, “AUTO_INCREMENT Handling in InnoDB, describes the characteristics of these modes.

    This variable has a default of 1 (“consecutive” lock mode).

  • innodb_buffer_pool_instances

    Version Introduced5.5.4
    Command-Line Format--innodb_buffer_pool_instances=#
    Option-File Formatinnodb_buffer_pool_instances
    Option Sets VariableYes, innodb_buffer_pool_instances
    Variable Nameinnodb_buffer_pool_instances
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typenumeric
    Default1
    Range1 .. 64

    The number of regions that the InnoDB buffer pool is divided into. For systems with buffer pools in the multi-gigabyte range, dividing the buffer pool into separate instances can improve concurrency, by reducing contention as different threads read and write to cached pages. Each page that is stored in or read from the buffer pool is assigned to one of the buffer pool instances randomly, using a hashing function. Each buffer pool manages its own free lists, flush lists, LRUs, and all other data structures connected to a buffer pool, and is protected by its own buffer pool mutex.

    This option only takes effect when you set the innodb_buffer_pool_size to a size of 1 gigabyte or more. The total size you specify is divided up among all the buffer pools. We recommend specifying a combination of innodb_buffer_pool_instances and innodb_buffer_pool_size so that each buffer pool instance is at least 1 gigabyte.

  • innodb_buffer_pool_size

    Command-Line Format--innodb_buffer_pool_size=#
    Option-File Formatinnodb_buffer_pool_size
    Option Sets VariableYes, innodb_buffer_pool_size
    Variable Nameinnodb_buffer_pool_size
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Platform Bit Size32
    Typenumeric
    Default134217728
    Range1048576 .. 2**32-1

    The size in bytes of the memory buffer InnoDB uses to cache data and indexes of its tables. The default value is 128MB, increased from a historical default of 8MB. The maximum value depends on the CPU architecture, 32-bit or 64-bit. For 32-bit systems, the CPU architecture and operating system sometimes impose a lower practical maximum size.

    The larger you set this value, the less disk I/O is needed to access data in tables. On a dedicated database server, you may set this to up to 80% of the machine physical memory size. Be prepared to scale back this value if these other issues occur:

    • Competition for physical memory might cause paging in the operating system.

    • InnoDB reserves additional memory for buffers and control structures, so that the total allocated space is approximately 10% greater than the specified size.

    • The address space must be contiguous, which can be an issue on Windows systems with DLLs that load at specific addresses.

    • The time to initialize the buffer pool is roughly proportional to its size. On large installations, this initialization time may be significant. For example, on a modern Linux x86_64 server, initialization of a 10GB buffer pool takes approximately 6 seconds. See Section 7.9.1, “The InnoDB Buffer Pool”.

  • innodb_change_buffering

    Command-Line Format--innodb_change_buffering=#
    Option-File Formatinnodb_change_buffering
    Option Sets VariableYes, innodb_change_buffering
    Variable Nameinnodb_change_buffering
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values (<= 5.5.3)
    Typeenumeration
    Defaultinserts
    Valid Values

    inserts

    none

     Permitted Values (>= 5.5.4)
    Typeenumeration
    Defaultall
    Valid Values

    inserts

    deletes

    purges

    changes

    all

    none

    Whether InnoDB performs change buffering, an optimization that delays write operations to secondary indexes so that the I/O operations can be performed sequentially. The permitted values are inserts (buffer insert operations), deletes (buffer delete operations; strictly speaking, the writes that mark index records for later deletion during a purge operation), changes (buffer insert and delete-marking operations), purges (buffer purge operations, the writes when deleted index entries are finally garbage-collected), all (buffer insert, delete-marking, and purge operations) and none (do not buffer any operations). The default is all. For details, see Section 13.4.7.4, “Controlling InnoDB Change Buffering”.

  • innodb_checksums

    Command-Line Format--innodb_checksums
    Option-File Formatinnodb_checksums
    Option Sets VariableYes, innodb_checksums
    Variable Nameinnodb_checksums
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typeboolean
    DefaultON

    InnoDB can use checksum validation on all pages read from the disk to ensure extra fault tolerance against broken hardware or data files. This validation is enabled by default. However, under some rare circumstances (such as when running benchmarks) this extra safety feature is unneeded and can be disabled with --skip-innodb-checksums.

  • innodb_commit_concurrency

    Command-Line Format--innodb_commit_concurrency=#
    Option-File Formatinnodb_commit_concurrency
    Option Sets VariableYes, innodb_commit_concurrency
    Variable Nameinnodb_commit_concurrency
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default0
    Range0 .. 1000

    The number of threads that can commit at the same time. A value of 0 (the default) permits any number of transactions to commit simultaneously.

    The value of innodb_commit_concurrency cannot be changed at runtime from zero to nonzero or vice versa. The value can be changed from one nonzero value to another.

  • innodb_concurrency_tickets

    Command-Line Format--innodb_concurrency_tickets=#
    Option-File Formatinnodb_concurrency_tickets
    Option Sets VariableYes, innodb_concurrency_tickets
    Variable Nameinnodb_concurrency_tickets
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default500
    Range1 .. 4294967295

    The number of threads that can enter InnoDB concurrently is determined by the innodb_thread_concurrency variable. A thread is placed in a queue when it tries to enter InnoDB if the number of threads has already reached the concurrency limit. When a thread is permitted to enter InnoDB, it is given a number of “free tickets” equal to the value of innodb_concurrency_tickets, and the thread can enter and leave InnoDB freely until it has used up its tickets. After that point, the thread again becomes subject to the concurrency check (and possible queuing) the next time it tries to enter InnoDB. The default value is 500.

  • innodb_data_file_path

    Command-Line Format--innodb_data_file_path=name
    Option-File Formatinnodb_data_file_path
    Variable Nameinnodb_data_file_path
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typefile name

    The paths to individual data files and their sizes. The full directory path to each data file is formed by concatenating innodb_data_home_dir to each path specified here. The file sizes are specified in KB, MB, or GB (1024MB) by appending K, M, or G to the size value. The sum of the sizes of the files must be at least 10MB. If you do not specify innodb_data_file_path, the default behavior is to create a single 10MB auto-extending data file named ibdata1. The size limit of individual files is determined by your operating system. You can set the file size to more than 4GB on those operating systems that support big files. You can also use raw disk partitions as data files. For detailed information on configuring InnoDB tablespace files, see Section 13.3.2, “Configuring InnoDB.

  • innodb_data_home_dir

    Command-Line Format--innodb_data_home_dir=path
    Option-File Formatinnodb_data_home_dir
    Option Sets VariableYes, innodb_data_home_dir
    Variable Nameinnodb_data_home_dir
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typefile name

    The common part of the directory path for all InnoDB data files in the shared tablespace. This setting does not affect the location of per-file tablespaces when innodb_file_per_table is enabled. The default value is the MySQL data directory. If you specify the value as an empty string, you can use absolute file paths in innodb_data_file_path.

  • innodb_doublewrite

    Command-Line Format--innodb-doublewrite
    Option-File Formatinnodb_doublewrite
    Option Sets VariableYes, innodb_doublewrite
    Variable Nameinnodb_doublewrite
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typeboolean

    If this variable is enabled (the default), InnoDB stores all data twice, first to the doublewrite buffer, and then to the actual data files. This variable can be turned off with --skip-innodb_doublewrite for benchmarks or cases when top performance is needed rather than concern for data integrity or possible failures.

  • innodb_fast_shutdown

    Command-Line Format--innodb_fast_shutdown[=#]
    Option-File Formatinnodb_fast_shutdown
    Option Sets VariableYes, innodb_fast_shutdown
    Variable Nameinnodb_fast_shutdown
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default1
    Valid Values

    0

    1

    2

    The InnoDB shutdown mode. If the value is 0, InnoDB does a slow shutdown, a full purge and an insert buffer merge before shutting down. If the value is 1 (the default), InnoDB skips these operations at shutdown, a process known as a fast shutdown. If the value is 2, InnoDB flushes its logs and shuts down cold, as if MySQL had crashed; no committed transactions are lost, but the crash recovery operation makes the next startup take longer.

    The slow shutdown can take minutes, or even hours in extreme cases where substantial amounts of data are still buffered. Use the slow shutdown technique before upgrading or downgrading between MySQL major releases, so that all data files are fully prepared in case the upgrade process updates the file format.

    Use innodb_fast_shutdown=2 in emergency or troubleshooting situations, to get the absolute fastest shutdown if data is at risk of corruption.

  • innodb_file_format

    Command-Line Format--innodb_file_format=#
    Option-File Formatinnodb_file_format
    Option Sets VariableYes, innodb_file_format
    Variable Nameinnodb_file_format
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values (>= 5.5.0, <= 5.5.6)
    Typestring
    DefaultBarracuda
    Valid Values

    Antelope

    Barracuda

     Permitted Values (>= 5.5.7)
    Typestring
    DefaultAntelope
    Valid Values

    Antelope

    Barracuda

    The file format to use for new InnoDB tables. Currently, Antelope and Barracuda are supported. This applies only for tables that have their own tablespace, so for it to have an effect, innodb_file_per_table must be enabled. The Barracuda file format is required for certain InnoDB features such as table compression.

  • innodb_file_format_check

    Command-Line Format--innodb_file_format_check=#
    Option-File Formatinnodb_file_format_check
    Option Sets VariableYes, innodb_file_format_check
    Variable Nameinnodb_file_format_check
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values (<= 5.5.0)
    Typestring
    DefaultAntelope
     Permitted Values (>= 5.5.4)
    Typestring
    DefaultBarracuda
     Permitted Values (>= 5.5.5)
    Typeboolean
    DefaultON

    As of MySQL 5.5.5, this variable can be set to 1 or 0 at server startup to enable or disable whether InnoDB checks the file format tag in the shared tablespace (for example, Antelope or Barracuda). If the tag is checked and is higher than that supported by the current version of InnoDB, an error occurs and InnoDB does not start. If the tag is not higher, InnoDB sets the value of innodb_file_format_max to the file format tag.

    Before MySQL 5.5.5, this variable can be set to 1 or 0 at server startup to enable or disable whether InnoDB checks the file format tag in the shared tablespace. If the tag is checked and is higher than that supported by the current version of InnoDB, an error occurs and InnoDB does not start. If the tag is not higher, InnoDB sets the value of innodb_file_format_check to the file format tag, which is the value seen at runtime.

  • innodb_file_format_max

    Version Introduced5.5.5
    Command-Line Format--innodb_file_format_max=#
    Option-File Formatinnodb_file_format_max
    Option Sets VariableYes, innodb_file_format_max
    Variable Nameinnodb_file_format_max
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typestring
    DefaultAntelope
    Valid Values

    Antelope

    Barracuda

    At server startup, InnoDB sets the value of innodb_file_format_max to the file format tag in the shared tablespace (for example, Antelope or Barracuda). If the server creates or opens a table with a “higher” file format, it sets the value of innodb_file_format_max to that format.

    This variable was added in MySQL 5.5.5.

  • innodb_file_per_table

    Command-Line Format--innodb_file_per_table
    Option-File Formatinnodb_file_per_table
    Variable Nameinnodb_file_per_table
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values (>= 5.5.0, <= 5.5.6)
    Typeboolean
    DefaultON
     Permitted Values (>= 5.5.7)
    Typeboolean
    DefaultOFF

    If innodb_file_per_table is disabled (the default), InnoDB creates tables in the system tablespace. If innodb_file_per_table is enabled, InnoDB creates each new table using its own .ibd file for storing data and indexes, rather than in the system tablespace. See Section 13.3.3, “Using Per-Table Tablespaces” for information about the features, such as InnoDB table compression, that only work for tables stored in separate tablespaces.

  • innodb_flush_log_at_trx_commit

    Command-Line Format--innodb_flush_log_at_trx_commit[=#]
    Option-File Formatinnodb_flush_log_at_trx_commit
    Option Sets VariableYes, innodb_flush_log_at_trx_commit
    Variable Nameinnodb_flush_log_at_trx_commit
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typeenumeration
    Default1
    Valid Values

    0

    1

    2

    If the value of innodb_flush_log_at_trx_commit is 0, the log buffer is written out to the log file once per second and the flush to disk operation is performed on the log file, but nothing is done at a transaction commit. When the value is 1 (the default), the log buffer is written out to the log file at each transaction commit and the flush to disk operation is performed on the log file. When the value is 2, the log buffer is written out to the file at each commit, but the flush to disk operation is not performed on it. However, the flushing on the log file takes place once per second also when the value is 2. Note that the once-per-second flushing is not 100% guaranteed to happen every second, due to process scheduling issues.

    The default value of 1 is required for full ACID compliance. You can achieve better performance by setting the value different from 1, but then you can lose up to one second worth of transactions in a crash. With a value of 0, any mysqld process crash can erase the last second of transactions. With a value of 2, only an operating system crash or a power outage can erase the last second of transactions. InnoDB's crash recovery works regardless of the value.

    For the greatest possible durability and consistency in a replication setup using InnoDB with transactions, use innodb_flush_log_at_trx_commit=1 and sync_binlog=1 in your master server my.cnf file.

    Caution

    Many operating systems and some disk hardware fool the flush-to-disk operation. They may tell mysqld that the flush has taken place, even though it has not. Then the durability of transactions is not guaranteed even with the setting 1, and in the worst case a power outage can even corrupt the InnoDB database. Using a battery-backed disk cache in the SCSI disk controller or in the disk itself speeds up file flushes, and makes the operation safer. You can also try using the Unix command hdparm to disable the caching of disk writes in hardware caches, or use some other command specific to the hardware vendor.

  • innodb_flush_method

    Command-Line Format--innodb_flush_method=name
    Option-File Formatinnodb_flush_method
    Option Sets VariableYes, innodb_flush_method
    Variable Nameinnodb_flush_method
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Type (solaris)enumeration
    Defaultfdatasync
    Valid Values

    O_DSYNC

    O_DIRECT

    By default, InnoDB uses the fsync() system call to flush both the data and log files. If innodb_flush_method option is set to O_DSYNC, InnoDB uses O_SYNC to open and flush the log files, and fsync() to flush the data files. If O_DIRECT is specified (available on some GNU/Linux versions, FreeBSD, and Solaris), InnoDB uses O_DIRECT (or directio() on Solaris) to open the data files, and uses fsync() to flush both the data and log files. Note that InnoDB uses fsync() instead of fdatasync(), and it does not use O_DSYNC by default because there have been problems with it on many varieties of Unix. This variable is relevant only for Unix. On Windows, the flush method is always async_unbuffered and cannot be changed.

    Depending on hardware configuration, setting innodb_flush_method to O_DIRECT can either have either a positive or negative effect on performance. Benchmark your particular configuration to decide which setting to use. The mix of read and write operations in your workload can also affect which setting performs better for you. For example, on a system with a hardware RAID controller and battery-backed write cache, O_DIRECT can help to avoid double buffering between the InnoDB buffer pool and the operating system's filesystem cache. On some systems where InnoDB data and log files are located on a SAN, the default value or O_DSYNC might be faster for a read-heavy workload with mostly SELECT statements. Always test this parameter with the same type of hardware and workload that reflects your production environment.

    Formerly, a value of fdatasync also specified the default behavior. This value was removed, due to confusion that a value of fdatasync caused fsync() system calls rather than fdatasync() for flushing. To obtain the default value now, do not set any value for innodb_flush_method at startup.

  • innodb_force_recovery

    Command-Line Format--innodb_force_recovery=#
    Option-File Formatinnodb_force_recovery
    Option Sets VariableYes, innodb_force_recovery
    Variable Nameinnodb_force_recovery
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typeenumeration
    Default0
    Valid Values

    0

    1

    2

    3

    4

    5

    6

    The crash recovery mode. Possible values are from 0 to 6. The meanings of these values are described in Section 13.3.7.2, “Forcing InnoDB Recovery”.

    Warning

    Only set this variable greater than 0 in an emergency situation, to dump your tables from a corrupt database. As a safety measure, InnoDB prevents any changes to its data when this variable is greater than 0. This restriction also prohibits some queries that use WHERE or ORDER BY clauses, because high values can prevent queries from using indexes.

  • innodb_io_capacity

    Command-Line Format--innodb_io_capacity=#
    Option-File Formatinnodb_io_capacity
    Option Sets VariableYes, innodb_io_capacity
    Variable Nameinnodb_io_capacity
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Platform Bit Size32
    Typenumeric
    Default200
    Range100 .. 2**32-1
     Permitted Values
    Platform Bit Size64
    Typenumeric
    Default200
    Range100 .. 2**64-1

    An upper limit on the I/O activity performed by the InnoDB background tasks, such as flushing pages from the buffer pool and merging data from the insert buffer. The default value is 200. For busy systems capable of higher I/O rates, you can set a higher value at server startup, to help the server handle the background maintenance work associated with a high rate of row changes. For systems with individual 5400 RPM or 7200 RPM drives, you might lower the value to the former default of 100.

    This parameter should be set to approximately the number of I/O operations that the system can perform per second. Ideally, keep this setting as low as practical, but not so low that these background activities fall behind. If the value is too high, data is removed from the buffer pool and insert buffer too quickly to provide significant benefit from the caching.

    The value represents an estimated proportion of the I/O operations per second (IOPS) available to older-generation disk drives that could perform about 100 IOPS. The current default of 200 reflects that modern storage devices are capable of much higher I/O rates.

    In general, you can increase the value as a function of the number of drives used for InnoDB I/O, particularly fast drives capable of high numbers of IOPS. For example, systems that use multiple disks or solid-state disks for InnoDB are likely to benefit from the ability to control this parameter.

    Although you can specify a very high number, in practice such large values cease to have any benefit; for example, a value of one million would be considered very high.

    You can set the innodb_io_capacity value to any number 100 or greater, and the default value is 200. You can set the value of this parameter in the MySQL option file (my.cnf or my.ini) or change it dynamically with the SET GLOBAL command, which requires the SUPER privilege.

    See Section 13.4.7.11, “Controlling the Master Thread I/O Rate” for more guidelines about this option. For general information about InnoDB I/O performance, see Section 7.5.7, “Optimizing InnoDB Disk I/O”.

  • innodb_large_prefix

    Version Introduced5.5.14
    Command-Line Format--innodb_large_prefix
    Option-File Formatinnodb_large_prefix
    Option Sets VariableYes, innodb_large_prefix
    Variable Nameinnodb_large_prefix
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typeboolean
    DefaultOFF

    Enable this option to allow index key prefixes longer than 767 bytes (up to 3072 bytes), for InnoDB tables that use the DYNAMIC and COMPRESSED row formats. (Creating such tables also requires the option values innodb_file_format=barracuda and innodb_file_per_table=true.) See Section 13.3.15, “Limits on InnoDB Tables” for the relevant maximums associated with index key prefixes under various settings.

    For tables using the REDUNDANT and COMPACT row formats, this option does not affect the allowed key prefix length. It does introduce a new error possibility. When this setting is enabled, attempting to create an index prefix with a key length greater than 3072 for a REDUNDANT or COMPACT table causes an error ER_INDEX_COLUMN_TOO_LONG (1727).

  • innodb_lock_wait_timeout

    Command-Line Format--innodb_lock_wait_timeout=#
    Option-File Formatinnodb_lock_wait_timeout
    Option Sets VariableYes, innodb_lock_wait_timeout
    Variable Nameinnodb_lock_wait_timeout
    Variable ScopeGlobal, Session
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default50
    Range1 .. 1073741824

    The timeout in seconds an InnoDB transaction waits for a row lock before giving up. The default value is 50 seconds. A transaction that tries to access a row that is locked by another InnoDB transaction waits at most this many seconds for write access to the row before issuing the following error:

    ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

    When a lock wait timeout occurs, the current statement is rolled back (not the entire transaction). To have the entire transaction roll back, start the server with the --innodb_rollback_on_timeout option. See also Section 13.3.13, “InnoDB Error Handling”.

    You might decrease this value for highly interactive applications or OLTP systems, to display user feedback quickly or put the update into a queue for processing later. You might increase this value for long-running back-end operations, such as a transform step in a data warehouse that waits for other large insert or update operations to finish.

    innodb_lock_wait_timeout applies to InnoDB row locks only. A MySQL table lock does not happen inside InnoDB and this timeout does not apply to waits for table locks.

    The lock wait timeout value does not apply to deadlocks, because InnoDB detects them immediately and rolls back one of the deadlocked transactions.

  • innodb_locks_unsafe_for_binlog

    Command-Line Format--innodb_locks_unsafe_for_binlog
    Option-File Formatinnodb_locks_unsafe_for_binlog
    Option Sets VariableYes, innodb_locks_unsafe_for_binlog
    Variable Nameinnodb_locks_unsafe_for_binlog
    Variable ScopeGlobal
    Dynamic VariableNo
    Deprecated5.6.3
     Permitted Values
    Typeboolean
    DefaultOFF

    This variable affects how InnoDB uses gap locking for searches and index scans. Normally, InnoDB uses an algorithm called next-key locking that combines index-row locking with gap locking. InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters. Thus, the row-level locks are actually index-record locks. In addition, a next-key lock on an index record also affects the “gap” before that index record. That is, a next-key lock is an index-record lock plus a gap lock on the gap preceding the index record. If one session has a shared or exclusive lock on record R in an index, another session cannot insert a new index record in the gap immediately before R in the index order. See Section 13.3.9.4, “InnoDB Record, Gap, and Next-Key Locks”.

    By default, the value of innodb_locks_unsafe_for_binlog is 0 (disabled), which means that gap locking is enabled: InnoDB uses next-key locks for searches and index scans. To enable the variable, set it to 1. This causes gap locking to be disabled: InnoDB uses only index-record locks for searches and index scans.

    Enabling innodb_locks_unsafe_for_binlog does not disable the use of gap locking for foreign-key constraint checking or duplicate-key checking.

    The effect of enabling innodb_locks_unsafe_for_binlog is similar to but not identical to setting the transaction isolation level to READ COMMITTED:

    • Enabling innodb_locks_unsafe_for_binlog is a global setting and affects all sessions, whereas the isolation level can be set globally for all sessions, or individually per session.

    • innodb_locks_unsafe_for_binlog can be set only at server startup, whereas the isolation level can be set at startup or changed at runtime.

    READ COMMITTED therefore offers finer and more flexible control than innodb_locks_unsafe_for_binlog. For additional details about the effect of isolation level on gap locking, see Section 12.3.6, “SET TRANSACTION Синтаксис”.

    Enabling innodb_locks_unsafe_for_binlog may cause phantom problems because other sessions can insert new rows into the gaps when gap locking is disabled. Suppose that there is an index on the id column of the child table and that you want to read and lock all rows from the table having an identifier value larger than 100, with the intention of updating some column in the selected rows later:

    SELECT * FROM child WHERE id > 100 FOR UPDATE;

    The query scans the index starting from the first record where id is greater than 100. If the locks set on the index records in that range do not lock out inserts made in the gaps, another session can insert a new row into the table. Consequently, if you were to execute the same SELECT again within the same transaction, you would see a new row in the result set returned by the query. This also means that if new items are added to the database, InnoDB does not guarantee serializability. Therefore, if innodb_locks_unsafe_for_binlog is enabled, InnoDB guarantees at most an isolation level of READ COMMITTED. (Conflict serializability is still guaranteed.) For additional information about phantoms, see Section 13.3.9.5, “Avoiding the Phantom Problem Using Next-Key Locking”.

    Enabling innodb_locks_unsafe_for_binlog has additional effects:

    • For UPDATE or DELETE statements, InnoDB holds locks only for rows that it updates or deletes. Record locks for nonmatching rows are released after MySQL has evaluated the WHERE condition. This greatly reduces the probability of deadlocks, but they can still happen.

    • For UPDATE statements, if a row is already locked, InnoDB performs a “semi-consistent” read, returning the latest committed version to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE. If the row matches (must be updated), MySQL reads the row again and this time InnoDB either locks it or waits for a lock on it.

    Consider the following example, beginning with this table:

    CREATE TABLE t (a INT NOT NULL, b INT) ENGINE = InnoDB;
    INSERT INTO t VALUES (1,2),(2,3),(3,2),(4,3),(5,2);
    COMMIT;

    In this case, table has no indexes, so searches and index scans use the hidden clustered index for record locking (see Section 13.3.11.1, “Clustered and Secondary Indexes”).

    Suppose that one client performs an UPDATE using these statements:

    SET autocommit = 0;
    UPDATE t SET b = 5 WHERE b = 3;

    Suppose also that a second client performs an UPDATE by executing these statements following those of the first client:

    SET autocommit = 0;
    UPDATE t SET b = 4 WHERE b = 2;

    As InnoDB executes each UPDATE, it first acquires an exclusive lock for each row, and then determines whether to modify it. If InnoDB does not modify the row and innodb_locks_unsafe_for_binlog is enabled, it releases the lock. Otherwise, InnoDB retains the lock until the end of the transaction. This affects transaction processing as follows.

    If innodb_locks_unsafe_for_binlog is disabled, the first UPDATE acquires x-locks and does not release any of them:

    x-lock(1,2); retain x-lock
    x-lock(2,3); update(2,3) to (2,5); retain x-lock
    x-lock(3,2); retain x-lock
    x-lock(4,3); update(4,3) to (4,5); retain x-lock
    x-lock(5,2); retain x-lock

    The second UPDATE blocks as soon as it tries to acquire any locks (because first update has retained locks on all rows), and does not proceed until the first UPDATE commits or rolls back:

    x-lock(1,2); block and wait for first UPDATE to commit or roll back

    If innodb_locks_unsafe_for_binlog is enabled, the first UPDATE acquires x-locks and releases those for rows that it does not modify:

    x-lock(1,2); unlock(1,2)
    x-lock(2,3); update(2,3) to (2,5); retain x-lock
    x-lock(3,2); unlock(3,2)
    x-lock(4,3); update(4,3) to (4,5); retain x-lock
    x-lock(5,2); unlock(5,2)

    For the second UPDATE, InnoDB does a “semi-consistent” read, returning the latest committed version of each row to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE:

    x-lock(1,2); update(1,2) to (1,4); retain x-lock
    x-lock(2,3); unlock(2,3)
    x-lock(3,2); update(3,2) to (3,4); retain x-lock
    x-lock(4,3); unlock(4,3)
    x-lock(5,2); update(5,2) to (5,4); retain x-lock
  • innodb_log_buffer_size

    Command-Line Format--innodb_log_buffer_size=#
    Option-File Formatinnodb_log_buffer_size
    Option Sets VariableYes, innodb_log_buffer_size
    Variable Nameinnodb_log_buffer_size
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typenumeric
    Default8388608
    Range262144 .. 4294967295

    The size in bytes of the buffer that InnoDB uses to write to the log files on disk. The default value is 8MB. A large log buffer enables large transactions to run without a need to write the log to disk before the transactions commit. Thus, if you have big transactions, making the log buffer larger saves disk I/O.

  • innodb_log_file_size

    Command-Line Format--innodb_log_file_size=#
    Option-File Formatinnodb_log_file_size
    Option Sets VariableYes, innodb_log_file_size
    Variable Nameinnodb_log_file_size
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typenumeric
    Default5242880
    Range108576 .. 4294967295

    The size in bytes of each log file in a log group. The combined size of log files must be less than 4GB. The default value is 5MB. Sensible values range from 1MB to 1/N-th of the size of the buffer pool, where N is the number of log files in the group. The larger the value, the less checkpoint flush activity is needed in the buffer pool, saving disk I/O. But larger log files also mean that recovery is slower in case of a crash.

  • innodb_log_files_in_group

    Command-Line Format--innodb_log_files_in_group=#
    Option-File Formatinnodb_log_files_in_group
    Option Sets VariableYes, innodb_log_files_in_group
    Variable Nameinnodb_log_files_in_group
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typenumeric
    Default2
    Range2 .. 100

    The number of log files in the log group. InnoDB writes to the files in a circular fashion. The default (and recommended) value is 2.

  • innodb_log_group_home_dir

    Command-Line Format--innodb_log_group_home_dir=path
    Option-File Formatinnodb_log_group_home_dir
    Option Sets VariableYes, innodb_log_group_home_dir
    Variable Nameinnodb_log_group_home_dir
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typefile name

    The directory path to the InnoDB redo log files. If you do not specify any InnoDB log variables, the default is to create two 5MB files named ib_logfile0 and ib_logfile1 in the MySQL data directory.

  • innodb_max_dirty_pages_pct

    Command-Line Format--innodb_max_dirty_pages_pct=#
    Option-File Formatinnodb_max_dirty_pages_pct
    Option Sets VariableYes, innodb_max_dirty_pages_pct
    Variable Nameinnodb_max_dirty_pages_pct
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default75
    Range0 .. 99

    This is an integer in the range from 0 to 99. The default value is 75. The main thread in InnoDB tries to write pages from the buffer pool so that the percentage of dirty (not yet written) pages will not exceed this value.

  • innodb_max_purge_lag

    Command-Line Format--innodb_max_purge_lag=#
    Option-File Formatinnodb_max_purge_lag
    Option Sets VariableYes, innodb_max_purge_lag
    Variable Nameinnodb_max_purge_lag
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default0
    Range0 .. 4294967295

    This variable controls how to delay INSERT, UPDATE, and DELETE operations when purge operations are lagging (see Section 13.3.10, “InnoDB Multi-Versioning”). The default value 0 (no delays).

    The InnoDB transaction system maintains a list of transactions that have index records delete-marked by UPDATE or DELETE operations. Let the length of this list be purge_lag. When purge_lag exceeds innodb_max_purge_lag, each INSERT, UPDATE, and DELETE operation is delayed by ((purge_lag/innodb_max_purge_lag)×10)–5 milliseconds. The delay is computed in the beginning of a purge batch, every ten seconds. The operations are not delayed if purge cannot run because of an old consistent read view that could see the rows to be purged.

    A typical setting for a problematic workload might be 1 million, assuming that transactions are small, only 100 bytes in size, and it is permissible to have 100MB of unpurged InnoDB table rows.

    The lag value is displayed as the history list length in the TRANSACTIONS section of InnoDB Monitor output. For example, if the output includes the following lines, the lag value is 20:

    ------------
    TRANSACTIONS
    ------------
    Trx id counter 0 290328385
    Purge done for trx's n:o < 0 290315608 undo n:o < 0 17
    History list length 20
  • innodb_mirrored_log_groups

    The number of identical copies of log groups to keep for the database. This should be set to 1.

  • innodb_old_blocks_pct

    Command-Line Format--innodb_old_blocks_pct=#
    Option-File Formatinnodb_old_blocks_pct
    Variable Nameinnodb_old_blocks_pct
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default37
    Range5 .. 95

    Specifies the approximate percentage of the InnoDB buffer pool used for the old block sublist. The range of values is 5 to 95. The default value is 37 (that is, 3/8 of the pool). See Section 7.9.1, “The InnoDB Buffer Pool”

  • innodb_old_blocks_time

    Command-Line Format--innodb_old_blocks_time=#
    Option-File Formatinnodb_old_blocks_time
    Variable Nameinnodb_old_blocks_time
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default0
    Range0 .. 2**32-1

    Specifies how long in milliseconds (ms) a block inserted into the old sublist must stay there after its first access before it can be moved to the new sublist. The default value is 0: A block inserted into the old sublist moves immediately to the new sublist the first time it is accessed, no matter how soon after insertion the access occurs. If the value is greater than 0, blocks remain in the old sublist until an access occurs at least that many ms after the first access. For example, a value of 1000 causes blocks to stay in the old sublist for 1 second after the first access before they become eligible to move to the new sublist. See Section 7.9.1, “The InnoDB Buffer Pool”

  • innodb_open_files

    Command-Line Format--innodb_open_files=#
    Option-File Formatinnodb_open_files
    Variable Nameinnodb_open_files
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typenumeric
    Default300
    Range10 .. 4294967295

    This variable is relevant only if you use multiple tablespaces in InnoDB. It specifies the maximum number of .ibd files that InnoDB can keep open at one time. The minimum value is 10. The default value is 300.

    The file descriptors used for .ibd files are for InnoDB only. They are independent of those specified by the --open-files-limit server option, and do not affect the operation of the table cache.

  • innodb_purge_batch_size

    Version Introduced5.5.4
    Command-Line Format--innodb_purge_batch_size=#
    Option-File Formatinnodb_purge_batch_size
    Variable Nameinnodb_purge_batch_size
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values (>= 5.5.4)
    Typenumeric
    Default20
    Range1 .. 5000

    The granularity of changes, expressed in units of redo log records, that trigger a purge operation, flushing the changed buffer pool blocks to disk. The default value is 20, and the range is 1-5000. This option is intended for tuning performance in combination with the setting innodb_purge_threads=1, and typical users do not need to modify it.

  • innodb_purge_threads

    Version Introduced5.5.4
    Command-Line Format--innodb_purge_threads=#
    Option-File Formatinnodb_purge_threads
    Variable Nameinnodb_purge_threads
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values (>= 5.5.4)
    Typenumeric
    Default0
    Range0 .. 1

    The number of background threads devoted to the InnoDB purge operation. Currently, can only be 0 (the default) or 1. The default value of 0 signifies that the purge operation is performed as part of the master thread. Running the purge operation in its own thread can reduce internal contention within InnoDB, improving scalability. Currently, the performance gain might be minimal because the background thread might encounter different kinds of contention than before. This feature primarily lays the groundwork for future performance work.

  • innodb_read_ahead_threshold

    Command-Line Format--innodb_read_ahead_threshold=#
    Option-File Formatinnodb_read_ahead_threshold
    Option Sets VariableYes, innodb_read_ahead_threshold
    Variable Nameinnodb_read_ahead_threshold
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default56
    Range0 .. 64

    Controls the sensitivity of linear read-ahead that InnoDB uses to prefetch pages into the buffer cache. If InnoDB reads at least innodb_read_ahead_threshold pages sequentially from an extent (64 pages), it initiates an asynchronous read for the entire following extent. The permissible range of values is 0 to 64. The default is 56: InnoDB must read at least 56 pages sequentially from an extent to initiate an asynchronous read for the following extent.

  • innodb_read_io_threads

    Command-Line Format--innodb_read_io_threads=#
    Option-File Formatinnodb_read_io_threads
    Option Sets VariableYes, innodb_read_io_threads
    Variable Nameinnodb_read_io_threads
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typenumeric
    Default4
    Range1 .. 64

    The number of I/O threads for read operations in InnoDB. The default value is 4.

  • innodb_replication_delay

    Command-Line Format--innodb_replication_delay=#
    Option-File Formatinnodb_replication_delay
    Option Sets VariableYes, innodb_replication_delay
    Variable Nameinnodb_replication_delay
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default0
    Range0 .. 4294967295

    The replication thread delay (in ms) on a slave server if innodb_thread_concurrency is reached.

  • innodb_rollback_on_timeout

    Command-Line Format--innodb_rollback_on_timeout
    Option-File Formatinnodb_rollback_on_timeout
    Option Sets VariableYes, innodb_rollback_on_timeout
    Variable Nameinnodb_rollback_on_timeout
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typeboolean
    DefaultOFF

    In MySQL 5.5, InnoDB rolls back only the last statement on a transaction timeout by default. If --innodb_rollback_on_timeout is specified, a transaction timeout causes InnoDB to abort and roll back the entire transaction (the same behavior as in MySQL 4.1).

  • innodb_rollback_segments

    Version Introduced5.5.11
    Command-Line Format--innodb_rollback_segments=#
    Option-File Formatinnodb_rollback_segments
    Option Sets VariableYes, innodb_rollback_segments
    Variable Nameinnodb_rollback_segments
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default128
    Range1 .. 128

    Defines how many of the rollback segments in the system tablespace that InnoDB uses within a transaction. You might reduce this value from its default of 128 if a smaller number of rollback segments performs better for your workload.

  • innodb_spin_wait_delay

    Command-Line Format--innodb_spin_wait_delay=#
    Option-File Formatinnodb_spin_wait_delay
    Option Sets VariableYes, innodb_spin_wait_delay
    Variable Nameinnodb_spin_wait_delay
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default6
    Range0 .. 4294967295

    The maximum delay between polls for a spin lock. The default value is 6.

  • innodb_stats_method

    Version Introduced5.5.10
    Command-Line Format--innodb_stats_method=name
    Option-File Formatinnodb_stats_method
    Option Sets VariableYes, innodb_stats_method
    Variable Nameinnodb_stats_method
    Variable ScopeGlobal, Session
    Dynamic VariableYes
     Permitted Values
    Typeenumeration
    Defaultnulls_equal

    How the server treats NULL values when collecting statistics about the distribution of index values for InnoDB tables. This variable has three possible values, nulls_equal, nulls_unequal, and nulls_ignored. For nulls_equal, all NULL index values are considered equal and form a single value group that has a size equal to the number of NULL values. For nulls_unequal, NULL values are considered unequal, and each NULL forms a distinct value group of size 1. For nulls_ignored, NULL values are ignored.

    The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 7.3.7, “InnoDB and MyISAM Index Statistics Collection”.

  • innodb_stats_on_metadata

    Version Introduced5.5.4
    Command-Line Format--innodb_stats_on_metadata
    Option-File Formatinnodb_stats_on_metadata
    Option Sets VariableYes, innodb_stats_on_metadata
    Variable Nameinnodb_stats_on_metadata
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typeboolean
    DefaultON

    When this variable is enabled (which is the default, as before the variable was created), InnoDB updates statistics during metadata statements such as SHOW TABLE STATUS or SHOW INDEX, or when accessing the INFORMATION_SCHEMA tables TABLES or STATISTICS. (These updates are similar to what happens for ANALYZE TABLE.) When disabled, InnoDB does not update statistics during these operations. Disabling this variable can improve access speed for schemas that have a large number of tables or indexes. It can also improve the stability of execution plans for queries that involve InnoDB tables.

  • innodb_stats_sample_pages

    Command-Line Format--innodb_stats_sample_pages=#
    Option-File Formatinnodb_stats_sample_pages
    Option Sets VariableYes, innodb_stats_sample_pages
    Variable Nameinnodb_stats_sample_pages
    Variable ScopeGlobal
    Dynamic VariableYes
    Deprecated5.6.3
     Permitted Values
    Typenumeric
    Default8
    Range1 .. 2**64-1

    The number of index pages to sample for index distribution statistics such as are calculated by ANALYZE TABLE. The default value is 8. For more information, see Section 13.4.8, “Changes for Flexibility, Ease of Use and Reliability”.

  • innodb_strict_mode

    Command-Line Format--innodb_strict_mode=#
    Option-File Formatinnodb_strict_mode
    Option Sets VariableYes, innodb_strict_mode
    Variable Nameinnodb_strict_mode
    Variable ScopeGlobal, Session
    Dynamic VariableYes
     Permitted Values
    Typeboolean
    DefaultOFF

    Whether InnoDB returns errors rather than warnings for certain conditions. This is analogous to strict SQL mode. The default value is OFF. See Section 13.4.8.4, “InnoDB Strict Mode” for a list of the conditions that are affected.

  • innodb_support_xa

    Command-Line Format--innodb_support_xa
    Option-File Formatinnodb_support_xa
    Option Sets VariableYes, innodb_support_xa
    Variable Nameinnodb_support_xa
    Variable ScopeGlobal, Session
    Dynamic VariableYes
     Permitted Values
    Typeboolean
    DefaultTRUE

    Enables InnoDB support for two-phase commit in XA transactions, causing an extra disk flush for transaction preparation. This setting is the default. The XA mechanism is used internally and is essential for any server that has its binary log turned on and is accepting changes to its data from more than one thread. If you turn it off, transactions can be written to the binary log in a different order from the one in which the live database is committing them. This can produce different data when the binary log is replayed in disaster recovery or on a replication slave. Do not turn it off on a replication master server unless you have an unusual setup where only one thread is able to change data.

    For a server that is accepting data changes from only one thread, it is safe and recommended to turn off this option to improve performance for InnoDB tables. For example, you can turn it off on replication slaves where only the replication SQL thread is changing data.

    You can also turn off this option if you do not need it for safe binary logging or replication, and you also do not use an external XA transaction manager.

  • innodb_sync_spin_loops

    Command-Line Format--innodb_sync_spin_loops=#
    Option-File Formatinnodb_sync_spin_loops
    Option Sets VariableYes, innodb_sync_spin_loops
    Variable Nameinnodb_sync_spin_loops
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default30
    Range0 .. 4294967295

    The number of times a thread waits for an InnoDB mutex to be freed before the thread is suspended. The default value is 30.

  • innodb_table_locks

    Command-Line Format--innodb_table_locks
    Option-File Formatinnodb_table_locks
    Option Sets VariableYes, innodb_table_locks
    Variable Nameinnodb_table_locks
    Variable ScopeGlobal, Session
    Dynamic VariableYes
     Permitted Values
    Typeboolean
    DefaultTRUE

    If autocommit = 0, InnoDB honors LOCK TABLES; MySQL does not return from LOCK TABLES ... WRITE until all other threads have released all their locks to the table. The default value of innodb_table_locks is 1, which means that LOCK TABLES causes InnoDB to lock a table internally if autocommit = 0.

    As of MySQL 5.5.3, innodb_table_locks = 0 has no effect for tables locked explicitly with LOCK TABLES ... WRITE. It still has an effect for tables locked for read or write by LOCK TABLES ... WRITE implicitly (for example, through triggers) or by LOCK TABLES ... READ.

  • innodb_thread_concurrency

    Command-Line Format--innodb_thread_concurrency=#
    Option-File Formatinnodb_thread_concurrency
    Option Sets VariableYes, innodb_thread_concurrency
    Variable Nameinnodb_thread_concurrency
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default0
    Range0 .. 1000

    InnoDB tries to keep the number of operating system threads concurrently inside InnoDB less than or equal to the limit given by this variable. Once the number of threads reaches this limit, additional threads are placed into a wait state within a FIFO queue for execution. Threads waiting for locks are not counted in the number of concurrently executing threads.

    The correct value for this variable is dependent on environment and workload. Try a range of different values to determine what value works for your applications. A recommended value is 2 times the number of CPUs plus the number of disks.

    The range of this variable is 0 to 1000. A value of 0 (the default) is interpreted as infinite concurrency (no concurrency checking). Disabling thread concurrency checking enables InnoDB to create as many threads as it needs.

  • innodb_thread_sleep_delay

    Command-Line Format--innodb_thread_sleep_delay=#
    Option-File Formatinnodb_thread_sleep_delay
    Option Sets VariableYes, innodb_thread_sleep_delay
    Variable Nameinnodb_thread_sleep_delay
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Typenumeric
    Default10000

    How long InnoDB threads sleep before joining the InnoDB queue, in microseconds. The default value is 10,000. A value of 0 disables sleep.

  • innodb_use_native_aio

    Version Introduced5.5.4
    Command-Line Format--innodb_use_native_aio=#
    Option-File Formatinnodb_use_native_aio
    Option Sets VariableYes, innodb_use_native_aio
    Variable Nameinnodb_use_native_aio
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typeboolean
    DefaultON

    Specifies whether to use the Linux asynchronous I/O subsystem. This variable applies to Linux systems only, and cannot be changed while the server is running.

    Normally, you do not need to touch this option, because it is enabled by default. If a problem with the asynchronous I/O subsystem in the OS prevents InnoDB from starting, start the server with this variable disabled (use innodb_use_native_aio=0 in the option file). This option could also be turned off automatically during startup, if InnoDB detects a potential problem such as a combination of tmpdir location, tmpfs filesystem, and Linux kernel that that does not support AIO on tmpfs.

    This variable was added in MySQL 5.5.4.

  • innodb_use_sys_malloc

    Command-Line Format--innodb_use_sys_malloc=#
    Option-File Formatinnodb_use_sys_malloc
    Option Sets VariableYes, innodb_use_sys_malloc
    Variable Nameinnodb_use_sys_malloc
    Variable ScopeGlobal
    Dynamic VariableNo
    Deprecated5.6.3
     Permitted Values
    Typeboolean
    DefaultON

    Whether InnoDB uses the operating system memory allocator (ON) or its own (OFF). The default value is ON.

  • innodb_version

    The InnoDB version number.

  • innodb_write_io_threads

    Command-Line Format--innodb_write_io_threads=#
    Option-File Formatinnodb_write_io_threads
    Option Sets VariableYes, innodb_write_io_threads
    Variable Nameinnodb_write_io_threads
    Variable ScopeGlobal
    Dynamic VariableNo
     Permitted Values
    Typenumeric
    Default4
    Range1 .. 64

    The number of I/O threads for write operations in InnoDB. The default value is 4.

  • sync_binlog

    Command-Line Format--sync-binlog=#
    Option-File Formatsync_binlog
    Option Sets VariableYes, sync_binlog
    Variable Namesync_binlog
    Variable ScopeGlobal
    Dynamic VariableYes
     Permitted Values
    Platform Bit Size32
    Typenumeric
    Default0
    Range0 .. 4294967295
     Permitted Values
    Platform Bit Size64
    Typenumeric
    Default0
    Range0 .. 18446744073709547520

    If the value of this variable is greater than 0, the MySQL server synchronizes its binary log to disk (using fdatasync()) after every sync_binlog writes to the binary log. There is one write to the binary log per statement if autocommit is enabled, and one write per transaction otherwise. The default value of sync_binlog is 0, which does no synchronizing to disk. A value of 1 is the safest choice, because in the event of a crash you lose at most one statement or transaction from the binary log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast).

13.3.5. Creating and Using InnoDB Tables

To create an InnoDB table, specify an ENGINE=InnoDB option in the CREATE TABLE statement:

CREATE TABLE customers (a INT, b CHAR (20), INDEX (a)) ENGINE=InnoDB;

The statement creates a table and an index on column a in the InnoDB tablespace that consists of the data files that you specified in my.cnf. In addition, MySQL creates a file customers.frm in the test directory under the MySQL database directory. Internally, InnoDB adds an entry for the table to its own data dictionary. The entry includes the database name. For example, if test is the database in which the customers table is created, the entry is for 'test/customers'. This means you can create a table of the same name customers in some other database, and the table names do not collide inside InnoDB.

You can query the amount of free space in the InnoDB tablespace by issuing a SHOW TABLE STATUS statement for any InnoDB table. The amount of free space in the tablespace appears in the Data_free section in the output of SHOW TABLE STATUS. For example:

SHOW TABLE STATUS FROM test LIKE 'customers'

The statistics SHOW displays for InnoDB tables are only approximate. They are used in SQL optimization. Table and index reserved sizes in bytes are accurate, though.

13.3.5.1. Using InnoDB Transactions

Transactions in SQL

By default, each client that connects to the MySQL server begins with autocommit mode enabled, which automatically commits every SQL statement as you execute it. To use multiple-statement transactions, you can switch autocommit off with the SQL statement SET autocommit = 0 and end each transaction with either COMMIT or ROLLBACK. If you want to leave autocommit on, you can begin your transactions within START TRANSACTION and end them with COMMIT or ROLLBACK. The following example shows two transactions. The first is committed; the second is rolled back.

shell> mysql test

mysql> CREATE TABLE customer (a INT, b CHAR (20), INDEX (a))
    -> ENGINE=InnoDB;
Query OK, 0 rows affected (0.00 sec)
mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO customer VALUES (10, 'Heikki');
Query OK, 1 row affected (0.00 sec)
mysql> COMMIT;
Query OK, 0 rows affected (0.00 sec)
mysql> SET autocommit=0;
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO customer VALUES (15, 'John');
Query OK, 1 row affected (0.00 sec)
mysql> ROLLBACK;
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT * FROM customer;
+------+--------+
| a    | b      |
+------+--------+
|   10 | Heikki |
+------+--------+
1 row in set (0.00 sec)
mysql>
Transactions in Client-Side Languages

In APIs such as PHP, Perl DBI, JDBC, ODBC, or the standard C call interface of MySQL, you can send transaction control statements such as COMMIT to the MySQL server as strings just like any other SQL statements such as SELECT or INSERT. Some APIs also offer separate special transaction commit and rollback functions or methods.

13.3.5.2. Converting Tables from Other Storage Engines to InnoDB

To convert a non-InnoDB table to use InnoDB use ALTER TABLE:

ALTER TABLE table_name ENGINE=InnoDB;
Important

Do not convert MySQL system tables in the mysql database (such as user or host) to the InnoDB type. This is an unsupported operation. The system tables must always be of the MyISAM type.

To make an InnoDB table that is a clone of a MyISAM table:

  • Create an empty InnoDB table with identical definitions.

  • Create the appropriate indexes.

  • Insert the rows with INSERT INTO innodb_table SELECT * FROM myisam_table.

You can also create the indexes after inserting the data. Historically, creating new secondary indexes was a slow operation for InnoDB, but this is no longer the case.

If you have UNIQUE constraints on secondary keys, you can speed up a table import by turning off the uniqueness checks temporarily during the import operation:

SET unique_checks=0;... import operation ...
SET unique_checks=1;

For big tables, this saves disk I/O because InnoDB can use its insert buffer to write secondary index records as a batch. Be certain that the data contains no duplicate keys. unique_checks permits but does not require storage engines to ignore duplicate keys.

To get better control over the insertion process, you might insert big tables in pieces:

INSERT INTO newtable SELECT * FROM oldtable
   WHERE yourkey > something AND yourkey <= somethingelse;

After all records have been inserted, you can rename the tables.

During the conversion of big tables, increase the size of the InnoDB buffer pool to reduce disk I/O, to a maximum of 80% of physical memory. You can also increase the sizes of the InnoDB log files.

Make sure that you do not fill up the tablespace: InnoDB tables require a lot more disk space than MyISAM tables. If an ALTER TABLE operation runs out of space, it starts a rollback, and that can take hours if it is disk-bound. For inserts, InnoDB uses the insert buffer to merge secondary index records to indexes in batches. That saves a lot of disk I/O. For rollback, no such mechanism is used, and the rollback can take 30 times longer than the insertion.

In the case of a runaway rollback, if you do not have valuable data in your database, it may be advisable to kill the database process rather than wait for millions of disk I/O operations to complete. For the complete procedure, see Section 13.3.7.2, “Forcing InnoDB Recovery”.

If you want all new user-created tables to use the InnoDB storage engine, add the line default-storage-engine=innodb to the [mysqld] section of your server option file.

13.3.5.3. AUTO_INCREMENT Handling in InnoDB

InnoDB provides an optimization that significantly improves scalability and performance of SQL statements that insert rows into tables with AUTO_INCREMENT columns. This section provides background information on the original (“traditional”) implementation of auto-increment locking in InnoDB, explains the configurable locking mechanism, documents the parameter for configuring the mechanism, and describes its behavior and interaction with replication.

13.3.5.3.1. “TraditionalInnoDB Auto-Increment Locking

The original implementation of auto-increment handling in InnoDB uses the following strategy to prevent problems when using the binary log for statement-based replication or for certain recovery scenarios.

If you specify an AUTO_INCREMENT column for an InnoDB table, the table handle in the InnoDB data dictionary contains a special counter called the auto-increment counter that is used in assigning new values for the column. This counter is stored only in main memory, not on disk.

InnoDB uses the following algorithm to initialize the auto-increment counter for a table t that contains an AUTO_INCREMENT column named ai_col: After a server startup, for the first insert into a table t, InnoDB executes the equivalent of this statement:

SELECT MAX(ai_col) FROM t FOR UPDATE;

InnoDB increments the value retrieved by the statement and assigns it to the column and to the auto-increment counter for the table. By default, the value is incremented by one. This default can be overridden by the auto_increment_increment configuration setting.

If the table is empty, InnoDB uses the value 1. This default can be overridden by the auto_increment_offset configuration setting.

If a SHOW TABLE STATUS statement examines the table t before the auto-increment counter is initialized, InnoDB initializes but does not increment the value and stores it for use by later inserts. This initialization uses a normal exclusive-locking read on the table and the lock lasts to the end of the transaction.

InnoDB follows the same procedure for initializing the auto-increment counter for a freshly created table.

After the auto-increment counter has been initialized, if a you do not explicitly specify a value for an AUTO_INCREMENT column, InnoDB increments the counter and assigns the new value to the column. If you insert a row that explicitly specifies the column value, and the value is bigger than the current counter value, the counter is set to the specified column value.

When accessing the auto-increment counter, InnoDB uses a special table-level AUTO-INC lock that it keeps to the end of the current SQL statement, not to the end of the transaction. The special lock release strategy was introduced to improve concurrency for inserts into a table containing an AUTO_INCREMENT column. Nevertheless, two transactions cannot have the AUTO-INC lock on the same table simultaneously, which can have a performance impact if the AUTO-INC lock is held for a long time. That might be the case for a statement such as INSERT INTO t1 ... SELECT ... FROM t2 that inserts all rows from one table into another.

InnoDB uses the in-memory auto-increment counter as long as the server runs. When the server is stopped and restarted, InnoDB reinitializes the counter for each table for the first INSERT to the table, as described earlier.

You may see gaps in the sequence of values assigned to the AUTO_INCREMENT column if you roll back transactions that have generated numbers using the counter.

If a user specifies NULL or 0 for the AUTO_INCREMENT column in an INSERT, InnoDB treats the row as if the value was not specified and generates a new value for it.

The behavior of the auto-increment mechanism is not defined if you assign a negative value to the column, or if the value becomes bigger than the maximum integer that can be stored in the specified integer type.

An AUTO_INCREMENT column must appear as the first column in an index on an InnoDB table.

InnoDB supports the AUTO_INCREMENT = N table option in CREATE TABLE and ALTER TABLE statements, to set the initial counter value or alter the current counter value. The effect of this option is canceled by a server restart, for reasons discussed earlier in this section.

13.3.5.3.2. Configurable InnoDB Auto-Increment Locking

As described in the previous section, InnoDB uses a special lock called the table-level AUTO-INC lock for inserts into tables with AUTO_INCREMENT columns. This lock is normally held to the end of the statement (not to the end of the transaction), to ensure that auto-increment numbers are assigned in a predictable and repeatable order for a given sequence of INSERT statements.

In the case of statement-based replication, this means that when an SQL statement is replicated on a slave server, the same values are used for the auto-increment column as on the master server. The result of execution of multiple INSERT statements is deterministic, and the slave reproduces the same data as on the master. If auto-increment values generated by multiple INSERT statements were interleaved, the result of two concurrent INSERT statements would be nondeterministic, and could not reliably be propagated to a slave server using statement-based replication.

To make this clear, consider an example that uses this table:

CREATE TABLE t1 (
  c1 INT(11) NOT NULL AUTO_INCREMENT,
  c2 VARCHAR(10) DEFAULT NULL,
  PRIMARY KEY (c1)
) ENGINE=InnoDB;

Suppose that there are two transactions running, each inserting rows into a table with an AUTO_INCREMENT column. One transaction is using an INSERT ... SELECT statement that inserts 1000 rows, and another is using a simple INSERT statement that inserts one row:

Tx1: INSERT INTO t1 (c2) SELECT 1000 rows from another table ...
Tx2: INSERT INTO t1 (c2) VALUES ('xxx');

InnoDB cannot tell in advance how many rows will be retrieved from the SELECT in the INSERT statement in Tx1, and it assigns the auto-increment values one at a time as the statement proceeds. With a table-level lock, held to the end of the statement, only one INSERT statement referring to table t1 can execute at a time, and the generation of auto-increment numbers by different statements is not interleaved. The auto-increment value generated by the Tx1 INSERT ... SELECT statement will be consecutive, and the (single) auto-increment value used by the INSERT statement in Tx2 will either be smaller or larger than all those used for Tx1, depending on which statement executes first.

As long as the SQL statements execute in the same order when replayed from the binary log (when using statement-based replication, or in recovery scenarios), the results will be the same as they were when Tx1 and Tx2 first ran. Thus, table-level locks held until the end of a statement make INSERT statements using auto-increment safe for use with statement-based replication. However, those locks limit concurrency and scalability when multiple transactions are executing insert statements at the same time.

In the preceding example, if there were no table-level lock, the value of the auto-increment column used for the INSERT in Tx2 depends on precisely when the statement executes. If the INSERT of Tx2 executes while the INSERT of Tx1 is running (rather than before it starts or after it completes), the specific auto-increment values assigned by the two INSERT statements are nondeterministic, and may vary from run to run.

InnoDB can avoid using the table-level AUTO-INC lock for a class of INSERT statements where the number of rows is known in advance, and still preserve deterministic execution and safety for statement-based replication. Further, if you are not using the binary log to replay SQL statements as part of recovery or replication, you can entirely eliminate use of the table-level AUTO-INC lock for even greater concurrency and performance—at the cost of permitting gaps in auto-increment numbers assigned by a statement and potentially having the numbers assigned by concurrently executing statements interleaved.

For INSERT statements where the number of rows to be inserted is known at the beginning of processing the statement, InnoDB quickly allocates the required number of auto-increment values without taking any lock, but only if there is no concurrent session already holding the table-level AUTO-INC lock (because that other statement will be allocating auto-increment values one-by-one as it proceeds). More precisely, such an INSERT statement obtains auto-increment values under the control of a mutex (a light-weight lock) that is not held until the statement completes, but only for the duration of the allocation process.

This new locking scheme enables much greater scalability, but it does introduce some subtle differences in how auto-increment values are assigned compared to the original mechanism. To describe the way auto-increment works in InnoDB, the following discussion defines some terms, and explains how InnoDB behaves using different settings of the new innodb_autoinc_lock_mode configuration parameter. Additional considerations are described following the explanation of auto-increment locking behavior.

First, some definitions:

  • INSERT-like” statements

    All statements that generate new rows in a table, including INSERT, INSERT ... SELECT, REPLACE, REPLACE ... SELECT, and LOAD DATA.

  • Simple inserts

    Statements for which the number of rows to be inserted can be determined in advance (when the statement is initially processed). This includes single-row and multiple-row INSERT and REPLACE statements that do not have a nested subquery, but not INSERT ... ON DUPLICATE KEY UPDATE.

  • Bulk inserts

    Statements for which the number of rows to be inserted (and the number of required auto-increment values) is not known in advance. This includes INSERT ... SELECT, REPLACE ... SELECT, and LOAD DATA statements, but not plain INSERT. InnoDB will assign new values for the AUTO_INCREMENT column one at a time as each row is processed.

  • Mixed-mode inserts

    These are “simple insert” statements that specify the auto-increment value for some (but not all) of the new rows. An example follows, where c1 is an AUTO_INCREMENT column of table t1:

    INSERT INTO t1 (c1,c2) VALUES (1,'a'), (NULL,'b'), (5,'c'), (NULL,'d');

    Another type of “mixed-mode insert” is INSERT ... ON DUPLICATE KEY UPDATE, which in the worst case is in effect an INSERT followed by a UPDATE, where the allocated value for the AUTO_INCREMENT column may or may not be used during the update phase.

In MySQL 5.5, there is a configuration parameter that controls how InnoDB uses locking when generating values for AUTO_INCREMENT columns. This parameter can be set using the --innodb-autoinc-lock-mode option at mysqld startup.

In general, if you encounter problems with the way auto-increment works (which will most likely involve replication), you can force use of the original behavior by setting the lock mode to 0.

There are three possible settings for the innodb_autoinc_lock_mode parameter:

  • innodb_autoinc_lock_mode = 0 (“traditional” lock mode)

    This lock mode provides the same behavior as before innodb_autoinc_lock_mode existed. For all “INSERT-like” statements, a special table-level AUTO-INC lock is obtained and held to the end of the statement. This assures that the auto-increment values assigned by any given statement are consecutive (although “gaps” can exist within a table if a transaction that generated auto-increment values is rolled back, as discussed later).

    This lock mode is provided only for backward compatibility and performance testing. There is little reason to use this lock mode unless you use “mixed-mode inserts” and care about the important difference in semantics described later.

  • innodb_autoinc_lock_mode = 1 (“consecutive” lock mode)

    This is the default lock mode. In this mode, “bulk inserts” use the special AUTO-INC table-level lock and hold it until the end of the statement. This applies to all INSERT ... SELECT, REPLACE ... SELECT, and LOAD DATA statements. Only one statement holding the AUTO-INC lock can execute at a time.

    With this lock mode, “simple inserts” (only) use a new locking model where a light-weight mutex is used during the allocation of auto-increment values, and no table-level AUTO-INC lock is used, unless an AUTO-INC lock is held by another transaction. If another transaction does hold an AUTO-INC lock, a “simple insert” waits for the AUTO-INC lock, as if it too were a “bulk insert.

    This lock mode ensures that, in the presence of INSERT statements where the number of rows is not known in advance (and where auto-increment numbers are assigned as the statement progresses), all auto-increment values assigned by any “INSERT-like” statement are consecutive, and operations are safe for statement-based replication.

    Simply put, the important impact of this lock mode is significantly better scalability. This mode is safe for use with statement-based replication. Further, as with “traditional” lock mode, auto-increment numbers assigned by any given statement are consecutive. In this mode, there is no change in semantics compared to “traditional” mode for any statement that uses auto-increment, with one important exception.

    The exception is for “mixed-mode inserts”, where the user provides explicit values for an AUTO_INCREMENT column for some, but not all, rows in a multiple-row “simple insert.” For such inserts, InnoDB will allocate more auto-increment values than the number of rows to be inserted. However, all values automatically assigned are consecutively generated (and thus higher than) the auto-increment value generated by the most recently executed previous statement. “Excess” numbers are lost.

    A similar situation exists if you use INSERT ... ON DUPLICATE KEY UPDATE. This statement is also classified as a “mixed-mode insert” since an auto-increment value is not necessarily generated for each row. Because InnoDB allocates the auto-increment value before the insert is actually attempted, it cannot know whether an inserted value will be a duplicate of an existing value and thus cannot know whether the auto-increment value it generates will be used for a new row. Therefore, if you are using statement-based replication, either avoid INSERT ... ON DUPLICATE KEY UPDATE or use innodb_autoinc_lock_mode = 0 (“traditional” lock mode).

  • innodb_autoinc_lock_mode = 2 (“interleaved” lock mode)

    In this lock mode, no “INSERT-like” statements use the table-level AUTO-INC lock, and multiple statements can execute at the same time. This is the fastest and most scalable lock mode, but it is not safe when using statement-based replication or recovery scenarios when SQL statements are replayed from the binary log.

    In this lock mode, auto-increment values are guaranteed to be unique and monotonically increasing across all concurrently executing “INSERT-like” statements. However, because multiple statements can be generating numbers at the same time (that is, allocation of numbers is interleaved across statements), the values generated for the rows inserted by any given statement may not be consecutive.

    If the only statements executing are “simple inserts” where the number of rows to be inserted is known ahead of time, there will be no gaps in the numbers generated for a single statement, except for “mixed-mode inserts.” However, when “bulk inserts” are executed, there may be gaps in the auto-increment values assigned by any given statement.

The auto-increment locking modes provided by innodb_autoinc_lock_mode have several usage implications:

  • Using auto-increment with replication

    If you are using statement-based replication, set innodb_autoinc_lock_mode to 0 or 1 and use the same value on the master and its slaves. Auto-increment values are not ensured to be the same on the slaves as on the master if you use innodb_autoinc_lock_mode = 2 (“interleaved”) or configurations where the master and slaves do not use the same lock mode.

    If you are using row-based replication, all of the auto-increment lock modes are safe. Row-based replication is not sensitive to the order of execution of the SQL statements.

  • Lost” auto-increment values and sequence gaps

    In all lock modes (0, 1, and 2), if a transaction that generated auto-increment values rolls back, those auto-increment values are “lost.” Once a value is generated for an auto-increment column, it cannot be rolled back, whether or not the “INSERT-like” statement is completed, and whether or not the containing transaction is rolled back. Such lost values are not reused. Thus, there may be gaps in the values stored in an AUTO_INCREMENT column of a table.

  • Gaps in auto-increment values for “bulk inserts

    With innodb_autoinc_lock_mode set to 0 (“traditional”) or 1 (“consecutive”), the auto-increment values generated by any given statement will be consecutive, without gaps, because the table-level AUTO-INC lock is held until the end of the statement, and only one such statement can execute at a time.

    With innodb_autoinc_lock_mode set to 2 (“interleaved”), there may be gaps in the auto-increment values generated by “bulk inserts,” but only if there are concurrently executing “INSERT-like” statements.

    For lock modes 1 or 2, gaps may occur between successive statements because for bulk inserts the exact number of auto-increment values required by each statement may not be known and overestimation is possible.

  • Auto-increment values assigned by “mixed-mode inserts

    Consider a “mixed-mode insert,” where a “simple insert” specifies the auto-increment value for some (but not all) resulting rows. Such a statement will behave differently in lock modes 0, 1, and 2. For example, assume c1 is an AUTO_INCREMENT column of table t1, and that the most recent automatically generated sequence number is 100. Consider the following “mixed-mode insert” statement:

    INSERT INTO t1 (c1,c2) VALUES (1,'a'), (NULL,'b'), (5,'c'), (NULL,'d');

    With innodb_autoinc_lock_mode set to 0 (“traditional”), the four new rows will be:

    +-----+------+
    | c1  | c2   |
    +-----+------+
    |   1 | a    |
    | 101 | b    |
    |   5 | c    |
    | 102 | d    |
    +-----+------+

    The next available auto-increment value will be 103 because the auto-increment values are allocated one at a time, not all at once at the beginning of statement execution. This result is true whether or not there are concurrently executing “INSERT-like” statements (of any type).

    With innodb_autoinc_lock_mode set to 1 (“consecutive”), the four new rows will also be:

    +-----+------+
    | c1  | c2   |
    +-----+------+
    |   1 | a    |
    | 101 | b    |
    |   5 | c    |
    | 102 | d    |
    +-----+------+

    However, in this case, the next available auto-increment value will be 105, not 103 because four auto-increment values are allocated at the time the statement is processed, but only two are used. This result is true whether or not there are concurrently executing “INSERT-like” statements (of any type).

    With innodb_autoinc_lock_mode set to mode 2 (“interleaved”), the four new rows will be:

    +-----+------+
    | c1  | c2   |
    +-----+------+
    |   1 | a    |
    |   x | b    |
    |   5 | c    |
    |   y | d    |
    +-----+------+
    

    The values of x and y will be unique and larger than any previously generated rows. However, the specific values of x and y will depend on the number of auto-increment values generated by concurrently executing statements.

    Finally, consider the following statement, issued when the most-recently generated sequence number was the value 4:

    INSERT INTO t1 (c1,c2) VALUES (1,'a'), (NULL,'b'), (5,'c'), (NULL,'d');

    With any innodb_autoinc_lock_mode setting, this statement will generate a duplicate-key error 23000 (Can't write; duplicate key in table) because 5 will be allocated for the row (NULL, 'b') and insertion of the row (5, 'c') will fail.

13.3.5.4. FOREIGN KEY Constraints

InnoDB supports foreign key constraints. The syntax for a foreign key constraint definition in InnoDB looks like this:

[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION

index_name represents a foreign key ID. If given, this is ignored if an index for the foreign key is defined explicitly. Otherwise, if InnoDB creates an index for the foreign key, it uses index_name for the index name.

Foreign keys definitions are subject to the following conditions:

  • Both tables must be InnoDB tables and they must not be TEMPORARY tables.

  • Corresponding columns in the foreign key and the referenced key must have similar internal data types inside InnoDB so that they can be compared without a type conversion. The size and sign of integer types must be the same. The length of string types need not be the same. For nonbinary (character) string columns, the character set and collation must be the same.

  • InnoDB requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist. This index might be silently dropped later, if you create another index that can be used to enforce the foreign key constraint. index_name, if given, is used as described previously.

  • InnoDB permits a foreign key to reference any index column or group of columns. However, in the referenced table, there must be an index where the referenced columns are listed as the first columns in the same order.

  • Index prefixes on foreign key columns are not supported. One consequence of this is that BLOB and TEXT columns cannot be included in a foreign key because indexes on those columns must always include a prefix length.

  • If the CONSTRAINT symbol clause is given, the symbol value must be unique in the database. If the clause is not given, InnoDB creates the name automatically.

InnoDB rejects any INSERT or UPDATE operation that attempts to create a foreign key value in a child table if there is no a matching candidate key value in the parent table. When an UPDATE or DELETE operation affects a key value in the parent table that has matching rows in the child table, the result depends on the referential action specified using ON UPDATE and ON DELETE subclauses of the FOREIGN KEY clause. InnoDB supports five options regarding the action to be taken. If ON DELETE or ON UPDATE are not specified, the default action is RESTRICT.

  • CASCADE: Delete or update the row from the parent table, and automatically delete or update the matching rows in the child table. Both ON DELETE CASCADE and ON UPDATE CASCADE are supported. Between two tables, do not define several ON UPDATE CASCADE clauses that act on the same column in the parent table or in the child table.

    Замечание

    Currently, cascaded foreign key actions do not activate triggers.

  • SET NULL: Delete or update the row from the parent table, and set the foreign key column or columns in the child table to NULL. Both ON DELETE SET NULL and ON UPDATE SET NULL clauses are supported.

    If you specify a SET NULL action, make sure that you have not declared the columns in the child table as NOT NULL.

  • RESTRICT: Rejects the delete or update operation for the parent table. Specifying RESTRICT (or NO ACTION) is the same as omitting the ON DELETE or ON UPDATE clause.

  • NO ACTION: A keyword from standard SQL. In MySQL, equivalent to RESTRICT. InnoDB rejects the delete or update operation for the parent table if there is a related foreign key value in the referenced table. Some database systems have deferred checks, and NO ACTION is a deferred check. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT.

  • SET DEFAULT: This action is recognized by the parser, but InnoDB rejects table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.

InnoDB supports foreign key references between one column and another within a table. (A column cannot have a foreign key reference to itself.) In these cases, “child table records” really refers to dependent records within the same table.

Examples of Foreign Key Clauses

Here is a simple example that relates parent and child tables through a single-column foreign key:

CREATE TABLE parent (id INT NOT NULL,
                     PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE child (id INT, parent_id INT,
                    INDEX par_ind (parent_id),
                    FOREIGN KEY (parent_id) REFERENCES parent(id)
                      ON DELETE CASCADE
) ENGINE=INNODB;

A more complex example in which a product_order table has foreign keys for two other tables. One foreign key references a two-column index in the product table. The other references a single-column index in the customer table:

CREATE TABLE product (category INT NOT NULL, id INT NOT NULL,
                      price DECIMAL,
                      PRIMARY KEY(category, id)) ENGINE=INNODB;
CREATE TABLE customer (id INT NOT NULL,
                       PRIMARY KEY (id)) ENGINE=INNODB;
CREATE TABLE product_order (no INT NOT NULL AUTO_INCREMENT,
                            product_category INT NOT NULL,
                            product_id INT NOT NULL,
                            customer_id INT NOT NULL,
                            PRIMARY KEY(no),
                            INDEX (product_category, product_id),
                            FOREIGN KEY (product_category, product_id)
                              REFERENCES product(category, id)
                              ON UPDATE CASCADE ON DELETE RESTRICT,
                            INDEX (customer_id),
                            FOREIGN KEY (customer_id)
                              REFERENCES customer(id)) ENGINE=INNODB;

InnoDB enables you to add a new foreign key constraint to a table by using ALTER TABLE:

ALTER TABLE tbl_name
    ADD [CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (index_col_name, ...)
    REFERENCES tbl_name (index_col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

The foreign key can be self referential (referring to the same table). When you add a foreign key constraint to a table using ALTER TABLE, remember to create the required indexes first.

Foreign Keys and ALTER TABLE

InnoDB supports the use of ALTER TABLE to drop foreign keys:

ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol;

If the FOREIGN KEY clause included a CONSTRAINT name when you created the foreign key, you can refer to that name to drop the foreign key. Otherwise, the fk_symbol value is internally generated by InnoDB when the foreign key is created. To find out the symbol value when you want to drop a foreign key, use the SHOW CREATE TABLE statement. For example:

mysql> SHOW CREATE TABLE ibtest11c\G
*************************** 1. row ***************************
       Table: ibtest11c
Create Table: CREATE TABLE `ibtest11c` (
  `A` int(11) NOT NULL auto_increment,
  `D` int(11) NOT NULL default '0',
  `B` varchar(200) NOT NULL default '',
  `C` varchar(175) default NULL,
  PRIMARY KEY  (`A`,`D`,`B`),
  KEY `B` (`B`,`C`),
  KEY `C` (`C`),
  CONSTRAINT `0_38775` FOREIGN KEY (`A`, `D`)
REFERENCES `ibtest11a` (`A`, `D`)
ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `0_38776` FOREIGN KEY (`B`, `C`)
REFERENCES `ibtest11a` (`B`, `C`)
ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=INNODB CHARSET=latin1
1 row in set (0.01 sec)

mysql> ALTER TABLE ibtest11c DROP FOREIGN KEY `0_38775`;

You cannot add a foreign key and drop a foreign key in separate clauses of a single ALTER TABLE statement. Separate statements are required.

If ALTER TABLE for an InnoDB table results in changes to column values (for example, because a column is truncated), InnoDB's FOREIGN KEY constraint checks do not notice possible violations caused by changing the values.

How Foreign Keys Work with Other MySQL Command

The InnoDB parser permits table and column identifiers in a FOREIGN KEY ... REFERENCES ... clause to be quoted within backticks. (Alternatively, double quotation marks can be used if the ANSI_QUOTES SQL mode is enabled.) The InnoDB parser also takes into account the setting of the lower_case_table_names system variable.

InnoDB returns a table's foreign key definitions as part of the output of the SHOW CREATE TABLE statement:

SHOW CREATE TABLE tbl_name;

mysqldump also produces correct definitions of tables in the dump file, and does not forget about the foreign keys.

To make it easier to reload dump files for tables that have foreign key relationships, mysqldump automatically includes a statement in the dump output to set foreign_key_checks to 0. This avoids problems with tables having to be reloaded in a particular order when the dump is reloaded. It is also possible to set this variable manually:

mysql> SET foreign_key_checks = 0;
mysql> SOURCE dump_file_name;
mysql> SET foreign_key_checks = 1;

This enables you to import the tables in any order if the dump file contains tables that are not correctly ordered for foreign keys. It also speeds up the import operation. Setting foreign_key_checks to 0 can also be useful for ignoring foreign key constraints during LOAD DATA and ALTER TABLE operations. However, even if foreign_key_checks = 0, InnoDB does not permit the creation of a foreign key constraint where a column references a nonmatching column type. Also, if an InnoDB table has foreign key constraints, ALTER TABLE cannot be used to change the table to use another storage engine. To alter the storage engine, drop any foreign key constraints first.

InnoDB does not permit you to drop a table that is referenced by a FOREIGN KEY constraint, unless you do SET foreign_key_checks = 0. When you drop a table, the constraints that were defined in its create statement are also dropped.

If you re-create a table that was dropped, it must have a definition that conforms to the foreign key constraints referencing it. It must have the right column names and types, and it must have indexes on the referenced keys, as stated earlier. If these are not satisfied, MySQL returns error number 1005 and refers to error 150 in the error message.

If MySQL reports an error number 1005 from a CREATE TABLE statement, and the error message refers to error 150, table creation failed because a foreign key constraint was not correctly formed. Similarly, if an ALTER TABLE fails and it refers to error 150, that means a foreign key definition would be incorrectly formed for the altered table. You can use SHOW ENGINE INNODB STATUS to display a detailed explanation of the most recent InnoDB foreign key error in the server.

Important

For users familiar with the ANSI/ISO SQL Standard, please note that no storage engine, including InnoDB, recognizes or enforces the MATCH clause used in referential-integrity constraint definitions. Use of an explicit MATCH clause will not have the specified effect, and also causes ON DELETE and ON UPDATE clauses to be ignored. For these reasons, specifying MATCH should be avoided.

The MATCH clause in the SQL standard controls how NULL values in a composite (multiple-column) foreign key are handled when comparing to a primary key. InnoDB essentially implements the semantics defined by MATCH SIMPLE, which permit a foreign key to be all or partially NULL. In that case, the (child table) row containing such a foreign key is permitted to be inserted, and does not match any row in the referenced (parent) table. It is possible to implement other semantics using triggers.

Additionally, MySQL and InnoDB require that the referenced columns be indexed for performance. However, the system does not enforce a requirement that the referenced columns be UNIQUE or be declared NOT NULL. The handling of foreign key references to nonunique keys or keys that contain NULL values is not well defined for operations such as UPDATE or DELETE CASCADE. You are advised to use foreign keys that reference only UNIQUE and NOT NULL keys.

Furthermore, InnoDB does not recognize or support “inline REFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification. InnoDB accepts REFERENCES clauses only when specified as part of a separate FOREIGN KEY specification. For other storage engines, MySQL Server parses and ignores foreign key specifications.

Deviation from SQL standards: If there are several rows in the parent table that have the same referenced key value, InnoDB acts in foreign key checks as if the other parent rows with the same key value do not exist. For example, if you have defined a RESTRICT type constraint, and there is a child row with several parent rows, InnoDB does not permit the deletion of any of those parent rows.

InnoDB performs cascading operations through a depth-first algorithm, based on records in the indexes corresponding to the foreign key constraints.

Deviation from SQL standards: A FOREIGN KEY constraint that references a non-UNIQUE key is not standard SQL. It is an InnoDB extension to standard SQL.

Deviation from SQL standards: If ON UPDATE CASCADE or ON UPDATE SET NULL recurses to update the same table it has previously updated during the cascade, it acts like RESTRICT. This means that you cannot use self-referential ON UPDATE CASCADE or ON UPDATE SET NULL operations. This is to prevent infinite loops resulting from cascaded updates. A self-referential ON DELETE SET NULL, on the other hand, is possible, as is a self-referential ON DELETE CASCADE. Cascading operations may not be nested more than 15 levels deep.

Deviation from SQL standards: Like MySQL in general, in an SQL statement that inserts, deletes, or updates many rows, InnoDB checks UNIQUE and FOREIGN KEY constraints row-by-row. When performing foreign key checks, InnoDB sets shared row-level locks on child or parent records it has to look at. InnoDB checks foreign key constraints immediately; the check is not deferred to transaction commit. According to the SQL standard, the default behavior should be deferred checking. That is, constraints are only checked after the entire SQL statement has been processed. Until InnoDB implements deferred constraint checking, some things will be impossible, such as deleting a record that refers to itself using a foreign key.

13.3.5.5. InnoDB and MySQL Replication

MySQL replication works for InnoDB tables as it does for MyISAM tables. It is also possible to use replication in a way where the storage engine on the slave is not the same as the original storage engine on the master. For example, you can replicate modifications to an InnoDB table on the master to a MyISAM table on the slave.

To set up a new slave for a master, you have to make a copy of the InnoDB tablespace and the log files, as well as the .frm files of the InnoDB tables, and move the copies to the slave. If the innodb_file_per_table variable is enabled, copy the .ibd files as well. For the proper procedure to do this, see Section 13.3.7, “Backing Up and Recovering an InnoDB Database”.

If you can shut down the master or an existing slave, you can take a cold backup of the InnoDB tablespace and log files and use that to set up a slave. To make a new slave without taking down any server you can also use the MySQL Enterprise Backup product.

Transactions that fail on the master do not affect replication at all. MySQL replication is based on the binary log where MySQL writes SQL statements that modify data. A transaction that fails (for example, because of a foreign key violation, or because it is rolled back) is not written to the binary log, so it is not sent to slaves. See Section 12.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Синтаксис”.

Replication and CASCADE Cascading actions for InnoDB tables on the master are replicated on the slave only if the tables sharing the foreign key relation use InnoDB on both the master and slave. This is true whether you are using statement-based or row-based replication. Suppose that you have started replication, and then create two tables on the master using the following CREATE TABLE statements:

CREATE TABLE fc1 (
    i INT PRIMARY KEY,
    j INT
) ENGINE = InnoDB;

CREATE TABLE fc2 (
    m INT PRIMARY KEY,
    n INT,
    FOREIGN KEY ni (n) REFERENCES fc1 (i)
        ON DELETE CASCADE
) ENGINE = InnoDB;

Suppose that the slave does not have InnoDB support enabled. If this is the case, then the tables on the slave are created, but they use the MyISAM storage engine, and the FOREIGN KEY option is ignored. Now we insert some rows into the tables on the master:

master> INSERT INTO fc1 VALUES (1, 1), (2, 2);
Query OK, 2 rows affected (0.09 sec)
Records: 2  Duplicates: 0  Warnings: 0

master> INSERT INTO fc2 VALUES (1, 1), (2, 2), (3, 1);
Query OK, 3 rows affected (0.19 sec)
Records: 3  Duplicates: 0  Warnings: 0

At this point, on both the master and the slave, table fc1 contains 2 rows, and table fc2 contains 3 rows, as shown here:

master> SELECT * FROM fc1;
+---+------+
| i | j    |
+---+------+
| 1 |    1 |
| 2 |    2 |
+---+------+
2 rows in set (0.00 sec)

master> SELECT * FROM fc2;
+---+------+
| m | n    |
+---+------+
| 1 |    1 |
| 2 |    2 |
| 3 |    1 |
+---+------+
3 rows in set (0.00 sec)

slave> SELECT * FROM fc1;
+---+------+
| i | j    |
+---+------+
| 1 |    1 |
| 2 |    2 |
+---+------+
2 rows in set (0.00 sec)

slave> SELECT * FROM fc2;
+---+------+
| m | n    |
+---+------+
| 1 |    1 |
| 2 |    2 |
| 3 |    1 |
+---+------+
3 rows in set (0.00 sec)

Now suppose that you perform the following DELETE statement on the master:

master> DELETE FROM fc1 WHERE i=1;
Query OK, 1 row affected (0.09 sec)

Due to the cascade, table fc2 on the master now contains only 1 row:

master> SELECT * FROM fc2;
+---+---+
| m | n |
+---+---+
| 2 | 2 |
+---+---+
1 row in set (0.00 sec)

However, the cascade does not propagate on the slave because on the slave the DELETE for fc1 deletes no rows from fc2. The slave's copy of fc2 still contains all of the rows that were originally inserted:

slave> SELECT * FROM fc2;
+---+---+
| m | n |
+---+---+
| 1 | 1 |
| 3 | 1 |
| 2 | 2 |
+---+---+
3 rows in set (0.00 sec)

This difference is due to the fact that the cascading deletes are handled internally by the InnoDB storage engine, which means that none of the changes are logged.

13.3.6. Adding, Removing, or Resizing InnoDB Data and Log Files

This section describes what you can do when your InnoDB tablespace runs out of room or when you want to change the size of the log files.

The easiest way to increase the size of the InnoDB tablespace is to configure it from the beginning to be auto-extending. Specify the autoextend attribute for the last data file in the tablespace definition. Then InnoDB increases the size of that file automatically in 8MB increments when it runs out of space. The increment size can be changed by setting the value of the innodb_autoextend_increment system variable, which is measured in MB.

Alternatively, you can increase the size of your tablespace by adding another data file. To do this, you have to shut down the MySQL server, change the tablespace configuration to add a new data file to the end of innodb_data_file_path, and start the server again.

If your last data file was defined with the keyword autoextend, the procedure for reconfiguring the tablespace must take into account the size to which the last data file has grown. Obtain the size of the data file, round it down to the closest multiple of 1024 × 1024 bytes (= 1MB), and specify the rounded size explicitly in innodb_data_file_path. Then you can add another data file. Remember that only the last data file in the innodb_data_file_path can be specified as auto-extending.

As an example, assume that the tablespace has just one auto-extending data file ibdata1:

innodb_data_home_dir =
innodb_data_file_path = /ibdata/ibdata1:10M:autoextend

Suppose that this data file, over time, has grown to 988MB. Here is the configuration line after modifying the original data file to not be auto-extending and adding another auto-extending data file:

innodb_data_home_dir =
innodb_data_file_path = /ibdata/ibdata1:988M;/disk2/ibdata2:50M:autoextend

When you add a new file to the tablespace configuration, make sure that it does not exist. InnoDB will create and initialize the file when you restart the server.

Currently, you cannot remove a data file from the tablespace. To decrease the size of your tablespace, use this procedure:

  1. Use mysqldump to dump all your InnoDB tables.

  2. Stop the server.

  3. Remove all the existing tablespace files, including the ibdata and ib_log files. If you want to keep a backup copy of the information, then copy all the ib* files to another location before the removing the files in your MySQL installation.

  4. Remove any .frm files for InnoDB tables.

  5. Configure a new tablespace.

  6. Restart the server.

  7. Import the dump files.

If you want to change the number or the size of your InnoDB log files, use the following instructions. The procedure to use depends on the value of innodb_fast_shutdown:

  • If innodb_fast_shutdown is not set to 2: Stop the MySQL server and make sure that it shuts down without errors (to ensure that there is no information for outstanding transactions in the log). Copy the old log files into a safe place in case something went wrong during the shutdown and you need them to recover the tablespace. Delete the old log files from the log file directory, edit my.cnf to change the log file configuration, and start the MySQL server again. mysqld sees that no InnoDB log files exist at startup and creates new ones.

  • If innodb_fast_shutdown is set to 2: Set innodb_fast_shutdown to 1:

    mysql> SET GLOBAL innodb_fast_shutdown = 1;
    

    Then follow the instructions in the previous item.

13.3.7. Backing Up and Recovering an InnoDB Database

The key to safe database management is making regular backups. Depending on your data volume, number of MySQL servers, and database workload, you can use these techniques, alone or in combination: hot backup with MySQL Enterprise Backup; cold backup by copying files while the MySQL server is shut down; physical backup for fast operation (especially for restore); logical backup with mysqldump for smaller data volumes or to record the structure of schema objects.

Hot Backups

The mysqlbackup command, part of the MySQL Enterprise Backup component, lets you back up a running MySQL instance, including InnoDB and MyISAM tables, with minimal disruption to operations while producing a consistent snapshot of the database. When mysqlbackup is copying InnoDB tables, reads and writes to both InnoDB and MyISAM tables can continue. During the copying of MyISAM tables, reads (but not writes) to those tables are permitted. MySQL Enterprise Backup can also create compressed backup files, and back up subsets of tables and databases. In conjunction with MySQL’s binary log, users can perform point-in-time recovery. MySQL Enterprise Backup is part of the MySQL Enterprise subscription. For more details, see Глава 24, MySQL Enterprise Backup.

Cold Backups

If you can shut down your MySQL server, you can make a binary backup that consists of all files used by InnoDB to manage its tables. Use the following procedure:

  1. Do a slow shutdown of the MySQL server and make sure that it stops without errors.

  2. Copy all InnoDB data files (ibdata files and .ibd files) into a safe place.

  3. Copy all the .frm files for InnoDB tables to a safe place.

  4. Copy all InnoDB log files (ib_logfile files) to a safe place.

  5. Copy your my.cnf configuration file or files to a safe place.

Alternative Backup Types

In addition to making binary backups as just described, regularly make dumps of your tables with mysqldump. A binary file might be corrupted without you noticing it. Dumped tables are stored into text files that are human-readable, so spotting table corruption becomes easier. Also, because the format is simpler, the chance for serious data corruption is smaller. mysqldump also has a --single-transaction option for making a consistent snapshot without locking out other clients. See Section 6.3.1, “Establishing a Backup Policy”.

Replication works with InnoDB tables, so you can use MySQL replication capabilities to keep a copy of your database at database sites requiring high availability.

Performing Recovery

To recover your InnoDB database to the present from the time at which the binary backup was made, you must run your MySQL server with binary logging turned on, even before taking the backup. To achieve point-in-time recovery after restoring a backup, you can apply changes from the binary log that occurred after the backup was made. See Section 6.5, “Point-in-Time (Incremental) Recovery Using the Binary Log”.

To recover from a crash of your MySQL server, the only requirement is to restart it. InnoDB automatically checks the logs and performs a roll-forward of the database to the present. InnoDB automatically rolls back uncommitted transactions that were present at the time of the crash. During recovery, mysqld displays output something like this:

InnoDB: Database was not shut down normally.
InnoDB: Starting recovery from log files...
InnoDB: Starting log scan based on checkpoint at
InnoDB: log sequence number 0 13674004
InnoDB: Doing recovery: scanned up to log sequence number 0 13739520
InnoDB: Doing recovery: scanned up to log sequence number 0 13805056
InnoDB: Doing recovery: scanned up to log sequence number 0 13870592
InnoDB: Doing recovery: scanned up to log sequence number 0 13936128
...
InnoDB: Doing recovery: scanned up to log sequence number 0 20555264
InnoDB: Doing recovery: scanned up to log sequence number 0 20620800
InnoDB: Doing recovery: scanned up to log sequence number 0 20664692
InnoDB: 1 uncommitted transaction(s) which must be rolled back
InnoDB: Starting rollback of uncommitted transactions
InnoDB: Rolling back trx no 16745
InnoDB: Rolling back of trx no 16745 completed
InnoDB: Rollback of uncommitted transactions completed
InnoDB: Starting an apply batch of log records to the database...
InnoDB: Apply batch completed
InnoDB: Started
mysqld: ready for connections

If your database becomes corrupted or disk failure occurs, you must perform the recovery using a backup. In the case of corruption, first find a backup that is not corrupted. After restoring the base backup, do a point-in-time recovery from the binary log files using mysqlbinlog and mysql to restore the changes that occurred after the backup was made.

In some cases of database corruption it is enough just to dump, drop, and re-create one or a few corrupt tables. You can use the CHECK TABLE SQL statement to check whether a table is corrupt, although CHECK TABLE naturally cannot detect every possible kind of corruption. You can use the Tablespace Monitor to check the integrity of the file space management inside the tablespace files.

In some cases, apparent database page corruption is actually due to the operating system corrupting its own file cache, and the data on disk may be okay. It is best first to try restarting your computer. Doing so may eliminate errors that appeared to be database page corruption.

13.3.7.1. The InnoDB Recovery Process

InnoDB crash recovery consists of several steps. The first step, redo log application, is performed during the initialization, before accepting any connections. If all changes were flushed from the buffer pool to the tablespaces (ibdata* and *.ibd files) at the time of the shutdown or crash, the redo log application can be skipped. If the redo log files are missing at startup, InnoDB skips the redo log application.

The remaining steps after redo log application do not depend on the redo log (other than for logging the writes) and are performed in parallel with normal processing. These include:

  • Rolling back incomplete transactions: Any transactions that were active at the time of crash or fast shutdown.

  • Insert buffer merge: Applying changes from the insert buffer tree (from the shared tablespace) to leaf pages of secondary indexes as the index pages are read to the buffer pool.

  • Purge: Deleting delete-marked records that are no longer visible for any active transaction.

Of these, only rollback of incomplete transactions is special to crash recovery. The insert buffer merge and the purge are performed during normal processing.

13.3.7.2. Forcing InnoDB Recovery

If there is database page corruption, you may want to dump your tables from the database with SELECT ... INTO OUTFILE. Usually, most of the data obtained in this way is intact. However, it is possible that the corruption might cause SELECT * FROM tbl_name statements or InnoDB background operations to crash or assert, or even cause InnoDB roll-forward recovery to crash. In such cases, you can use the innodb_force_recovery option to force the InnoDB storage engine to start up while preventing background operations from running, so that you can dump your tables. For example, you can add the following line to the [mysqld] section of your option file before restarting the server:

[mysqld]
innodb_force_recovery = 4

innodb_force_recovery is 0 by default (normal startup without forced recovery). The permissible nonzero values for innodb_force_recovery follow. A larger number includes all precautions of smaller numbers. If you can dump your tables with an option value of at most 4, then you are relatively safe that only some data on corrupt individual pages is lost. A value of 6 is more drastic because database pages are left in an obsolete state, which in turn may introduce more corruption into B-trees and other database structures.

  • 1 (SRV_FORCE_IGNORE_CORRUPT)

    Let the server run even if it detects a corrupt page. Try to make SELECT * FROM tbl_name jump over corrupt index records and pages, which helps in dumping tables.

  • 2 (SRV_FORCE_NO_BACKGROUND)

    Prevent the main thread from running. If a crash would occur during the purge operation, this recovery value prevents it.

  • 3 (SRV_FORCE_NO_TRX_UNDO)

    Do not run transaction rollbacks after recovery.

  • 4 (SRV_FORCE_NO_IBUF_MERGE)

    Prevent insert buffer merge operations. If they would cause a crash, do not do them. Do not calculate table statistics.

  • 5 (SRV_FORCE_NO_UNDO_LOG_SCAN)

    Do not look at undo logs when starting the database: InnoDB treats even incomplete transactions as committed.

  • 6 (SRV_FORCE_NO_LOG_REDO)

    Do not do the log roll-forward in connection with recovery.

    With this value, you might not be able to do queries other than a basic SELECT * FROM t, with no WHERE, ORDER BY, or other clauses. More complex queries could encounter corrupted data structures and fail.

    If corruption within the table data prevents you from dumping the entire table contents, a query with an ORDER BY primary_key DESC clause might be able to dump the portion of the table after the corrupted part.

The database must not otherwise be used with any nonzero value of innodb_force_recovery. As a safety measure, InnoDB prevents users from performing INSERT, UPDATE, or DELETE operations when innodb_force_recovery is greater than 0.

You can SELECT from tables to dump them, or DROP or CREATE tables even if forced recovery is used. If you know that a given table is causing a crash on rollback, you can drop it. You can also use this to stop a runaway rollback caused by a failing mass import or ALTER TABLE. You can kill the mysqld process and set innodb_force_recovery to 3 to bring the database up without the rollback, then DROP the table that is causing the runaway rollback.

13.3.7.3. InnoDB Checkpoints

Making your log files very large may reduce disk I/O during checkpointing. It often makes sense to set the total size of the log files as large as the buffer pool or even larger. Although in the past large log files could make crash recovery take excessive time, starting with MySQL 5.5, performance enhancements to crash recovery make it possible to use large log files with fast startup after a crash. (Strictly speaking, this performance improvement is available for MySQL 5.1 with the InnoDB Plugin 1.0.7 and higher. It is with MySQL 5.5 and InnoDB 1.1 that this improvement is available in the default InnoDB storage engine.)

How Checkpoint Processing Works

InnoDB implements a checkpoint mechanism known as “fuzzy” checkpointing. InnoDB flushes modified database pages from the buffer pool in small batches. There is no need to flush the buffer pool in one single batch, which would in practice stop processing of user SQL statements during the checkpointing process.

During crash recovery, InnoDB looks for a checkpoint label written to the log files. It knows that all modifications to the database before the label are present in the disk image of the database. Then InnoDB scans the log files forward from the checkpoint, applying the logged modifications to the database.

InnoDB writes to its log files on a rotating basis. It also writes checkpoint information to the first log file at each checkpoint. All committed modifications that make the database pages in the buffer pool different from the images on disk must be available in the log files in case InnoDB has to do a recovery. This means that when InnoDB starts to reuse a log file, it has to make sure that the database page images on disk contain the modifications logged in the log file that InnoDB is going to reuse. In other words, InnoDB must create a checkpoint and this often involves flushing of modified database pages to disk.

13.3.8. Moving an InnoDB Database to Another Machine

On Windows, InnoDB always stores database and table names internally in lowercase. To move databases in a binary format from Unix to Windows or from Windows to Unix, create all databases and tables using lowercase names. A convenient way to accomplish this is to add the following line to the [mysqld] section of your my.cnf or my.ini file before creating any databases or tables:

[mysqld]
lower_case_table_names=1

Like MyISAM data files, InnoDB data and log files are binary-compatible on all platforms having the same floating-point number format. You can move an InnoDB database simply by copying all the relevant files listed in Section 13.3.7, “Backing Up and Recovering an InnoDB Database”. If the floating-point formats differ but you have not used FLOAT or DOUBLE data types in your tables, then the procedure is the same: simply copy the relevant files. If you use mysqldump to dump your tables on one machine and then import the dump files on the other machine, it does not matter whether the formats differ or your tables contain floating-point data.

One way to increase performance is to switch off autocommit mode when importing data, assuming that the tablespace has enough space for the big rollback segment that the import transactions generate. Do the commit only after importing a whole table or a segment of a table.

13.3.9. The InnoDB Transaction Model and Locking

To implement a large-scale, busy, or highly reliable database application, to port substantial code from a different database system, or to push MySQL performance to the limits of the laws of physics, you must understand the notions of transactions and locking as they relate to the InnoDB storage engine.

In the InnoDB transaction model, the goal is to combine the best properties of a multi-versioning database with traditional two-phase locking. InnoDB does locking on the row level and runs queries as nonlocking consistent reads by default, in the style of Oracle. The lock information in InnoDB is stored so space-efficiently that lock escalation is not needed: Typically, several users are permitted to lock every row in InnoDB tables, or any random subset of the rows, without causing InnoDB memory exhaustion.

In InnoDB, all user activity occurs inside a transaction. If autocommit mode is enabled, each SQL statement forms a single transaction on its own. By default, MySQL starts the session for each new connection with autocommit enabled, so MySQL does a commit after each SQL statement if that statement did not return an error. If a statement returns an error, the commit or rollback behavior depends on the error. See Section 13.3.13, “InnoDB Error Handling”.

A session that has autocommit enabled can perform a multiple-statement transaction by starting it with an explicit START TRANSACTION or BEGIN statement and ending it with a COMMIT or ROLLBACK statement. See Section 12.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Синтаксис”.

If autocommit mode is disabled within a session with SET autocommit = 0, the session always has a transaction open. A COMMIT or ROLLBACK statement ends the current transaction and a new one starts.

A COMMIT means that the changes made in the current transaction are made permanent and become visible to other sessions. A ROLLBACK statement, on the other hand, cancels all modifications made by the current transaction. Both COMMIT and ROLLBACK release all InnoDB locks that were set during the current transaction.

In terms of the SQL:1992 transaction isolation levels, the default InnoDB level is REPEATABLE READ. InnoDB offers all four transaction isolation levels described by the SQL standard: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE.

A user can change the isolation level for a single session or for all subsequent connections with the SET TRANSACTION statement. To set the server's default isolation level for all connections, use the --transaction-isolation option on the command line or in an option file. For detailed information about isolation levels and level-setting syntax, see Section 12.3.6, “SET TRANSACTION Синтаксис”.

In row-level locking, InnoDB normally uses next-key locking. That means that besides index records, InnoDB can also lock the “gap” preceding an index record to block insertions by other sessions in the gap immediately before the index record. A next-key lock refers to a lock that locks an index record and the gap before it. A gap lock refers to a lock that locks only the gap before some index record.

For more information about row-level locking, and the circumstances under which gap locking is disabled, see Section 13.3.9.4, “InnoDB Record, Gap, and Next-Key Locks”.

13.3.9.1. InnoDB Lock Modes

InnoDB implements standard row-level locking where there are two types of locks:

  • A shared (S) lock permits a transaction to read a row.

  • An exclusive (X) lock permits a transaction to update or delete a row.

If transaction T1 holds a shared (S) lock on row r, then requests from some distinct transaction T2 for a lock on row r are handled as follows:

  • A request by T2 for an S lock can be granted immediately. As a result, both T1 and T2 hold an S lock on r.

  • A request by T2 for an X lock cannot be granted immediately.

If a transaction T1 holds an exclusive (X) lock on row r, a request from some distinct transaction T2 for a lock of either type on r cannot be granted immediately. Instead, transaction T2 has to wait for transaction T1 to release its lock on row r.

Additionally, InnoDB supports multiple granularity locking which permits coexistence of record locks and locks on entire tables. To make locking at multiple granularity levels practical, additional types of locks called intention locks are used. Intention locks are table locks in InnoDB. The idea behind intention locks is for a transaction to indicate which type of lock (shared or exclusive) it will require later for a row in that table. There are two types of intention locks used in InnoDB (assume that transaction T has requested a lock of the indicated type on table t):

  • Intention shared (IS): Transaction T intends to set S locks on individual rows in table t.

  • Intention exclusive (IX): Transaction T intends to set X locks on those rows.

For example, SELECT ... LOCK IN SHARE MODE sets an IS lock and SELECT ... FOR UPDATE sets an IX lock.

The intention locking protocol is as follows:

  • Before a transaction can acquire an S lock on a row in table t, it must first acquire an IS or stronger lock on t.

  • Before a transaction can acquire an X lock on a row, it must first acquire an IX lock on t.

These rules can be conveniently summarized by means of the following lock type compatibility matrix.

 XIXSIS
XConflictConflictConflictConflict
IXConflictCompatibleConflictCompatible
SConflictConflictCompatibleCompatible
ISConflictCompatibleCompatibleCompatible

A lock is granted to a requesting transaction if it is compatible with existing locks, but not if it conflicts with existing locks. A transaction waits until the conflicting existing lock is released. If a lock request conflicts with an existing lock and cannot be granted because it would cause deadlock, an error occurs.

Thus, intention locks do not block anything except full table requests (for example, LOCK TABLES ... WRITE). The main purpose of IX and IS locks is to show that someone is locking a row, or going to lock a row in the table.

The following example illustrates how an error can occur when a lock request would cause a deadlock. The example involves two clients, A and B.

First, client A creates a table containing one row, and then begins a transaction. Within the transaction, A obtains an S lock on the row by selecting it in share mode:

mysql> CREATE TABLE t (i INT) ENGINE = InnoDB;
Query OK, 0 rows affected (1.07 sec)

mysql> INSERT INTO t (i) VALUES(1);
Query OK, 1 row affected (0.09 sec)

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM t WHERE i = 1 LOCK IN SHARE MODE;
+------+
| i    |
+------+
|    1 |
+------+
1 row in set (0.10 sec)

Next, client B begins a transaction and attempts to delete the row from the table:

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> DELETE FROM t WHERE i = 1;

The delete operation requires an X lock. The lock cannot be granted because it is incompatible with the S lock that client A holds, so the request goes on the queue of lock requests for the row and client B blocks.

Finally, client A also attempts to delete the row from the table:

mysql> DELETE FROM t WHERE i = 1;
ERROR 1213 (40001): Deadlock found when trying to get lock;
try restarting transaction

Deadlock occurs here because client A needs an X lock to delete the row. However, that lock request cannot be granted because client B already has a request for an X lock and is waiting for client A to release its S lock. Nor can the S lock held by A be upgraded to an X lock because of the prior request by B for an X lock. As a result, InnoDB generates an error for client A and releases its locks. At that point, the lock request for client B can be granted and B deletes the row from the table.

13.3.9.2. Consistent Nonlocking Reads

A consistent read means that InnoDB uses multi-versioning to present to a query a snapshot of the database at a point in time. The query sees the changes made by transactions that committed before that point of time, and no changes made by later or uncommitted transactions. The exception to this rule is that the query sees the changes made by earlier statements within the same transaction. This exception causes the following anomaly: If you update some rows in a table, a SELECT sees the latest version of the updated rows, but it might also see older versions of any rows. If other sessions simultaneously update the same table, the anomaly means that you might see the table in a state that never existed in the database.

If the transaction isolation level is REPEATABLE READ (the default level), all consistent reads within the same transaction read the snapshot established by the first such read in that transaction. You can get a fresher snapshot for your queries by committing the current transaction and after that issuing new queries.

With READ COMMITTED isolation level, each consistent read within a transaction sets and reads its own fresh snapshot.

Consistent read is the default mode in which InnoDB processes SELECT statements in READ COMMITTED and REPEATABLE READ isolation levels. A consistent read does not set any locks on the tables it accesses, and therefore other sessions are free to modify those tables at the same time a consistent read is being performed on the table.

Suppose that you are running in the default REPEATABLE READ isolation level. When you issue a consistent read (that is, an ordinary SELECT statement), InnoDB gives your transaction a timepoint according to which your query sees the database. If another transaction deletes a row and commits after your timepoint was assigned, you do not see the row as having been deleted. Inserts and updates are treated similarly.

You can advance your timepoint by committing your transaction and then doing another SELECT or START TRANSACTION WITH CONSISTENT SNAPSHOT.

This is called multi-versioned concurrency control.

In the following example, session A sees the row inserted by B only when B has committed the insert and A has committed as well, so that the timepoint is advanced past the commit of B.

             Session A              Session B

           SET autocommit=0;      SET autocommit=0;
time
|          SELECT * FROM t;
|          empty set
|                                 INSERT INTO t VALUES (1, 2);
|
v          SELECT * FROM t;
           empty set
                                  COMMIT;

           SELECT * FROM t;
           empty set

           COMMIT;

           SELECT * FROM t;
           ---------------------
           |    1    |    2    |
           ---------------------
           1 row in set

If you want to see the “freshest” state of the database, use either the READ COMMITTED isolation level or a locking read:

SELECT * FROM t LOCK IN SHARE MODE;

With READ COMMITTED isolation level, each consistent read within a transaction sets and reads its own fresh snapshot. With LOCK IN SHARE MODE, a locking read occurs instead: A SELECT blocks until the transaction containing the freshest rows ends (see Section 13.3.9.3, “SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE Locking Reads”).

Consistent read does not work over certain DDL statements:

  • Consistent read does not work over DROP TABLE, because MySQL cannot use a table that has been dropped and InnoDB destroys the table.

  • Consistent read does not work over ALTER TABLE, because that statement makes a temporary copy of the original table and deletes the original table when the temporary copy is built. When you reissue a consistent read within a transaction, rows in the new table are not visible because those rows did not exist when the transaction's snapshot was taken.

The type of read varies for selects in clauses like INSERT INTO ... SELECT, UPDATE ... (SELECT), and CREATE TABLE ... SELECT that do not specify FOR UPDATE or LOCK IN SHARE MODE:

13.3.9.3. SELECT ... FOR UPDATE and SELECT ... LOCK IN SHARE MODE Locking Reads

If you query data and then insert or update related data within the same transaction, the regular SELECT statement does not give enough protection. Other transactions can update or delete the same rows you just queried. InnoDB supports two types of locking reads that offer extra safety:

  • SELECT ... LOCK IN SHARE MODE sets a shared mode lock on any rows that are read. Other sessions can read the rows, but cannot modify them until your transaction commits. If any of these rows were changed by another transaction that has not yet committed, your query waits until that transaction ends and then uses the latest values.

  • SELECT ... FOR UPDATE locks the rows and any associated index entries, the same as if you issued an UPDATE statement for those rows. Other transactions are blocked from updating those rows, from doing SELECT ... LOCK IN SHARE MODE, or from reading the data in certain transaction isolation levels. Consistent reads ignore any locks set on the records that exist in the read view. (Old versions of a record cannot be locked; they are reconstructed by applying undo logs on an in-memory copy of the record.)

These clauses are primarily useful when dealing with tree-structured or graph-structured data, either in a single table or split across multiple tables.

All locks set by LOCK IN SHARE MODE and FOR UPDATE queries are released when the transaction is committed or rolled back.

Замечание

Locking of rows for update using SELECT FOR UPDATE only applies when autocommit is disabled (either by beginning transaction with START TRANSACTION or by setting autocommit to 0. If autocommit is enabled, the rows matching the specification are not locked.

Usage Examples

Suppose that you want to insert a new row into a table child, and make sure that the child row has a parent row in table parent. Your application code can ensure referential integrity throughout this sequence of operations.

First, use a consistent read to query the table PARENT and verify that the parent row exists. Can you safely insert the child row to table CHILD? No, because some other session could delete the parent row in the moment between your SELECT and your INSERT, without you being aware of it.

To avoid this potential issue, perform the SELECT using LOCK IN SHARE MODE:

SELECT * FROM parent WHERE NAME = 'Jones' LOCK IN SHARE MODE;

After the LOCK IN SHARE MODE query returns the parent 'Jones', you can safely add the child record to the CHILD table and commit the transaction. Any transaction that tries to read or write to the applicable row in the PARENT table waits until you are finished, that is, the data in all tables is in a consistent state.

For another example, consider an integer counter field in a table CHILD_CODES, used to assign a unique identifier to each child added to table CHILD. Do not use either consistent read or a shared mode read to read the present value of the counter, because two users of the database could see the same value for the counter, and a duplicate-key error occurs if two transactions attempt to add rows with the same identifier to the CHILD table.

Here, LOCK IN SHARE MODE is not a good solution because if two users read the counter at the same time, at least one of them ends up in deadlock when it attempts to update the counter.

Here are two ways to implement reading and incrementing the counter without interference from another transaction:

  • First update the counter by incrementing it by 1, then read it and use the new value in the CHILD table. Any other transaction that tries to read the counter waits until your transaction commits. If another transaction is in the middle of this same sequence, your transaction waits until the other one commits.

  • First perform a locking read of the counter using FOR UPDATE, and then increment the counter:

    SELECT counter_field FROM child_codes FOR UPDATE;
    UPDATE child_codes SET counter_field = counter_field + 1;

A SELECT ... FOR UPDATE reads the latest available data, setting exclusive locks on each row it reads. Thus, it sets the same locks a searched SQL UPDATE would set on the rows.

The preceding description is merely an example of how SELECT ... FOR UPDATE works. In MySQL, the specific task of generating a unique identifier actually can be accomplished using only a single access to the table:

UPDATE child_codes SET counter_field = LAST_INSERT_ID(counter_field + 1);
SELECT LAST_INSERT_ID();

The SELECT statement merely retrieves the identifier information (specific to the current connection). It does not access any table.

13.3.9.4. InnoDB Record, Gap, and Next-Key Locks

InnoDB has several types of record-level locks:

  • Record lock: This is a lock on an index record.

  • Gap lock: This is a lock on a gap between index records, or a lock on the gap before the first or after the last index record.

  • Next-key lock: This is a combination of a record lock on the index record and a gap lock on the gap before the index record.

Record locks always lock index records, even if a table is defined with no indexes. For such cases, InnoDB creates a hidden clustered index and uses this index for record locking. See Section 13.3.11.1, “Clustered and Secondary Indexes”.

By default, InnoDB operates in REPEATABLE READ transaction isolation level and with the innodb_locks_unsafe_for_binlog system variable disabled. In this case, InnoDB uses next-key locks for searches and index scans, which prevents phantom rows (see Section 13.3.9.5, “Avoiding the Phantom Problem Using Next-Key Locking”).

Next-key locking combines index-row locking with gap locking. InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters. Thus, the row-level locks are actually index-record locks. In addition, a next-key lock on an index record also affects the “gap” before that index record. That is, a next-key lock is an index-record lock plus a gap lock on the gap preceding the index record. If one session has a shared or exclusive lock on record R in an index, another session cannot insert a new index record in the gap immediately before R in the index order.

Suppose that an index contains the values 10, 11, 13, and 20. The possible next-key locks for this index cover the following intervals, where ( or ) denote exclusion of the interval endpoint and [ or ] denote inclusion of the endpoint:

(negative infinity, 10]
(10, 11]
(11, 13]
(13, 20]
(20, positive infinity)

For the last interval, the next-key lock locks the gap above the largest value in the index and the “supremum” pseudo-record having a value higher than any value actually in the index. The supremum is not a real index record, so, in effect, this next-key lock locks only the gap following the largest index value.

The preceding example shows that a gap might span a single index value, multiple index values, or even be empty.

Gap locking is not needed for statements that lock rows using a unique index to search for a unique row. (This does not include the case that the search condition includes only some columns of a multiple-column unique index; in that case, gap locking does occur.) For example, if the id column has a unique index, the following statement uses only an index-record lock for the row having id value 100 and it does not matter whether other sessions insert rows in the preceding gap:

SELECT * FROM child WHERE id = 100;

If id is not indexed or has a nonunique index, the statement does lock the preceding gap.

A type of gap lock called an insertion intention gap lock is set by INSERT operations prior to row insertion. This lock signals the intent to insert in such a way that multiple transactions inserting into the same index gap need not wait for each other if they are not inserting at the same position within the gap. Suppose that there are index records with values of 4 and 7. Separate transactions that attempt to insert values of 5 and 6 each lock the gap between 4 and 7 with insert intention locks prior to obtaining the exclusive lock on the inserted row, but do not block each other because the rows are nonconflicting.

Gap locking can be disabled explicitly. This occurs if you change the transaction isolation level to READ COMMITTED or enable the innodb_locks_unsafe_for_binlog system variable. Under these circumstances, gap locking is disabled for searches and index scans and is used only for foreign-key constraint checking and duplicate-key checking.

There are also other effects of using the READ COMMITTED isolation level or enabling innodb_locks_unsafe_for_binlog: Record locks for nonmatching rows are released after MySQL has evaluated the WHERE condition. For UPDATE statements, InnoDB does a “semi-consistent” read, such that it returns the latest committed version to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE.

13.3.9.5. Avoiding the Phantom Problem Using Next-Key Locking

The so-called phantom problem occurs within a transaction when the same query produces different sets of rows at different times. For example, if a SELECT is executed twice, but returns a row the second time that was not returned the first time, the row is a “phantom” row.

Suppose that there is an index on the id column of the child table and that you want to read and lock all rows from the table having an identifier value larger than 100, with the intention of updating some column in the selected rows later:

SELECT * FROM child WHERE id > 100 FOR UPDATE;

The query scans the index starting from the first record where id is bigger than 100. Let the table contain rows having id values of 90 and 102. If the locks set on the index records in the scanned range do not lock out inserts made in the gaps (in this case, the gap between 90 and 102), another session can insert a new row into the table with an id of 101. If you were to execute the same SELECT within the same transaction, you would see a new row with an id of 101 (a “phantom”) in the result set returned by the query. If we regard a set of rows as a data item, the new phantom child would violate the isolation principle of transactions that a transaction should be able to run so that the data it has read does not change during the transaction.

To prevent phantoms, InnoDB uses an algorithm called next-key locking that combines index-row locking with gap locking. InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters. Thus, the row-level locks are actually index-record locks. In addition, a next-key lock on an index record also affects the “gap” before that index record. That is, a next-key lock is an index-record lock plus a gap lock on the gap preceding the index record. If one session has a shared or exclusive lock on record R in an index, another session cannot insert a new index record in the gap immediately before R in the index order.

When InnoDB scans an index, it can also lock the gap after the last record in the index. Just that happens in the preceding example: To prevent any insert into the table where id would be bigger than 100, the locks set by InnoDB include a lock on the gap following id value 102.

You can use next-key locking to implement a uniqueness check in your application: If you read your data in share mode and do not see a duplicate for a row you are going to insert, then you can safely insert your row and know that the next-key lock set on the successor of your row during the read prevents anyone meanwhile inserting a duplicate for your row. Thus, the next-key locking enables you to “lock” the nonexistence of something in your table.

Gap locking can be disabled as discussed in Section 13.3.9.4, “InnoDB Record, Gap, and Next-Key Locks”. This may cause phantom problems because other sessions can insert new rows into the gaps when gap locking is disabled.

13.3.9.6. Locks Set by Different SQL Statements in InnoDB

A locking read, an UPDATE, or a DELETE generally set record locks on every index record that is scanned in the processing of the SQL statement. It does not matter whether there are WHERE conditions in the statement that would exclude the row. InnoDB does not remember the exact WHERE condition, but only knows which index ranges were scanned. The locks are normally next-key locks that also block inserts into the “gap” immediately before the record. However, gap locking can be disabled explicitly, which causes next-key locking not to be used. For more information, see Section 13.3.9.4, “InnoDB Record, Gap, and Next-Key Locks”. The transaction isolation level also can affect which locks are set; see Section 12.3.6, “SET TRANSACTION Синтаксис”.

If a secondary index is used in a search and index record locks to be set are exclusive, InnoDB also retrieves the corresponding clustered index records and sets locks on them.

Differences between shared and exclusive locks are described in Section 13.3.9.1, “InnoDB Lock Modes”.

If you have no indexes suitable for your statement and MySQL must scan the entire table to process the statement, every row of the table becomes locked, which in turn blocks all inserts by other users to the table. It is important to create good indexes so that your queries do not unnecessarily scan many rows.

For SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE, locks are acquired for scanned rows, and expected to be released for rows that do not qualify for inclusion in the result set (for example, if they do not meet the criteria given in the WHERE clause). However, in some cases, rows might not be unlocked immediately because the relationship between a result row and its original source is lost during query execution. For example, in a UNION, scanned (and locked) rows from a table might be inserted into a temporary table before evaluation whether they qualify for the result set. In this circumstance, the relationship of the rows in the temporary table to the rows in the original table is lost and the latter rows are not unlocked until the end of query execution.

InnoDB sets specific types of locks as follows.

  • SELECT ... FROM is a consistent read, reading a snapshot of the database and setting no locks unless the transaction isolation level is set to SERIALIZABLE. For SERIALIZABLE level, the search sets shared next-key locks on the index records it encounters.

  • SELECT ... FROM ... LOCK IN SHARE MODE sets shared next-key locks on all index records the search encounters.

  • For index records the search encounters, SELECT ... FROM ... FOR UPDATE blocks other sessions from doing SELECT ... FROM ... LOCK IN SHARE MODE or from reading in certain transaction isolation levels. Consistent reads will ignore any locks set on the records that exist in the read view.

  • UPDATE ... WHERE ... sets an exclusive next-key lock on every record the search encounters.

  • DELETE FROM ... WHERE ... sets an exclusive next-key lock on every record the search encounters.

  • INSERT sets an exclusive lock on the inserted row. This lock is an index-record lock, not a next-key lock (that is, there is no gap lock) and does not prevent other sessions from inserting into the gap before the inserted row.

    Prior to inserting the row, a type of gap lock called an insertion intention gap lock is set. This lock signals the intent to insert in such a way that multiple transactions inserting into the same index gap need not wait for each other if they are not inserting at the same position within the gap. Suppose that there are index records with values of 4 and 7. Separate transactions that attempt to insert values of 5 and 6 each lock the gap between 4 and 7 with insert intention locks prior to obtaining the exclusive lock on the inserted row, but do not block each other because the rows are nonconflicting.

    If a duplicate-key error occurs, a shared lock on the duplicate index record is set. This use of a shared lock can result in deadlock should there be multiple sessions trying to insert the same row if another session already has an exclusive lock. This can occur if another session deletes the row. Suppose that an InnoDB table t1 has the following structure:

    CREATE TABLE t1 (i INT, PRIMARY KEY (i)) ENGINE = InnoDB;

    Now suppose that three sessions perform the following operations in order:

    Session 1:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);

    Session 2:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);

    Session 3:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);

    Session 1:

    ROLLBACK;

    The first operation by session 1 acquires an exclusive lock for the row. The operations by sessions 2 and 3 both result in a duplicate-key error and they both request a shared lock for the row. When session 1 rolls back, it releases its exclusive lock on the row and the queued shared lock requests for sessions 2 and 3 are granted. At this point, sessions 2 and 3 deadlock: Neither can acquire an exclusive lock for the row because of the shared lock held by the other.

    A similar situation occurs if the table already contains a row with key value 1 and three sessions perform the following operations in order:

    Session 1:

    START TRANSACTION;
    DELETE FROM t1 WHERE i = 1;

    Session 2:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);

    Session 3:

    START TRANSACTION;
    INSERT INTO t1 VALUES(1);

    Session 1:

    COMMIT;

    The first operation by session 1 acquires an exclusive lock for the row. The operations by sessions 2 and 3 both result in a duplicate-key error and they both request a shared lock for the row. When session 1 commits, it releases its exclusive lock on the row and the queued shared lock requests for sessions 2 and 3 are granted. At this point, sessions 2 and 3 deadlock: Neither can acquire an exclusive lock for the row because of the shared lock held by the other.

  • INSERT ... ON DUPLICATE KEY UPDATE differs from a simple INSERT in that an exclusive next-key lock rather than a shared lock is placed on the row to be updated when a duplicate-key error occurs.

  • REPLACE is done like an INSERT if there is no collision on a unique key. Otherwise, an exclusive next-key lock is placed on the row to be replaced.

  • INSERT INTO T SELECT ... FROM S WHERE ... sets an exclusive index record without a gap lock on each row inserted into T. If the transaction isolation level is READ COMMITTED or innodb_locks_unsafe_for_binlog is enabled, and the transaction isolation level is not SERIALIZABLE, InnoDB does the search on S as a consistent read (no locks). Otherwise, InnoDB sets shared next-key locks on rows from S. InnoDB has to set locks in the latter case: In roll-forward recovery from a backup, every SQL statement must be executed in exactly the same way it was done originally.

    CREATE TABLE ... SELECT ... performs the SELECT with shared next-key locks or as a consistent read, as for INSERT ... SELECT.

    When a SELECT is used in the constructs REPLACE INTO t SELECT ... FROM s WHERE ... or UPDATE t ... WHERE col IN (SELECT ... FROM s ...), InnoDB sets shared next-key locks on rows from table s.

  • While initializing a previously specified AUTO_INCREMENT column on a table, InnoDB sets an exclusive lock on the end of the index associated with the AUTO_INCREMENT column. In accessing the auto-increment counter, InnoDB uses a specific AUTO-INC table lock mode where the lock lasts only to the end of the current SQL statement, not to the end of the entire transaction. Other sessions cannot insert into the table while the AUTO-INC table lock is held; see Section 13.3.9, “The InnoDB Transaction Model and Locking”.

    InnoDB fetches the value of a previously initialized AUTO_INCREMENT column without setting any locks.

  • If a FOREIGN KEY constraint is defined on a table, any insert, update, or delete that requires the constraint condition to be checked sets shared record-level locks on the records that it looks at to check the constraint. InnoDB also sets these locks in the case where the constraint fails.

  • LOCK TABLES sets table locks, but it is the higher MySQL layer above the InnoDB layer that sets these locks. InnoDB is aware of table locks if innodb_table_locks = 1 (the default) and autocommit = 0, and the MySQL layer above InnoDB knows about row-level locks.

    Otherwise, InnoDB's automatic deadlock detection cannot detect deadlocks where such table locks are involved. Also, because in this case the higher MySQL layer does not know about row-level locks, it is possible to get a table lock on a table where another session currently has row-level locks. However, this does not endanger transaction integrity, as discussed in Section 13.3.9.8, “Deadlock Detection and Rollback”. See also Section 13.3.15, “Limits on InnoDB Tables”.

13.3.9.7. Implicit Transaction Commit and Rollback

By default, MySQL starts the session for each new connection with autocommit mode enabled, so MySQL does a commit after each SQL statement if that statement did not return an error. If a statement returns an error, the commit or rollback behavior depends on the error. See Section 13.3.13, “InnoDB Error Handling”.

If a session that has autocommit disabled ends without explicitly committing the final transaction, MySQL rolls back that transaction.

Some statements implicitly end a transaction, as if you had done a COMMIT before executing the statement. For details, see Section 12.3.3, “Statements That Cause an Implicit Commit”.

13.3.9.8. Deadlock Detection and Rollback

InnoDB automatically detects transaction deadlocks and rolls back a transaction or transactions to break the deadlock. InnoDB tries to pick small transactions to roll back, where the size of a transaction is determined by the number of rows inserted, updated, or deleted.

InnoDB is aware of table locks if innodb_table_locks = 1 (the default) and autocommit = 0, and the MySQL layer above it knows about row-level locks. Otherwise, InnoDB cannot detect deadlocks where a table lock set by a MySQL LOCK TABLES statement or a lock set by a storage engine other than InnoDB is involved. Resolve these situations by setting the value of the innodb_lock_wait_timeout system variable.

When InnoDB performs a complete rollback of a transaction, all locks set by the transaction are released. However, if just a single SQL statement is rolled back as a result of an error, some of the locks set by the statement may be preserved. This happens because InnoDB stores row locks in a format such that it cannot know afterward which lock was set by which statement.

If a SELECT calls a stored function in a transaction, and a statement within the function fails, that statement rolls back. Furthermore, if ROLLBACK is executed after that, the entire transaction rolls back.

13.3.9.9. How to Cope with Deadlocks

Deadlocks are a classic problem in transactional databases, but they are not dangerous unless they are so frequent that you cannot run certain transactions at all. Normally, you must write your applications so that they are always prepared to re-issue a transaction if it gets rolled back because of a deadlock.

InnoDB uses automatic row-level locking. You can get deadlocks even in the case of transactions that just insert or delete a single row. That is because these operations are not really “atomic”; they automatically set locks on the (possibly several) index records of the row inserted or deleted.

You can cope with deadlocks and reduce the likelihood of their occurrence with the following techniques:

  • Use SHOW ENGINE INNODB STATUS to determine the cause of the latest deadlock. That can help you to tune your application to avoid deadlocks.

  • Always be prepared to re-issue a transaction if it fails due to deadlock. Deadlocks are not dangerous. Just try again.

  • Commit your transactions often. Small transactions are less prone to collision.

  • If you are using locking reads (SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE), try using a lower isolation level such as READ COMMITTED.

  • Access your tables and rows in a fixed order. Then transactions form well-defined queues and do not deadlock.

  • Add well-chosen indexes to your tables. Then your queries need to scan fewer index records and consequently set fewer locks. Use EXPLAIN SELECT to determine which indexes the MySQL server regards as the most appropriate for your queries.

  • Use less locking. If you can afford to permit a SELECT to return data from an old snapshot, do not add the clause FOR UPDATE or LOCK IN SHARE MODE to it. Using the READ COMMITTED isolation level is good here, because each consistent read within the same transaction reads from its own fresh snapshot.

  • If nothing else helps, serialize your transactions with table-level locks. The correct way to use LOCK TABLES with transactional tables, such as InnoDB tables, is to begin a transaction with SET autocommit = 0 (not START TRANSACTION) followed by LOCK TABLES, and to not call UNLOCK TABLES until you commit the transaction explicitly. For example, if you need to write to table t1 and read from table t2, you can do this:

    SET autocommit=0;
    LOCK TABLES t1 WRITE, t2 READ, ...;... do something with tables t1 and t2 here ...
    COMMIT;
    UNLOCK TABLES;
    

    Table-level locks make your transactions queue nicely and avoid deadlocks.

  • Another way to serialize transactions is to create an auxiliary “semaphore” table that contains just a single row. Have each transaction update that row before accessing other tables. In that way, all transactions happen in a serial fashion. Note that the InnoDB instant deadlock detection algorithm also works in this case, because the serializing lock is a row-level lock. With MySQL table-level locks, the timeout method must be used to resolve deadlocks.

13.3.10. InnoDB Multi-Versioning

InnoDB is a multi-versioned storage engine: it keeps information about old versions of changed rows, to support transactional features such as concurrency and rollback. This information is stored in the tablespace in a data structure called a rollback segment (after an analogous data structure in Oracle). InnoDB uses the information in the rollback segment to perform the undo operations needed in a transaction rollback. It also uses the information to build earlier versions of a row for a consistent read.

Internal Details of Multi-Versioning

Internally, InnoDB adds three fields to each row stored in the database. A 6-byte DB_TRX_ID field indicates the transaction identifier for the last transaction that inserted or updated the row. Also, a deletion is treated internally as an update where a special bit in the row is set to mark it as deleted. Each row also contains a 7-byte DB_ROLL_PTR field called the roll pointer. The roll pointer points to an undo log record written to the rollback segment. If the row was updated, the undo log record contains the information necessary to rebuild the content of the row before it was updated. A 6-byte DB_ROW_ID field contains a row ID that increases monotonically as new rows are inserted. If InnoDB generates a clustered index automatically, the index contains row ID values. Otherwise, the DB_ROW_ID column does not appear in any index.

Undo logs in the rollback segment are divided into insert and update undo logs. Insert undo logs are needed only in transaction rollback and can be discarded as soon as the transaction commits. Update undo logs are used also in consistent reads, but they can be discarded only after there is no transaction present for which InnoDB has assigned a snapshot that in a consistent read could need the information in the update undo log to build an earlier version of a database row.

Guidelines for Managing Rollback Segments

Commit your transactions regularly, including those transactions that issue only consistent reads. Otherwise, InnoDB cannot discard data from the update undo logs, and the rollback segment may grow too big, filling up your tablespace.

The physical size of an undo log record in the rollback segment is typically smaller than the corresponding inserted or updated row. You can use this information to calculate the space needed for your rollback segment.

In the InnoDB multi-versioning scheme, a row is not physically removed from the database immediately when you delete it with an SQL statement. InnoDB only physically removes the corresponding row and its index records when it discards the update undo log record written for the deletion. This removal operation is called a purge, and it is quite fast, usually taking the same order of time as the SQL statement that did the deletion.

If you insert and delete rows in smallish batches at about the same rate in the table, the purge thread can start to lag behind and the table can grow bigger and bigger because of all the “dead” rows, making everything disk-bound and very slow In such a case, throttle new row operations, and allocate more resources to the purge thread by tuning the innodb_max_purge_lag system variable. See Section 13.3.4, “InnoDB Startup Options and System Variables” for more information.

13.3.11. InnoDB Table and Index Structures

Role of the .frm File

MySQL stores its data dictionary information for tables in .frm files in database directories. This is true for all MySQL storage engines, but every InnoDB table also has its own entry in the InnoDB internal data dictionary inside the tablespace. When MySQL drops a table or a database, it has to delete one or more .frm files as well as the corresponding entries inside the InnoDB data dictionary. Consequently, you cannot move InnoDB tables between databases simply by moving the .frm files.

13.3.11.1. Clustered and Secondary Indexes

Every InnoDB table has a special index called the clustered index where the data for the rows is stored. Typically, the clustered index is synonymous with the primary key. To get the best performance from queries, inserts, and other database operations, you must understand how InnoDB uses the clustered index to optimize the most common lookup and DML operations for each table.

  • If you define a PRIMARY KEY on your table, InnoDB uses it as the clustered index. Define a primary key for each table that you create. If there is no logical unique and non-null column or set of columns, add a new auto-increment column, whose values are filled in automatically.

  • If you do not define a PRIMARY KEY for your table, MySQL locates the first UNIQUE index where all the key columns are NOT NULL and InnoDB uses it as the clustered index.

  • If the table has no PRIMARY KEY or suitable UNIQUE index, InnoDB internally generates a hidden clustered index on a synthetic column containing row ID values. The rows are ordered by the ID that InnoDB assigns to the rows in such a table. The row ID is a 6-byte field that increases monotonically as new rows are inserted. Thus, the rows ordered by the row ID are physically in insertion order.

How the Clustered Index Speeds Up Queries

Accessing a row through the clustered index is fast because the row data is on the same page where the index search leads. If a table is large, the clustered index architecture often saves a disk I/O operation when compared to storage organizations that store row data using a different page from the index record. (For example, MyISAM uses one file for data rows and another for index records.)

How Secondary Indexes Relate to the Clustered Index

All indexes other than the clustered index are known as secondary indexes. In InnoDB, each record in a secondary index contains the primary key columns for the row, as well as the columns specified for the secondary index. InnoDB uses this primary key value to search for the row in the clustered index.

If the primary key is long, the secondary indexes use more space, so it is advantageous to have a short primary key.

13.3.11.2. Physical Structure of an InnoDB Index

All InnoDB indexes are B-trees where the index records are stored in the leaf pages of the tree. The default size of an index page is 16KB. When new records are inserted, InnoDB tries to leave 1/16 of the page free for future insertions and updates of the index records.

If index records are inserted in a sequential order (ascending or descending), the resulting index pages are about 15/16 full. If records are inserted in a random order, the pages are from 1/2 to 15/16 full. If the fill factor of an index page drops below 1/2, InnoDB tries to contract the index tree to free the page.

Замечание

Changing the page size is not a supported operation and there is no guarantee that InnoDB will function normally with a page size other than 16KB. Problems compiling or running InnoDB may occur. In particular, ROW_FORMAT=COMPRESSED in the Barracuda file format assumes that the page size is at most 16KB and uses 14-bit pointers.

A version of InnoDB built for one page size cannot use data files or log files from a version built for a different page size.

13.3.11.3. Insert Buffering

Database applications often insert new rows in the ascending order of the primary key. In this case, due to the layout of the clustered index in the same order as the primary key, insertions into an InnoDB table do not require random reads from a disk.

On the other hand, secondary indexes are usually nonunique, and insertions into secondary indexes happen in a relatively random order. In the same way, deletes and updates can affect data pages that are not adjacent in secondary indexes. This would cause a lot of random disk I/O operations without a special mechanism used in InnoDB.

When an index record is inserted, marked for deletion, or deleted from a nonunique secondary index, InnoDB checks whether the secondary index page is in the buffer pool. If that is the case, InnoDB applies the change directly to the index page. If the index page is not found in the buffer pool, InnoDB records the change in a special structure known as the insert buffer. The insert buffer is kept small so that it fits entirely in the buffer pool, and changes can be applied very quickly. This process is known as change buffering. (Formerly, it applied only to inserts and was called insert buffering. The data structure is still called the insert buffer.)

Disk I/O for Flushing the Insert Buffer

Periodically, the insert buffer is merged into the secondary index trees in the database. Often, it is possible to merge several changes into the same page of the index tree, saving disk I/O operations. It has been measured that the insert buffer can speed up insertions into a table up to 15 times.

The insert buffer merging may continue to happen after the transaction has been committed. In fact, it may continue to happen after a server shutdown and restart (see Section 13.3.7.2, “Forcing InnoDB Recovery”).

Insert buffer merging may take many hours when many secondary indexes must be updated and many rows have been inserted. During this time, disk I/O will be increased, which can cause significant slowdown on disk-bound queries. Another significant background I/O operation is the purge thread (see Section 13.3.10, “InnoDB Multi-Versioning”).

13.3.11.4. Adaptive Hash Indexes

The feature known as the adaptive hash index lets InnoDB perform more like an in-memory database on systems with appropriate combinations of workload and ample memory for the buffer pool, without sacrificing any transactional features or reliability.

If a table fits almost entirely in main memory, a hash index can speed up queries by enabling direct lookup of any element, turning the index value into a sort of pointer. InnoDB has a mechanism that monitors index searches. If InnoDB notices that queries could benefit from building a hash index, it does so automatically.

The hash index is always built based on an existing B-tree index on the table. InnoDB can build a hash index on a prefix of any length of the key defined for the B-tree, depending on the pattern of searches that InnoDB observes for the B-tree index. A hash index can be partial, covering only those pages of the index that are often accessed.

13.3.11.5. Physical Row Structure

The physical row structure for an InnoDB table depends on the row format specified when the table was created. InnoDB uses the COMPACT format by default, but the REDUNDANT format is available to retain compatibility with older versions of MySQL. To check the row format of an InnoDB table, use SHOW TABLE STATUS.

The compact row format decreases row storage space by about 20% at the cost of increasing CPU use for some operations. If your workload is a typical one that is limited by cache hit rates and disk speed, compact format is likely to be faster. If the workload is a rare case that is limited by CPU speed, compact format might be slower.

Rows in InnoDB tables that use REDUNDANT row format have the following characteristics:

  • Each index record contains a six-byte header. The header is used to link together consecutive records, and also in row-level locking.

  • Records in the clustered index contain fields for all user-defined columns. In addition, there is a six-byte transaction ID field and a seven-byte roll pointer field.

  • If no primary key was defined for a table, each clustered index record also contains a six-byte row ID field.

  • Each secondary index record also contains all the primary key fields defined for the clustered index key that are not in the secondary index.

  • A record contains a pointer to each field of the record. If the total length of the fields in a record is less than 128 bytes, the pointer is one byte; otherwise, two bytes. The array of these pointers is called the record directory. The area where these pointers point is called the data part of the record.

  • Internally, InnoDB stores fixed-length character columns such as CHAR(10) in a fixed-length format. InnoDB does not truncate trailing spaces from VARCHAR columns.

  • An SQL NULL value reserves one or two bytes in the record directory. Besides that, an SQL NULL value reserves zero bytes in the data part of the record if stored in a variable length column. In a fixed-length column, it reserves the fixed length of the column in the data part of the record. Reserving the fixed space for NULL values enables an update of the column from NULL to a non-NULL value to be done in place without causing fragmentation of the index page.

Rows in InnoDB tables that use COMPACT row format have the following characteristics:

  • Each index record contains a five-byte header that may be preceded by a variable-length header. The header is used to link together consecutive records, and also in row-level locking.

  • The variable-length part of the record header contains a bit vector for indicating NULL columns. If the number of columns in the index that can be NULL is N, the bit vector occupies CEILING(N/8) bytes. (For example, if there are anywhere from 9 to 15 columns that can be NULL, the bit vector uses two bytes.) Columns that are NULL do not occupy space other than the bit in this vector. The variable-length part of the header also contains the lengths of variable-length columns. Each length takes one or two bytes, depending on the maximum length of the column. If all columns in the index are NOT NULL and have a fixed length, the record header has no variable-length part.

  • For each non-NULL variable-length field, the record header contains the length of the column in one or two bytes. Two bytes will only be needed if part of the column is stored externally in overflow pages or the maximum length exceeds 255 bytes and the actual length exceeds 127 bytes. For an externally stored column, the two-byte length indicates the length of the internally stored part plus the 20-byte pointer to the externally stored part. The internal part is 768 bytes, so the length is 768+20. The 20-byte pointer stores the true length of the column.

  • The record header is followed by the data contents of the non-NULL columns.

  • Records in the clustered index contain fields for all user-defined columns. In addition, there is a six-byte transaction ID field and a seven-byte roll pointer field.

  • If no primary key was defined for a table, each clustered index record also contains a six-byte row ID field.

  • Each secondary index record also contains all the primary key fields defined for the clustered index key that are not in the secondary index. If any of these primary key fields are variable length, the record header for each secondary index will have a variable-length part to record their lengths, even if the secondary index is defined on fixed-length columns.

  • Internally, InnoDB stores fixed-length, fixed-width character columns such as CHAR(10) in a fixed-length format. InnoDB does not truncate trailing spaces from VARCHAR columns.

  • Internally, InnoDB attempts to store UTF-8 CHAR(N) columns in N bytes by trimming trailing spaces. (With REDUNDANT row format, such columns occupy 3 × N bytes.) Reserving the minimum space N in many cases enables column updates to be done in place without causing fragmentation of the index page.

13.3.12. InnoDB Disk I/O and File Space Management

The laws of physics dictate that it takes time and effort to read and write data. The ACID design model requires a certain amount of I/O that might seem redundant, but helps to ensure data reliability. Within these constraints, InnoDB tries to optimize the database work and the organization of disk files to minimize the amount of disk I/O. Sometimes, I/O is postponed until the database is not busy, or until everything needs to be brought to a consistent state, such as during a database restart after a crash.

13.3.12.1. InnoDB Disk I/O

InnoDB uses asynchronous disk I/O where possible, by creating a number of threads to handle I/O operations, while permitting other database operations to proceed while the I/O is still in progress. On Linux and Windows platforms, InnoDB uses the available OS and library functions to perform “native” asynchronous I/O. On other platforms, InnoDB still uses I/O threads, but the threads may actually wait for I/O requests to complete; this technique is known as “simulated” asynchronous I/O.

Read-Ahead

If InnoDB can determine there is a high probability that data might be needed soon, it performs read-ahead operations to bring that data into the buffer pool so that it is available in memory. Making a few large read requests for contiguous data can be more efficient than making several small, spread-out requests. There are two read-ahead heuristics in InnoDB:

  • In sequential read-ahead, if InnoDB notices that the access pattern to a segment in the tablespace is sequential, it posts in advance a batch of reads of database pages to the I/O system.

  • In random read-ahead, if InnoDB notices that some area in a tablespace seems to be in the process of being fully read into the buffer pool, it posts the remaining reads to the I/O system.

Doublewrite Buffer

InnoDB uses a novel file flush technique involving a structure called the doublewrite buffer. It adds safety to recovery following an operating system crash or a power outage, and improves performance on most varieties of Unix by reducing the need for fsync() operations.

Before writing pages to a data file, InnoDB first writes them to a contiguous tablespace area called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer has completed does InnoDB write the pages to their proper positions in the data file. If the operating system crashes in the middle of a page write, InnoDB can later find a good copy of the page from the doublewrite buffer during recovery.

13.3.12.2. File Space Management

The data files that you define in the configuration file form the InnoDB system tablespace. The files are logically concatenated to form the tablespace. There is no striping in use. Currently, you cannot define where within the tablespace your tables are allocated. However, in a newly created tablespace, InnoDB allocates space starting from the first data file.

To avoid the issues that come with storing all tables and indexes inside the system tablespace, you can turn on the innodb_file_per_table configuration option, which stores each newly created table in a separate tablespace file (with extension .ibd). For tables stored this way, there is less fragmentation within the disk file, and when the table is truncated, the space is returned to the operating system rather than still being reserved by InnoDB within the system tablespace.

Pages, Extents, Segments, and Tablespaces

Each tablespace consists of database pages with a default size of 16KB. The pages are grouped into extents of size 1MB (64 consecutive pages). The “files” inside a tablespace are called segments in InnoDB. (These segments are different from the “rollback segment”, which actually contains many tablespace segments.)

When a segment grows inside the tablespace, InnoDB allocates the first 32 pages to it individually. After that, InnoDB starts to allocate whole extents to the segment. InnoDB can add up to 4 extents at a time to a large segment to ensure good sequentiality of data.

Two segments are allocated for each index in InnoDB. One is for nonleaf nodes of the B-tree, the other is for the leaf nodes. Keeping the leaf nodes contiguous on disk enables better sequential I/O operations, because these leaf nodes contain the actual table data.

Some pages in the tablespace contain bitmaps of other pages, and therefore a few extents in an InnoDB tablespace cannot be allocated to segments as a whole, but only as individual pages.

When you ask for available free space in the tablespace by issuing a SHOW TABLE STATUS statement, InnoDB reports the extents that are definitely free in the tablespace. InnoDB always reserves some extents for cleanup and other internal purposes; these reserved extents are not included in the free space.

When you delete data from a table, InnoDB contracts the corresponding B-tree indexes. Whether the freed space becomes available for other users depends on whether the pattern of deletes frees individual pages or extents to the tablespace. Dropping a table or deleting all rows from it is guaranteed to release the space to other users, but remember that deleted rows are physically removed only in an (automatic) purge operation after they are no longer needed for transaction rollbacks or consistent reads. (See Section 13.3.10, “InnoDB Multi-Versioning”.)

To see information about the tablespace, use the Tablespace Monitor. See Section 13.3.14.2, “SHOW ENGINE INNODB STATUS and the InnoDB Monitors”.

How Pages Relate to Table Rows

The maximum row length, except for variable-length columns (VARBINARY, VARCHAR, BLOB and TEXT), is slightly less than half of a database page. That is, the maximum row length is about 8000 bytes. LONGBLOB and LONGTEXT columns must be less than 4GB, and the total row length, including BLOB and TEXT columns, must be less than 4GB.

If a row is less than half a page long, all of it is stored locally within the page. If it exceeds half a page, variable-length columns are chosen for external off-page storage until the row fits within half a page. For a column chosen for off-page storage, InnoDB stores the first 768 bytes locally in the row, and the rest externally into overflow pages. Each such column has its own list of overflow pages. The 768-byte prefix is accompanied by a 20-byte value that stores the true length of the column and points into the overflow list where the rest of the value is stored.

13.3.12.3. Defragmenting a Table

Random insertions into or deletions from a secondary index may cause the index to become fragmented. Fragmentation means that the physical ordering of the index pages on the disk is not close to the index ordering of the records on the pages, or that there are many unused pages in the 64-page blocks that were allocated to the index.

One symptom of fragmentation is that a table takes more space than it “should” take. How much that is exactly, is difficult to determine. All InnoDB data and indexes are stored in B-trees, and their fill factor may vary from 50% to 100%. Another symptom of fragmentation is that a table scan such as this takes more time than it “should” take:

SELECT COUNT(*) FROM t WHERE a_non_indexed_column <> 12345;

The preceding query requires MySQL to scan the clustered index rather than a secondary index. Most disks can read 10MB/s to 50MB/s, which can be used to estimate how fast a table scan should be.

To speed up index scans, you can periodically perform a “nullALTER TABLE operation, which causes MySQL to rebuild the table:

ALTER TABLE tbl_name ENGINE=INNODB

Another way to perform a defragmentation operation is to use mysqldump to dump the table to a text file, drop the table, and reload it from the dump file.

If the insertions into an index are always ascending and records are deleted only from the end, the InnoDB filespace management algorithm guarantees that fragmentation in the index does not occur.

13.3.13. InnoDB Error Handling

Error handling in InnoDB is not always the same as specified in the SQL standard. According to the standard, any error during an SQL statement should cause rollback of that statement. InnoDB sometimes rolls back only part of the statement, or the whole transaction. The following items describe how InnoDB performs error handling:

  • If you run out of file space in the tablespace, a MySQL Table is full error occurs and InnoDB rolls back the SQL statement.

  • A transaction deadlock causes InnoDB to roll back the entire transaction. Retry the whole transaction when this happens.

    A lock wait timeout causes InnoDB to roll back only the single statement that was waiting for the lock and encountered the timeout. (To have the entire transaction roll back, start the server with the --innodb_rollback_on_timeout option.) Retry the statement if using the current behavior, or the entire transaction if using --innodb_rollback_on_timeout.

    Both deadlocks and lock wait timeouts are normal on busy servers and it is necessary for applications to be aware that they may happen and handle them by retrying. You can make them less likely by doing as little work as possible between the first change to data during a transaction and the commit, so the locks are held for the shortest possible time and for the smallest possible number of rows. Sometimes splitting work between different transactions may be practical and helpful.

    When a transaction rollback occurs due to a deadlock or lock wait timeout, it cancels the effect of the statements within the transaction. But if the start-transaction statement was START TRANSACTION or BEGIN statement, rollback does not cancel that statement. Further SQL statements become part of the transaction until the occurrence of COMMIT, ROLLBACK, or some SQL statement that causes an implicit commit.

  • A duplicate-key error rolls back the SQL statement, if you have not specified the IGNORE option in your statement.

  • A row too long error rolls back the SQL statement.

  • Other errors are mostly detected by the MySQL layer of code (above the InnoDB storage engine level), and they roll back the corresponding SQL statement. Locks are not released in a rollback of a single SQL statement.

During implicit rollbacks, as well as during the execution of an explicit ROLLBACK SQL statement, SHOW PROCESSLIST displays Rolling back in the State column for the relevant connection.

13.3.13.1. InnoDB Error Codes

The following is a nonexhaustive list of common InnoDB-specific errors that you may encounter, with information about why each occurs and how to resolve the problem.

  • 1005 (ER_CANT_CREATE_TABLE)

    Cannot create table. If the error message refers to error 150, table creation failed because a foreign key constraint was not correctly formed. If the error message refers to error –1, table creation probably failed because the table includes a column name that matched the name of an internal InnoDB table.

  • 1016 (ER_CANT_OPEN_FILE)

    Cannot find the InnoDB table from the InnoDB data files, although the .frm file for the table exists. See Section 13.3.14.4, “Troubleshooting InnoDB Data Dictionary Operations”.

  • 1114 (ER_RECORD_FILE_FULL)

    InnoDB has run out of free space in the tablespace. Reconfigure the tablespace to add a new data file.

  • 1205 (ER_LOCK_WAIT_TIMEOUT)

    Lock wait timeout expired. Transaction was rolled back.

  • 1206 (ER_LOCK_TABLE_FULL)

    The total number of locks exceeds the lock table size. To avoid this error, increase the value of innodb_buffer_pool_size. Within an individual application, a workaround may be to break a large operation into smaller pieces. For example, if the error occurs for a large INSERT, perform several smaller INSERT operations.

  • 1213 (ER_LOCK_DEADLOCK)

    Transaction deadlock. Rerun the transaction.

  • 1216 (ER_NO_REFERENCED_ROW)

    You are trying to add a row but there is no parent row, and a foreign key constraint fails. Add the parent row first.

  • 1217 (ER_ROW_IS_REFERENCED)

    You are trying to delete a parent row that has children, and a foreign key constraint fails. Delete the children first.

13.3.13.2. Operating System Error Codes

To print the meaning of an operating system error number, use the perror program that comes with the MySQL distribution.

  • Linux System Error Codes

    The following table provides a list of some common Linux system error codes. For a more complete list, see Linux source code.

    NumberMacroОписание
    1EPERMOperation not permitted
    2ENOENTNo such file or directory
    3ESRCHNo such process
    4EINTRInterrupted system call
    5EIOI/O error
    6ENXIONo such device or address
    7E2BIGArg list too long
    8ENOEXECExec format error
    9EBADFBad file number
    10ECHILDNo child processes
    11EAGAINTry again
    12ENOMEMOut of memory
    13EACCESPermission denied
    14EFAULTBad address
    15ENOTBLKBlock device required
    16EBUSYDevice or resource busy
    17EEXISTFile exists
    18EXDEVCross-device link
    19ENODEVNo such device
    20ENOTDIRNot a directory
    21EISDIRIs a directory
    22EINVALInvalid argument
    23ENFILEFile table overflow
    24EMFILEToo many open files
    25ENOTTYInappropriate ioctl for device
    26ETXTBSYText file busy
    27EFBIGFile too large
    28ENOSPCNo space left on device
    29ESPIPEIllegal seek
    30EROFSRead-only file system
    31EMLINKToo many links
  • Windows System Error Codes

    The following table provides a list of some common Windows system error codes. For a complete list, see the Microsoft Web site.

    NumberMacroОписание
    1ERROR_INVALID_FUNCTIONIncorrect function.
    2ERROR_FILE_NOT_FOUNDThe system cannot find the file specified.
    3ERROR_PATH_NOT_FOUNDThe system cannot find the path specified.
    4ERROR_TOO_MANY_OPEN_FILESThe system cannot open the file.
    5ERROR_ACCESS_DENIEDAccess is denied.
    6ERROR_INVALID_HANDLEThe handle is invalid.
    7ERROR_ARENA_TRASHEDThe storage control blocks were destroyed.
    8ERROR_NOT_ENOUGH_MEMORYNot enough storage is available to process this command.
    9ERROR_INVALID_BLOCKThe storage control block address is invalid.
    10ERROR_BAD_ENVIRONMENTThe environment is incorrect.
    11ERROR_BAD_FORMATAn attempt was made to load a program with an incorrect format.
    12ERROR_INVALID_ACCESSThe access code is invalid.
    13ERROR_INVALID_DATAThe data is invalid.
    14ERROR_OUTOFMEMORYNot enough storage is available to complete this operation.
    15ERROR_INVALID_DRIVEThe system cannot find the drive specified.
    16ERROR_CURRENT_DIRECTORYThe directory cannot be removed.
    17ERROR_NOT_SAME_DEVICEThe system cannot move the file to a different disk drive.
    18ERROR_NO_MORE_FILESThere are no more files.
    19ERROR_WRITE_PROTECTThe media is write protected.
    20ERROR_BAD_UNITThe system cannot find the device specified.
    21ERROR_NOT_READYThe device is not ready.
    22ERROR_BAD_COMMANDThe device does not recognize the command.
    23ERROR_CRCData error (cyclic redundancy check).
    24ERROR_BAD_LENGTHThe program issued a command but the command length is incorrect.
    25ERROR_SEEKThe drive cannot locate a specific area or track on the disk.
    26ERROR_NOT_DOS_DISKThe specified disk or diskette cannot be accessed.
    27ERROR_SECTOR_NOT_FOUNDThe drive cannot find the sector requested.
    28ERROR_OUT_OF_PAPERThe printer is out of paper.
    29ERROR_WRITE_FAULTThe system cannot write to the specified device.
    30ERROR_READ_FAULTThe system cannot read from the specified device.
    31ERROR_GEN_FAILUREA device attached to the system is not functioning.
    32ERROR_SHARING_VIOLATIONThe process cannot access the file because it is being used by another process.
    33ERROR_LOCK_VIOLATIONThe process cannot access the file because another process has locked a portion of the file.
    34ERROR_WRONG_DISKThe wrong diskette is in the drive. Insert %2 (Volume Serial Number: %3) into drive %1.
    36ERROR_SHARING_BUFFER_EXCEEDEDToo many files opened for sharing.
    38ERROR_HANDLE_EOFReached the end of the file.
    39ERROR_HANDLE_DISK_FULLThe disk is full.
    87ERROR_INVALID_PARAMETERThe parameter is incorrect.
    112ERROR_DISK_FULLThe disk is full.
    123ERROR_INVALID_NAMEThe file name, directory name, or volume label syntax is incorrect.
    1450ERROR_NO_SYSTEM_RESOURCESInsufficient system resources exist to complete the requested service.

13.3.14. InnoDB Performance Tuning and Troubleshooting

13.3.14.1. InnoDB Performance Tuning Tips

With InnoDB becoming the default storage engine in MySQL 5.5 and higher, the tips and guidelines for InnoDB tables are now part of the main optimization chapter. See Section 7.5, “Optimizing for InnoDB Tables”.

13.3.14.2. SHOW ENGINE INNODB STATUS and the InnoDB Monitors

InnoDB Monitors provide information about the InnoDB internal state. This information is useful for performance tuning. Each Monitor can be enabled by creating a table with a special name, which causes InnoDB to write Monitor output periodically. Also, output for the standard InnoDB Monitor is available on demand through the SHOW ENGINE INNODB STATUS SQL statement.

There are several types of InnoDB Monitors:

  • The standard InnoDB Monitor displays the following types of information:

    • Table and record locks held by each active transaction

    • Lock waits of a transactions

    • Semaphore waits of threads

    • Pending file I/O requests

    • Buffer pool statistics

    • Purge and insert buffer merge activity of the main InnoDB thread

    For a discussion of InnoDB lock modes, see Section 13.3.9.1, “InnoDB Lock Modes”.

    To enable the standard InnoDB Monitor for periodic output, create a table named innodb_monitor. To obtain Monitor output on demand, use the SHOW ENGINE INNODB STATUS SQL statement to fetch the output to your client program. If you are using the mysql interactive client, the output is more readable if you replace the usual semicolon statement terminator with \G:

    mysql> SHOW ENGINE INNODB STATUS\G
    
  • The InnoDB Lock Monitor is like the standard Monitor but also provides extensive lock information. To enable this Monitor for periodic output, create a table named innodb_lock_monitor.

  • The InnoDB Tablespace Monitor prints a list of file segments in the shared tablespace and validates the tablespace allocation data structures. To enable this Monitor for periodic output, create a table named innodb_tablespace_monitor.

  • The InnoDB Table Monitor prints the contents of the InnoDB internal data dictionary. To enable this Monitor for periodic output, create a table named innodb_table_monitor.

To enable an InnoDB Monitor for periodic output, use a CREATE TABLE statement to create the table associated with the Monitor. For example, to enable the standard InnoDB Monitor, create the innodb_monitor table:

CREATE TABLE innodb_monitor (a INT) ENGINE=INNODB;

To stop the Monitor, drop the table:

DROP TABLE innodb_monitor;

The CREATE TABLE syntax is just a way to pass a command to the InnoDB engine through MySQL's SQL parser: The only things that matter are the table name innodb_monitor and that it be an InnoDB table. The structure of the table is not relevant at all for the InnoDB Monitor. If you shut down the server, the Monitor does not restart automatically when you restart the server. Drop the Monitor table and issue a new CREATE TABLE statement to start the Monitor. (This syntax may change in a future release.)

The PROCESS privilege is required to start or stop the InnoDB Monitor tables.

When you enable InnoDB Monitors for periodic output, InnoDB writes their output to the mysqld server standard error output (stderr). In this case, no output is sent to clients. When switched on, InnoDB Monitors print data about every 15 seconds. Server output usually is directed to the error log (see Section 5.2.2, “The Error Log”). This data is useful in performance tuning. On Windows, start the server from a command prompt in a console window with the --console option if you want to direct the output to the window rather than to the error log.

InnoDB sends diagnostic output to stderr or to files rather than to stdout or fixed-size memory buffers, to avoid potential buffer overflows. As a side effect, the output of SHOW ENGINE INNODB STATUS is written to a status file in the MySQL data directory every fifteen seconds. The name of the file is innodb_status.pid, where pid is the server process ID. InnoDB removes the file for a normal shutdown. If abnormal shutdowns have occurred, instances of these status files may be present and must be removed manually. Before removing them, you might want to examine them to see whether they contain useful information about the cause of abnormal shutdowns. The innodb_status.pid file is created only if the configuration option innodb-status-file=1 is set.

InnoDB Monitors should be enabled only when you actually want to see Monitor information because output generation does result in some performance decrement. Also, if you enable monitor output by creating the associated table, your error log may become quite large if you forget to remove the table later.

For additional information about InnoDB monitors, see:

Each monitor begins with a header containing a timestamp and the monitor name. For example:

================================================
090407 12:06:19 INNODB TABLESPACE MONITOR OUTPUT
================================================

The header for the standard Monitor (INNODB MONITOR OUTPUT) is also used for the Lock Monitor because the latter produces the same output with the addition of extra lock information.

The following sections describe the output for each Monitor.

13.3.14.2.1. InnoDB Standard Monitor and Lock Monitor Output

The Lock Monitor is the same as the standard Monitor except that it includes additional lock information. Enabling either monitor for periodic output by creating the associated InnoDB table turns on the same output stream, but the stream includes the extra information if the Lock Monitor is enabled. For example, if you create the innodb_monitor and innodb_lock_monitor tables, that turns on a single output stream. The stream includes extra lock information until you disable the Lock Monitor by removing the innodb_lock_monitor table.

Пример InnoDB Monitor output:

mysql> SHOW ENGINE INNODB STATUS\G
*************************** 1. row ***************************
Status:
=====================================
030709 13:00:59 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 18 seconds
----------
BACKGROUND THREAD
----------
srv_master_thread loops: 53 1_second, 44 sleeps, 5 10_second, 7 background,
  7 flush
srv_master_thread log flush and writes: 48
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 413452, signal count 378357
--Thread 32782 has waited at btr0sea.c line 1477 for 0.00 seconds the
semaphore: X-lock on RW-latch at 41a28668 created in file btr0sea.c line 135
a writer (thread id 32782) has reserved it in mode wait exclusive
number of readers 1, waiters flag 1
Last time read locked in file btr0sea.c line 731
Last time write locked in file btr0sea.c line 1347
Mutex spin waits 0, rounds 0, OS waits 0
RW-shared spins 2, rounds 60, OS waits 2
RW-excl spins 0, rounds 0, OS waits 0
Spin rounds per wait: 0.00 mutex, 20.00 RW-shared, 0.00 RW-excl
------------------------
LATEST FOREIGN KEY ERROR
------------------------
030709 13:00:59 Transaction:
TRANSACTION 0 290328284, ACTIVE 0 sec, process no 3195
inserting
15 lock struct(s), heap size 2496, undo log entries 9
MySQL thread id 25, query id 4668733 localhost heikki update
insert into ibtest11a (D, B, C) values (5, 'khDk' ,'khDk')
Foreign key constraint fails for table test/ibtest11a:
,
  CONSTRAINT `0_219242` FOREIGN KEY (`A`, `D`) REFERENCES `ibtest11b` (`A`,
  `D`) ON DELETE CASCADE ON UPDATE CASCADE
Trying to add in child table, in index PRIMARY tuple:
 0: len 4; hex 80000101; asc ....;; 1: len 4; hex 80000005; asc ....;; 2:
 len 4; hex 6b68446b; asc khDk;; 3: len 6; hex 0000114e0edc; asc ...N..;; 4:
 len 7; hex 00000000c3e0a7; asc .......;; 5: len 4; hex 6b68446b; asc khDk;;
But in parent table test/ibtest11b, in index PRIMARY,
the closest match we can find is record:
RECORD: info bits 0 0: len 4; hex 8000015b; asc ...[;; 1: len 4; hex
80000005; asc ....;; 2: len 3; hex 6b6864; asc khd;; 3: len 6; hex
0000111ef3eb; asc ......;; 4: len 7; hex 800001001e0084; asc .......;; 5:
len 3; hex 6b6864; asc khd;;
------------------------
LATEST DETECTED DEADLOCK
------------------------
030709 12:59:58
*** (1) TRANSACTION:
TRANSACTION 0 290252780, ACTIVE 1 sec, process no 3185
inserting
LOCK WAIT 3 lock struct(s), heap size 320, undo log entries 146
MySQL thread id 21, query id 4553379 localhost heikki update
INSERT INTO alex1 VALUES(86, 86, 794,'aA35818','bb','c79166','d4766t',
'e187358f','g84586','h794',date_format('2001-04-03 12:54:22','%Y-%m-%d
%H:%i'),7
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 48310 n bits 568 table test/alex1 index
symbole trx id 0 290252780 lock mode S waiting
Record lock, heap no 324 RECORD: info bits 0 0: len 7; hex 61613335383138;
asc aa35818;; 1:
*** (2) TRANSACTION:
TRANSACTION 0 290251546, ACTIVE 2 sec, process no 3190
inserting
130 lock struct(s), heap size 11584, undo log entries 437
MySQL thread id 23, query id 4554396 localhost heikki update
REPLACE INTO alex1 VALUES(NULL, 32, NULL,'aa3572','','c3572','d6012t','',
NULL,'h396', NULL, NULL, 7.31,7.31,7.31,200)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 0 page no 48310 n bits 568 table test/alex1 index
symbole trx id 0 290251546 lock_mode X locks rec but not gap
Record lock, heap no 324 RECORD: info bits 0 0: len 7; hex 61613335383138;
asc aa35818;; 1:
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 48310 n bits 568 table test/alex1 index
symbole trx id 0 290251546 lock_mode X locks gap before rec insert intention
waiting
Record lock, heap no 82 RECORD: info bits 0 0: len 7; hex 61613335373230;
asc aa35720;; 1:
*** WE ROLL BACK TRANSACTION (1)
------------
TRANSACTIONS
------------
Trx id counter 0 290328385
Purge done for trx's n:o < 0 290315608 undo n:o < 0 17
History list length 20
Total number of lock structs in row lock hash table 70
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0 0, not started, process no 3491
MySQL thread id 32, query id 4668737 localhost heikki
show innodb status
---TRANSACTION 0 290328384, ACTIVE 0 sec, process no 3205
38929 inserting
1 lock struct(s), heap size 320
MySQL thread id 29, query id 4668736 localhost heikki update
insert into speedc values (1519229,1, 'hgjhjgghggjgjgjgjgjggjgjgjgjgjgggjgjg
jlhhgghggggghhjhghgggggghjhghghghghghhhhghghghjhhjghjghjkghjghjghjghjfhjfh
---TRANSACTION 0 290328383, ACTIVE 0 sec, process no 3180
28684 committing
1 lock struct(s), heap size 320, undo log entries 1
MySQL thread id 19, query id 4668734 localhost heikki update
insert into speedcm values (1603393,1, 'hgjhjgghggjgjgjgjgjggjgjgjgjgjgggjgj
gjlhhgghggggghhjhghgggggghjhghghghghghhhhghghghjhhjghjghjkghjghjghjghjfhjf
---TRANSACTION 0 290328327, ACTIVE 0 sec, process no 3200
36880 starting index read
LOCK WAIT 2 lock struct(s), heap size 320
MySQL thread id 27, query id 4668644 localhost heikki Searching rows for
update
update ibtest11a set B = 'kHdkkkk' where A = 89572
------- TRX HAS BEEN WAITING 0 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 65556 n bits 232 table test/ibtest11a index
PRIMARY trx id 0 290328327 lock_mode X waiting
Record lock, heap no 1 RECORD: info bits 0 0: len 9; hex 73757072656d756d00;
asc supremum.;;
------------------
---TRANSACTION 0 290328284, ACTIVE 0 sec, process no 3195
34831 rollback of SQL statement
ROLLING BACK 14 lock struct(s), heap size 2496, undo log entries 9
MySQL thread id 25, query id 4668733 localhost heikki update
insert into ibtest11a (D, B, C) values (5, 'khDk' ,'khDk')
---TRANSACTION 0 290327208, ACTIVE 1 sec, process no 3190
32782
58 lock struct(s), heap size 5504, undo log entries 159
MySQL thread id 23, query id 4668732 localhost heikki update
REPLACE INTO alex1 VALUES(86, 46, 538,'aa95666','bb','c95666','d9486t',
'e200498f','g86814','h538',date_format('2001-04-03 12:54:22','%Y-%m-%d
%H:%i'),
---TRANSACTION 0 290323325, ACTIVE 3 sec, process no 3185
30733 inserting
4 lock struct(s), heap size 1024, undo log entries 165
MySQL thread id 21, query id 4668735 localhost heikki update
INSERT INTO alex1 VALUES(NULL, 49, NULL,'aa42837','','c56319','d1719t','',
NULL,'h321', NULL, NULL, 7.31,7.31,7.31,200)
--------
FILE I/O
--------
I/O thread 0 state: waiting for i/o request (insert buffer thread)
I/O thread 1 state: waiting for i/o request (log thread)
I/O thread 2 state: waiting for i/o request (read thread)
I/O thread 3 state: waiting for i/o request (write thread)
Pending normal aio reads: 0, aio writes: 0,
 ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0
Pending flushes (fsync) log: 0; buffer pool: 0
151671 OS file reads, 94747 OS file writes, 8750 OS fsyncs
25.44 reads/s, 18494 avg bytes/read, 17.55 writes/s, 2.33 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf for space 0: size 1, free list len 19, seg size 21,
85004 inserts, 85004 merged recs, 26669 merges
Hash table size 207619, used cells 14461, node heap has 16 buffer(s)
1877.67 hash searches/s, 5121.10 non-hash searches/s
---
LOG
---
Log sequence number 18 1212842764
Log flushed up to   18 1212665295
Last checkpoint at  18 1135877290
0 pending log writes, 0 pending chkp writes
4341 log i/o's done, 1.22 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total memory allocated 84966343; in additional pool allocated 1402624
Buffer pool size   3200
Free buffers       110
Database pages     3074
Modified db pages  2674
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages read 171380, created 51968, written 194688
28.72 reads/s, 20.72 creates/s, 47.55 writes/s
Buffer pool hit rate 999 / 1000
--------------
ROW OPERATIONS
--------------
0 queries inside InnoDB, 0 queries in queue
Main thread process no. 3004, id 7176, state: purging
Number of rows inserted 3738558, updated 127415, deleted 33707, read 755779
1586.13 inserts/s, 50.89 updates/s, 28.44 deletes/s, 107.88 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
============================

InnoDB Monitor output is limited to 1MB when produced using the SHOW ENGINE INNODB STATUS statement. This limit does not apply to output written to the server's error output.

Some notes on the output sections:

BACKGROUND THREAD

The srv_master_thread lines shows work done by the main background thread.

SEMAPHORES

This section reports threads waiting for a semaphore and statistics on how many times threads have needed a spin or a wait on a mutex or a rw-lock semaphore. A large number of threads waiting for semaphores may be a result of disk I/O, or contention problems inside InnoDB. Contention can be due to heavy parallelism of queries or problems in operating system thread scheduling. Setting the innodb_thread_concurrency system variable smaller than the default value might help in such situations. The Spin rounds per wait line shows the number of spinlock rounds per OS wait for a mutex.

LATEST FOREIGN KEY ERROR

This section provides information about the most recent foreign key constraint error. It is not present if no such error has occurred. The contents include the statement that failed as well as information about the constraint that failed and the referenced and referencing tables.

LATEST DETECTED DEADLOCK

This section provides information about the most recent deadlock. It is not present if no deadlock has occurred. The contents show which transactions are involved, the statement each was attempting to execute, the locks they have and need, and which transaction InnoDB decided to roll back to break the deadlock. The lock modes reported in this section are explained in Section 13.3.9.1, “InnoDB Lock Modes”.

TRANSACTIONS

If this section reports lock waits, your applications might have lock contention. The output can also help to trace the reasons for transaction deadlocks.

FILE I/O

This section provides information about threads that InnoDB uses to perform various types of I/O. The first few of these are dedicated to general InnoDB processing. The contents also display information for pending I/O operations and statistics for I/O performance.

The number of these threads are controlled by the innodb_read_io_threads and innodb_write_io_threads parameters. See Section 13.3.4, “InnoDB Startup Options and System Variables”.

INSERT BUFFER AND ADAPTIVE HASH INDEX

This section shows the status of the InnoDB insert buffer and adaptive hash index. (See Section 13.3.11.3, “Insert Buffering”, and Section 13.3.11.4, “Adaptive Hash Indexes”.) The contents include the number of operations performed for each, plus statistics for hash index performance.

LOG

This section displays information about the InnoDB log. The contents include the current log sequence number, how far the log has been flushed to disk, and the position at which InnoDB last took a checkpoint. (See Section 13.3.7.3, “InnoDB Checkpoints”.) The section also displays information about pending writes and write performance statistics.

BUFFER POOL AND MEMORY

This section gives you statistics on pages read and written. You can calculate from these numbers how many data file I/O operations your queries currently are doing.

For additional information about the operation of the buffer pool, see Section 7.9.1, “The InnoDB Buffer Pool”.

ROW OPERATIONS

This section shows what the main thread is doing, including the number and performance rate for each type of row operation.

In MySQL 5.5, output from the standard Monitor includes additional sections compared to the output for previous versions. For details, see Section 1.4.3, “Diagnostic and Monitoring Capabilities”.

13.3.14.2.2. InnoDB Tablespace Monitor Output

The InnoDB Tablespace Monitor prints information about the file segments in the shared tablespace and validates the tablespace allocation data structures. If you use individual tablespaces by enabling innodb_file_per_table, the Tablespace Monitor does not describe those tablespaces.

Пример InnoDB Tablespace Monitor output:

================================================
090408 21:28:09 INNODB TABLESPACE MONITOR OUTPUT
================================================
FILE SPACE INFO: id 0
size 13440, free limit 3136, free extents 28
not full frag extents 2: used pages 78, full frag extents 3
first seg id not used 0 23845
SEGMENT id 0 1 space 0; page 2; res 96 used 46; full ext 0
fragm pages 32; free extents 0; not full extents 1: pages 14
SEGMENT id 0 2 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
SEGMENT id 0 3 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
...
SEGMENT id 0 15 space 0; page 2; res 160 used 160; full ext 2
fragm pages 32; free extents 0; not full extents 0: pages 0
SEGMENT id 0 488 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
SEGMENT id 0 17 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
...
SEGMENT id 0 171 space 0; page 2; res 592 used 481; full ext 7
fragm pages 16; free extents 0; not full extents 2: pages 17
SEGMENT id 0 172 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
SEGMENT id 0 173 space 0; page 2; res 96 used 44; full ext 0
fragm pages 32; free extents 0; not full extents 1: pages 12
...
SEGMENT id 0 601 space 0; page 2; res 1 used 1; full ext 0
fragm pages 1; free extents 0; not full extents 0: pages 0
NUMBER of file segments: 73
Validating tablespace
Validation ok
---------------------------------------
END OF INNODB TABLESPACE MONITOR OUTPUT
=======================================

The Tablespace Monitor output includes information about the shared tablespace as a whole, followed by a list containing a breakdown for each segment within the tablespace.

The tablespace consists of database pages with a default size of 16KB. The pages are grouped into extents of size 1MB (64 consecutive pages).

The initial part of the output that displays overall tablespace information has this format:

FILE SPACE INFO: id 0
size 13440, free limit 3136, free extents 28
not full frag extents 2: used pages 78, full frag extents 3
first seg id not used 0 23845

Overall tablespace information includes these values:

  • id: The tablespace ID. A value of 0 refers to the shared tablespace.

  • size: The current tablespace size in pages.

  • free limit: The minimum page number for which the free list has not been initialized. Pages at or above this limit are free.

  • free extents: The number of free extents.

  • not full frag extents, used pages: The number of fragment extents that are not completely filled, and the number of pages in those extents that have been allocated.

  • full frag extents: The number of completely full fragment extents.

  • first seg id not used: The first unused segment ID.

Individual segment information has this format:

SEGMENT id 0 15 space 0; page 2; res 160 used 160; full ext 2
fragm pages 32; free extents 0; not full extents 0: pages 0

Segment information includes these values:

id: The segment ID.

space, page: The tablespace number and page within the tablespace where the segment “inode” is located. A tablespace number of 0 indicates the shared tablespace. InnoDB uses inodes to keep track of segments in the tablespace. The other fields displayed for a segment (id, res, and so forth) are derived from information in the inode.

res: The number of pages allocated (reserved) for the segment.

used: The number of allocated pages in use by the segment.

full ext: The number of extents allocated for the segment that are completely used.

fragm pages: The number of initial pages that have been allocated to the segment.

free extents: The number of extents allocated for the segment that are completely unused.

not full extents: The number of extents allocated for the segment that are partially used.

pages: The number of pages used within the not-full extents.

When a segment grows, it starts as a single page, and InnoDB allocates the first pages for it individually, up to 32 pages (this is the fragm pages value). After that, InnoDB allocates complete 64-page extents. InnoDB can add up to 4 extents at a time to a large segment to ensure good sequentiality of data.

For the example segment shown earlier, it has 32 fragment pages, plus 2 full extents (64 pages each), for a total of 160 pages used out of 160 pages allocated. The following segment has 32 fragment pages and one partially full extent using 14 pages for a total of 46 pages used out of 96 pages allocated:

SEGMENT id 0 1 space 0; page 2; res 96 used 46; full ext 0
fragm pages 32; free extents 0; not full extents 1: pages 14

It is possible for a segment that has extents allocated to it to have a fragm pages value less than 32 if some of the individual pages have been deallocated subsequent to extent allocation.

13.3.14.2.3. InnoDB Table Monitor Output

The InnoDB Table Monitor prints the contents of the InnoDB internal data dictionary.

The output contains one section per table. The SYS_FOREIGN and SYS_FOREIGN_COLS sections are for internal data dictionary tables that maintain information about foreign keys. There are also sections for the Table Monitor table and each user-created InnoDB table. Suppose that the following two tables have been created in the test database:

CREATE TABLE parent
(
  par_id    INT NOT NULL,
  fname      CHAR(20),
  lname      CHAR(20),
  PRIMARY KEY (par_id),
  UNIQUE INDEX (lname, fname)
) ENGINE = INNODB;

CREATE TABLE child
(
  par_id      INT NOT NULL,
  child_id    INT NOT NULL,
  name        VARCHAR(40),
  birth       DATE,
  weight      DECIMAL(10,2),
  misc_info   VARCHAR(255),
  last_update TIMESTAMP,
  PRIMARY KEY (par_id, child_id),
  INDEX (name),
  FOREIGN KEY (par_id) REFERENCES parent (par_id)
    ON DELETE CASCADE
    ON UPDATE CASCADE
) ENGINE = INNODB;

Then the Table Monitor output will look something like this (reformatted slightly):

===========================================
090420 12:09:32 INNODB TABLE MONITOR OUTPUT
===========================================
--------------------------------------
TABLE: name SYS_FOREIGN, id 0 11, columns 7, indexes 3, appr.rows 1
  COLUMNS: ID: DATA_VARCHAR DATA_ENGLISH len 0;
           FOR_NAME: DATA_VARCHAR DATA_ENGLISH len 0;
           REF_NAME: DATA_VARCHAR DATA_ENGLISH len 0;
           N_COLS: DATA_INT len 4;
           DB_ROW_ID: DATA_SYS prtype 256 len 6;
           DB_TRX_ID: DATA_SYS prtype 257 len 6;
  INDEX: name ID_IND, id 0 11, fields 1/6, uniq 1, type 3
   root page 46, appr.key vals 1, leaf pages 1, size pages 1
   FIELDS:  ID DB_TRX_ID DB_ROLL_PTR FOR_NAME REF_NAME N_COLS
  INDEX: name FOR_IND, id 0 12, fields 1/2, uniq 2, type 0
   root page 47, appr.key vals 1, leaf pages 1, size pages 1
   FIELDS:  FOR_NAME ID
  INDEX: name REF_IND, id 0 13, fields 1/2, uniq 2, type 0
   root page 48, appr.key vals 1, leaf pages 1, size pages 1
   FIELDS:  REF_NAME ID
--------------------------------------
TABLE: name SYS_FOREIGN_COLS, id 0 12, columns 7, indexes 1, appr.rows 1
  COLUMNS: ID: DATA_VARCHAR DATA_ENGLISH len 0;
           POS: DATA_INT len 4;
           FOR_COL_NAME: DATA_VARCHAR DATA_ENGLISH len 0;
           REF_COL_NAME: DATA_VARCHAR DATA_ENGLISH len 0;
           DB_ROW_ID: DATA_SYS prtype 256 len 6;
           DB_TRX_ID: DATA_SYS prtype 257 len 6;
  INDEX: name ID_IND, id 0 14, fields 2/6, uniq 2, type 3
   root page 49, appr.key vals 1, leaf pages 1, size pages 1
   FIELDS:  ID POS DB_TRX_ID DB_ROLL_PTR FOR_COL_NAME REF_COL_NAME
--------------------------------------
TABLE: name test/child, id 0 14, columns 10, indexes 2, appr.rows 201
  COLUMNS: par_id: DATA_INT DATA_BINARY_TYPE DATA_NOT_NULL len 4;
           child_id: DATA_INT DATA_BINARY_TYPE DATA_NOT_NULL len 4;
           name: DATA_VARCHAR prtype 524303 len 40;
           birth: DATA_INT DATA_BINARY_TYPE len 3;
           weight: DATA_FIXBINARY DATA_BINARY_TYPE len 5;
           misc_info: DATA_VARCHAR prtype 524303 len 255;
           last_update: DATA_INT DATA_UNSIGNED DATA_BINARY_TYPE DATA_NOT_NULL len 4;
           DB_ROW_ID: DATA_SYS prtype 256 len 6;
           DB_TRX_ID: DATA_SYS prtype 257 len 6;
  INDEX: name PRIMARY, id 0 17, fields 2/9, uniq 2, type 3
   root page 52, appr.key vals 201, leaf pages 5, size pages 6
   FIELDS:  par_id child_id DB_TRX_ID DB_ROLL_PTR name birth weight misc_info last_update
  INDEX: name name, id 0 18, fields 1/3, uniq 3, type 0
   root page 53, appr.key vals 210, leaf pages 1, size pages 1
   FIELDS:  name par_id child_id
  FOREIGN KEY CONSTRAINT test/child_ibfk_1: test/child ( par_id )
             REFERENCES test/parent ( par_id )
--------------------------------------
TABLE: name test/innodb_table_monitor, id 0 15, columns 4, indexes 1, appr.rows 0
  COLUMNS: i: DATA_INT DATA_BINARY_TYPE len 4;
           DB_ROW_ID: DATA_SYS prtype 256 len 6;
           DB_TRX_ID: DATA_SYS prtype 257 len 6;
  INDEX: name GEN_CLUST_INDEX, id 0 19, fields 0/4, uniq 1, type 1
   root page 193, appr.key vals 0, leaf pages 1, size pages 1
   FIELDS:  DB_ROW_ID DB_TRX_ID DB_ROLL_PTR i
--------------------------------------
TABLE: name test/parent, id 0 13, columns 6, indexes 2, appr.rows 299
  COLUMNS: par_id: DATA_INT DATA_BINARY_TYPE DATA_NOT_NULL len 4;
           fname: DATA_CHAR prtype 524542 len 20;
           lname: DATA_CHAR prtype 524542 len 20;
           DB_ROW_ID: DATA_SYS prtype 256 len 6;
           DB_TRX_ID: DATA_SYS prtype 257 len 6;
  INDEX: name PRIMARY, id 0 15, fields 1/5, uniq 1, type 3
   root page 50, appr.key vals 299, leaf pages 2, size pages 3
   FIELDS:  par_id DB_TRX_ID DB_ROLL_PTR fname lname
  INDEX: name lname, id 0 16, fields 2/3, uniq 2, type 2
   root page 51, appr.key vals 300, leaf pages 1, size pages 1
   FIELDS:  lname fname par_id
  FOREIGN KEY CONSTRAINT test/child_ibfk_1: test/child ( par_id )
             REFERENCES test/parent ( par_id )
-----------------------------------
END OF INNODB TABLE MONITOR OUTPUT
==================================

For each table, Table Monitor output contains a section that displays general information about the table and specific information about its columns, indexes, and foreign keys.

The general information for each table includes the table name (in db_name/tbl_name format except for internal tables), its ID, the number of columns and indexes, and an approximate row count.

The COLUMNS part of a table section lists each column in the table. Information for each column indicates its name and data type characteristics. Some internal columns are added by InnoDB, such as DB_ROW_ID (row ID), DB_TRX_ID (transaction ID), and DB_ROLL_PTR (a pointer to the rollback/undo data).

  • DATA_xxx: These symbols indicate the data type. There may be multiple DATA_xxx symbols for a given column.

  • prtype: The column's “precise” type. This field includes information such as the column data type, character set code, nullability, signedness, and whether it is a binary string. This field is described in the innobase/include/data0type.h source file.

  • len: The column length in bytes.

Each INDEX part of the table section provides the name and characteristics of one table index:

  • name: The index name. If the name is PRIMARY, the index is a primary key. If the name is GEN_CLUST_INDEX, the index is the clustered index that is created automatically if the table definition doesn't include a primary key or non-NULL unique index. See Section 13.3.11.1, “Clustered and Secondary Indexes”.

  • id: The index ID.

  • fields: The number of fields in the index, as a value in m/n format:

    • m is the number of user-defined columns; that is, the number of columns you would see in the index definition in a CREATE TABLE statement.

    • n is the total number of index columns, including those added internally. For the clustered index, the total includes the other columns in the table definition, plus any columns added internally. For a secondary index, the total includes the columns from the primary key that are not part of the secondary index.

  • uniq: The number of leading fields that are enough to determine index values uniquely.

  • type: The index type. This is a bit field. For example, 1 indicates a clustered index and 2 indicates a unique index, so a clustered index (which always contains unique values), will have a type value of 3. An index with a type value of 0 is neither clustered nor unique. The flag values are defined in the innobase/include/dict0mem.h source file.

  • root page: The index root page number.

  • appr. key vals: The approximate index cardinality.

  • leaf pages: The approximate number of leaf pages in the index.

  • size pages: The approximate total number of pages in the index.

  • FIELDS: The names of the fields in the index. For a clustered index that was generated automatically, the field list begins with the internal DB_ROW_ID (row ID) field. DB_TRX_ID and DB_ROLL_PTR are always added internally to the clustered index, following the fields that comprise the primary key. For a secondary index, the final fields are those from the primary key that are not part of the secondary index.

The end of the table section lists the FOREIGN KEY definitions that apply to the table. This information appears whether the table is a referencing or referenced table.

13.3.14.3. InnoDB General Troubleshooting

The following general guidelines apply to troubleshooting InnoDB problems:

  • When an operation fails or you suspect a bug, look at the MySQL server error log (see Section 5.2.2, “The Error Log”).

  • Issues relating to the InnoDB data dictionary include failed CREATE TABLE statements (orphaned table files), inability to open .InnoDB files, and system cannot find the path specified errors. For information about these sorts of problems and errors, see Section 13.3.14.4, “Troubleshooting InnoDB Data Dictionary Operations”.

  • When troubleshooting, it is usually best to run the MySQL server from the command prompt, rather than through mysqld_safe or as a Windows service. You can then see what mysqld prints to the console, and so have a better grasp of what is going on. On Windows, start mysqld with the --console option to direct the output to the console window.

  • Use the InnoDB Monitors to obtain information about a problem (see Section 13.3.14.2, “SHOW ENGINE INNODB STATUS and the InnoDB Monitors”). If the problem is performance-related, or your server appears to be hung, you should use the standard Monitor to print information about the internal state of InnoDB. If the problem is with locks, use the Lock Monitor. If the problem is in creation of tables or other data dictionary operations, use the Table Monitor to print the contents of the InnoDB internal data dictionary. To see tablespace information use the Tablespace Monitor.

  • If you suspect that a table is corrupt, run CHECK TABLE on that table.

13.3.14.4. Troubleshooting InnoDB Data Dictionary Operations

Information about table definitions is stored both in the .frm files, and in the InnoDB data dictionary. If you move .frm files around, or if the server crashes in the middle of a data dictionary operation, these sources of information can become inconsistent.

Problem with CREATE TABLE

A symptom of an out-of-sync data dictionary is that a CREATE TABLE statement fails. If this occurs, look in the server's error log. If the log says that the table already exists inside the InnoDB internal data dictionary, you have an orphaned table inside the InnoDB tablespace files that has no corresponding .frm file. The error message looks like this:

InnoDB: Error: table test/parent already exists in InnoDB internal
InnoDB: data dictionary. Have you deleted the .frm file
InnoDB: and not used DROP TABLE? Have you used DROP DATABASE
InnoDB: for InnoDB tables in MySQL version <= 3.23.43?
InnoDB: See the Restrictions section of the InnoDB manual.
InnoDB: You can drop the orphaned table inside InnoDB by
InnoDB: creating an InnoDB table with the same name in another
InnoDB: database and moving the .frm file to the current database.
InnoDB: Then MySQL thinks the table exists, and DROP TABLE will
InnoDB: succeed.

You can drop the orphaned table by following the instructions given in the error message. If you are still unable to use DROP TABLE successfully, the problem may be due to name completion in the mysql client. To work around this problem, start the mysql client with the --skip-auto-rehash option and try DROP TABLE again. (With name completion on, mysql tries to construct a list of table names, which fails when a problem such as just described exists.)

Problem Opening Table

Another symptom of an out-of-sync data dictionary is that MySQL prints an error that it cannot open a .InnoDB file:

ERROR 1016: Can't open file: 'child2.InnoDB'. (errno: 1)

In the error log you can find a message like this:

InnoDB: Cannot find table test/child2 from the internal data dictionary
InnoDB: of InnoDB though the .frm file for the table exists. Maybe you
InnoDB: have deleted and recreated InnoDB data files but have forgotten
InnoDB: to delete the corresponding .frm files of InnoDB tables?

This means that there is an orphaned .frm file without a corresponding table inside InnoDB. You can drop the orphaned .frm file by deleting it manually.

Problem with Temporary Table

If MySQL crashes in the middle of an ALTER TABLE operation, you may end up with an orphaned temporary table inside the InnoDB tablespace. Using the Table Monitor, you can see listed a table with a name that begins with #sql-. You can perform SQL statements on tables whose name contains the character “#” if you enclose the name within backticks. Thus, you can drop such an orphaned table like any other orphaned table using the method described earlier. To copy or rename a file in the Unix shell, you need to put the file name in double quotation marks if the file name contains “#”.

Problem with Missing Tablespace

With innodb_file_per_table enabled, the following message might occur if the .frm or .ibd files (or both) are missing:

InnoDB: in InnoDB data dictionary has tablespace id N,
InnoDB: but tablespace with that id or name does not exist. Have
InnoDB: you deleted or moved .ibd files?
InnoDB: This may also be a table created with CREATE TEMPORARY TABLE
InnoDB: whose .ibd and .frm files MySQL automatically removed, but the
InnoDB: table still exists in the InnoDB internal data dictionary.

If this occurs, try the following procedure to resolve the problem:

  1. Create a matching .frm file in some other database directory and copy it to the database directory where the orphan table is located.

  2. Issue DROP TABLE for the original table. That should successfully drop the table and InnoDB should print a warning to the error log that the .ibd file was missing.

13.3.15. Limits on InnoDB Tables

Warning

Do not convert MySQL system tables in the mysql database from MyISAM to InnoDB tables! This is an unsupported operation. If you do this, MySQL does not restart until you restore the old system tables from a backup or re-generate them with the mysql_install_db script.

Warning

It is not a good idea to configure InnoDB to use data files or log files on NFS volumes. Otherwise, the files might be locked by other processes and become unavailable for use by MySQL.

Maximums and Minimums

  • A table can contain a maximum of 1000 columns.

  • By default, an index key for a single-column index can be up to 767 bytes. The same length limit applies to any index key prefix. See Section 12.1.13, “CREATE INDEX Синтаксис”. For example, you might hit this limit with a column prefix index of more than 255 characters on a TEXT or VARCHAR column, assuming a UTF-8 character set and the maximum of 3 bytes for each character. When the innodb_large_prefix configuration option is enabled, this length limit is raised to 3072 bytes, for InnoDB tables that use the DYNAMIC and COMPRESSED row formats.

    When you attempt to specify an index prefix length longer than allowed, the length is silently reduced to the maximum length. This configuration option changes the error handling for some combinations of row format and prefix length longer than the maximum allowed. See innodb_large_prefix for details.

  • The InnoDB internal maximum key length is 3500 bytes, but MySQL itself restricts this to 3072 bytes. This limit applies to the length of the combined index key in a multi-column index.

  • The maximum row length, except for variable-length columns (VARBINARY, VARCHAR, BLOB and TEXT), is slightly less than half of a database page. That is, the maximum row length is about 8000 bytes. LONGBLOB and LONGTEXT columns must be less than 4GB, and the total row length, including BLOB and TEXT columns, must be less than 4GB.

    If a row is less than half a page long, all of it is stored locally within the page. If it exceeds half a page, variable-length columns are chosen for external off-page storage until the row fits within half a page, as described in Section 13.3.12.2, “File Space Management”.

  • Although InnoDB supports row sizes larger than 65,535 bytes internally, MySQL inself imposes a row-size limit of 65,535 for the combined size of all columns:

    mysql> CREATE TABLE t (a VARCHAR(8000), b VARCHAR(10000),
        -> c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),
        -> f VARCHAR(10000), g VARCHAR(10000)) ENGINE=InnoDB;
    ERROR 1118 (42000): Row size too large. The maximum row size for the
    used table type, not counting BLOBs, is 65535. You have to change some
    columns to TEXT or BLOBs
    

    See Section E.10.4, “Table Column-Count and Row-Size Limits”.

  • On some older operating systems, files must be less than 2GB. This is not a limitation of InnoDB itself, but if you require a large tablespace, you will need to configure it using several smaller data files rather than one or a file large data files.

  • The combined size of the InnoDB log files must be less than 4GB.

  • The minimum tablespace size is 10MB. The maximum tablespace size is four billion database pages (64TB). This is also the maximum size for a table.

  • The default database page size in InnoDB is 16KB.

    Замечание

    Changing the page size is not a supported operation and there is no guarantee that InnoDB will function normally with a page size other than 16KB. Problems compiling or running InnoDB may occur. In particular, ROW_FORMAT=COMPRESSED in the Barracuda file format assumes that the page size is at most 16KB and uses 14-bit pointers.

    A version of InnoDB built for one page size cannot use data files or log files from a version built for a different page size. This limitation could affect restore or downgrade operations using data from MySQL 5.6, which does support page sizes other than 16KB.

Index Types

  • InnoDB tables do not support FULLTEXT indexes.

  • InnoDB tables support spatial data types, but not indexes on them.

Restrictions on InnoDB Tables

  • ANALYZE TABLE determines index cardinality (as displayed in the Cardinality column of SHOW INDEX output) by doing eight random dives to each of the index trees and updating index cardinality estimates accordingly. Because these are only estimates, repeated runs of ANALYZE TABLE may produce different numbers. This makes ANALYZE TABLE fast on InnoDB tables but not 100% accurate because it does not take all rows into account.

    You can change the number of random dives by modifying the innodb_stats_sample_pages system variable. For more information, see Section 13.4.8, “Changes for Flexibility, Ease of Use and Reliability”.

    MySQL uses index cardinality estimates only in join optimization. If some join is not optimized in the right way, you can try using ANALYZE TABLE. In the few cases that ANALYZE TABLE does not produce values good enough for your particular tables, you can use FORCE INDEX with your queries to force the use of a particular index, or set the max_seeks_for_key system variable to ensure that MySQL prefers index lookups over table scans. See Section 5.1.3, “Server System Variables”, and Section C.5.6, “Optimizer-Related Issues”.

  • SHOW TABLE STATUS does not give accurate statistics on InnoDB tables, except for the physical size reserved by the table. The row count is only a rough estimate used in SQL optimization.

  • InnoDB does not keep an internal count of rows in a table because concurrent transactions might “see” different numbers of rows at the same time. To process a SELECT COUNT(*) FROM t statement, InnoDB scans an index of the table, which takes some time if the index is not entirely in the buffer pool. If your table does not change often, using the MySQL query cache is a good solution. To get a fast count, you have to use a counter table you create yourself and let your application update it according to the inserts and deletes it does. If an approximate row count is sufficient, SHOW TABLE STATUS can be used. See Section 13.3.14.1, “InnoDB Performance Tuning Tips”.

  • On Windows, InnoDB always stores database and table names internally in lowercase. To move databases in a binary format from Unix to Windows or from Windows to Unix, create all databases and tables using lowercase names.

  • For an AUTO_INCREMENT column, you must always define an index for the table, and that index must contain just the AUTO_INCREMENT column. In MyISAM tables, the AUTO_INCREMENT column may be part of a multiple-column index.

  • While initializing a previously specified AUTO_INCREMENT column on a table, InnoDB sets an exclusive lock on the end of the index associated with the AUTO_INCREMENT column. While accessing the auto-increment counter, InnoDB uses a specific AUTO-INC table lock mode where the lock lasts only to the end of the current SQL statement, not to the end of the entire transaction. Other clients cannot insert into the table while the AUTO-INC table lock is held. See Section 13.3.5.3, “AUTO_INCREMENT Handling in InnoDB.

  • When you restart the MySQL server, InnoDB may reuse an old value that was generated for an AUTO_INCREMENT column but never stored (that is, a value that was generated during an old transaction that was rolled back).

  • When an AUTO_INCREMENT integer column runs out of values, a subsequent INSERT operation returns a duplicate-key error. This is general MySQL behavior, similar to how MyISAM works.

  • DELETE FROM tbl_name does not regenerate the table but instead deletes all rows, one by one.

  • Under some conditions, TRUNCATE tbl_name for an InnoDB table is mapped to DELETE FROM tbl_name. See Section 12.1.33, “TRUNCATE TABLE Синтаксис”.

  • Currently, cascaded foreign key actions do not activate triggers.

  • You cannot create a table with a column name that matches the name of an internal InnoDB column (including DB_ROW_ID, DB_TRX_ID, DB_ROLL_PTR, and DB_MIX_ID). The server reports error 1005 and refers to error –1 in the error message. This restriction applies only to use of the names in uppercase.

Locking and Transactions

  • LOCK TABLES acquires two locks on each table if innodb_table_locks=1 (the default). In addition to a table lock on the MySQL layer, it also acquires an InnoDB table lock. Versions of MySQL before 4.1.2 did not acquire InnoDB table locks; the old behavior can be selected by setting innodb_table_locks=0. If no InnoDB table lock is acquired, LOCK TABLES completes even if some records of the tables are being locked by other transactions.

    As of MySQL 5.5.3, innodb_table_locks=0 has no effect for tables locked explicitly with LOCK TABLES ... WRITE. It still has an effect for tables locked for read or write by LOCK TABLES ... WRITE implicitly (for example, through triggers) or by LOCK TABLES ... READ.

  • All InnoDB locks held by a transaction are released when the transaction is committed or aborted. Thus, it does not make much sense to invoke LOCK TABLES on InnoDB tables in autocommit=1 mode because the acquired InnoDB table locks would be released immediately.

  • You cannot lock additional tables in the middle of a transaction because LOCK TABLES performs an implicit COMMIT and UNLOCK TABLES.

  • The limit of 1023 concurrent data-modifying transactions has been raised in MySQL 5.5 and above. The limit is now 128 * 1023 concurrent transactions that generate undo records. You can remove any workarounds that require changing the proper structure of your transactions, such as committing more frequently.

13.4. New Features of InnoDB 1.1

13.4.1. Introduction to InnoDB 1.1

InnoDB 1.1 combines the familiar reliability and performance of the InnoDB storage engine, with new performance and usability enhancements. InnoDB 1.1 includes all the features that were part of the InnoDB Plugin for MySQL 5.1, plus new features specific to MySQL 5.5 and higher.

Beginning with MySQL version 5.5, InnoDB is the default storage engine, rather than MyISAM, to promote greater data reliability and reducing the chance of corruption.

13.4.1.1. Features of the InnoDB Storage Engine

The InnoDB Storage Engine for MySQL contains several important new features:

Upward and Downward Compatibility

Note that the ability to use data compression and the new row format require the use of a new InnoDB file format called Barracuda. The previous file format, used by the built-in InnoDB in MySQL 5.1 and earlier, is now called Antelope and does not support these features, but does support the other features introduced with the InnoDB storage engine.

The InnoDB storage engine is upward compatible from standard InnoDB as built in to, and distributed with, MySQL. Existing databases can be used with the InnoDB Storage Engine for MySQL. The new parameter innodb_file_format can help protect upward and downward compatibility between InnoDB versions and database files, allowing users to enable or disable use of new features that can only be used with certain versions of InnoDB.

InnoDB since version 5.0.21 has a safety feature that prevents it from opening tables that are in an unknown format. However, the system tablespace may contain references to new-format tables that confuse the built-in InnoDB in MySQL 5.1 and earlier. These references are cleared in a slow shutdown.

With previous versions of InnoDB, no error would be returned until you try to access a table that is in a format “too new” for the software. To provide early feedback, InnoDB 1.1 checks the system tablespace before startup to ensure that the file format used in the database is supported by the storage engine. See Section 13.4.4.2.1, “Compatibility Check When InnoDB Is Started” for the details.

13.4.1.2. Obtaining and Installing the InnoDB Storage Engine

Starting with MySQL 5.4.2, you do not need to do anything special to get or install the most up-to-date InnoDB storage engine. From that version forward, the InnoDB storage engine in the server is what was formerly known as the InnoDB Plugin. Earlier versions of MySQL required some extra build and configuration steps to get the Plugin-specific features such as fast index creation and table compression.

Report any bugs in the InnoDB storage engine using the My Oracle Support site. For general discussions about InnoDB Storage Engine for MySQL, see http://forums.mysql.com/list.php?22.

13.4.1.3. Viewing the InnoDB Storage Engine Version Number

InnoDB storage engine releases are numbered with version numbers independent of MySQL release numbers. The initial release of the InnoDB storage engine is version 1.0, and it is designed to work with MySQL 5.1. Version 1.1 of the InnoDB storage engine is for MySQL 5.5 and up.

  • The first component of the InnoDB storage engine version number designates a major release level.

  • The second component corresponds to the MySQL release. The digit 0 corresponds to MySQL 5.1. The digit 1 corresponds to MySQL 5.5.

  • The third component indicates the specific release of the InnoDB storage engine (at a given major release level and for a specific MySQL release); only bug fixes and minor functional changes are introduced at this level.

Once you have installed the InnoDB storage engine, you can check its version number in three ways:

  • In the error log, it is printed during startup

  • SELECT * FROM information_schema.plugins;

  • SELECT @@innodb_version;

The InnoDB storage engine writes its version number to the error log, which can be helpful in diagnosis of errors:

091105 12:28:06 InnoDB Plugin 1.0.5 started; log sequence number 46509

Note that the PLUGIN_VERSION column in the table INFORMATION_SCHEMA.PLUGINS does not display the third component of the version number, only the first and second components, as in 1.0.

13.4.1.4. Compatibility Considerations for Downgrade and Backup

Because InnoDB 1.1 supports the “Barracuda” file format, with new on-disk data structures within both the database and log files, pay special attention to file format compatibility with respect to the following scenarios:

  • Downgrading from MySQL 5.5 to the MySQL 5.1 or earlier (without the InnoDB Plugin enabled), or otherwise using earlier versions of MySQL with database files created by MySQL 5.5 and higher.

  • Using mysqldump.

  • Using MySQL replication.

  • Using MySQL Enterprise Backup or InnoDB Hot Backup.

WARNING: Once you create any tables with the Barracuda file format, take care to avoid crashes and corruptions when using those files with an earlier version of MySQL. It is strongly recommended that you use a “slow shutdown” (SET GLOBAL innodb_fast_shutdown=0) when stopping the MySQL server before downgrading to MySQL 5.1 or earlier. This ensures that the log files and other system information do not cause consistency issues or startup problems when using a prior version of MySQL.

WARNING: If you dump a database containing compressed tables with mysqldump, the dump file may contain CREATE TABLE statements that attempt to create compressed tables, or those using ROW_FORMAT=DYNAMIC in the new database. Therefore, be sure the new database is running the InnoDB storage engine, with the proper settings for innodb_file_format and innodb_file_per_table, if you want to have the tables re-created as they exist in the original database. Typically, when the mysqldump file is loaded, MySQL and InnoDB ignore CREATE TABLE options they do not recognize, and the table(s) are created in a format used by the running server.

WARNING: If you use MySQL replication, ensure all slaves are configured with the InnoDB storage engine, with the same settings for innodb_file_format and innodb_file_per_table. If you do not do so, and you create tables that require the new Barracuda file format, replication errors may occur. If a slave MySQL server is running an older version of MySQL, it ignores the CREATE TABLE options to create a compressed table or one with ROW_FORMAT=DYNAMIC, and creates the table uncompressed, with ROW_FORMAT=COMPACT.

WARNING: Version 3.0 of InnoDB Hot Backup does not support the new Barracuda file format. Using InnoDB Hot Backup Version 3 to backup databases in this format causes unpredictable behavior. MySQL Enterprise Backup, the successor product to InnoDB Hot Backup, does support tables with the Barracuda file format. You can also back up such databases with mysqldump.

13.4.2. Fast Index Creation in the InnoDB Storage Engine

In MySQL 5.5 and higher, or in MySQL 5.1 with the InnoDB Plugin, creating and dropping secondary indexes does not copy the contents of the entire table, making this operation much more efficient than with prior releases.

13.4.2.1. Overview of Fast Index Creation

With MySQL 5.5 and higher, or MySQL 5.1 with the InnoDB Plugin, creating and dropping secondary indexes for InnoDB tables is much faster than before. Historically, adding or dropping an index on a table with existing data could be very slow. The CREATE INDEX and DROP INDEX statements worked by creating a new, empty table defined with the requested set of indexes, then copying the existing rows to the new table one-by-one, updating the indexes as the rows are inserted. After all rows from the original table were copied, the old table was dropped and the copy was renamed with the name of the original table.

The performance speedup for fast index creation applies to secondary indexes, not to the primary key index. The rows of an InnoDB table are stored in a clustered index organized based on the primary key, forming what some database systems call an “index-organized table”. Because the table structure is so closely tied to the primary key, redefining the primary key still requires copying the data.

This new mechanism also means that you can generally speed the overall process of creating and loading an indexed table by creating the table with only the clustered index, and adding the secondary indexes after the data is loaded.

Although no syntax changes are required in the CREATE INDEX or DROP INDEX commands, some factors affect the performance, space usage, and semantics of this operation (see Section 13.4.2.6, “Limitations of Fast Index Creation”).

13.4.2.2. Examples of Fast Index Creation

It is possible to create multiple indexes on a table with one ALTER TABLE statement. This is relatively efficient, because the clustered index of the table needs to be scanned only once (although the data is sorted separately for each new index). For example:

CREATE TABLE T1(A INT PRIMARY KEY, B INT, C CHAR(1)) ENGINE=InnoDB;
INSERT INTO T1 VALUES (1,2,'a'), (2,3,'b'), (3,2,'c'), (4,3,'d'), (5,2,'e');
COMMIT;
ALTER TABLE T1 ADD INDEX (B), ADD UNIQUE INDEX (C);

The above statements create table T1 with the clustered index (primary key) on column A, insert several rows, and then build two new indexes on columns B and C. If there were many rows inserted into T1 before the ALTER TABLE statement, this approach is much more efficient than creating all the secondary indexes before loading the data.

You can also create the indexes one at a time, but then the clustered index of the table is scanned (as well as sorted) once for each CREATE INDEX statement. Thus, the following statements are not as efficient as the ALTER TABLE statement above, even though neither requires recreating the clustered index for table T1.

CREATE INDEX B ON T1 (B);
CREATE UNIQUE INDEX C ON T1 (C);

Dropping InnoDB secondary indexes also does not require any copying of table data. You can equally quickly drop multiple indexes with a single ALTER TABLE statement or multiple DROP INDEX statements:

ALTER TABLE T1 DROP INDEX B, DROP INDEX C;

or:

DROP INDEX B ON T1;
DROP INDEX C ON T1;

Restructuring the clustered index in InnoDB always requires copying the data in the table. For example, if you create a table without a primary key, InnoDB chooses one for you, which may be the first UNIQUE key defined on NOT NULL columns, or a system-generated key. Defining a PRIMARY KEY later causes the data to be copied, as in the following example:

CREATE TABLE T2 (A INT, B INT) ENGINE=InnoDB;
INSERT INTO T2 VALUES (NULL, 1);
ALTER TABLE T2 ADD PRIMARY KEY (B);

When you create a UNIQUE or PRIMARY KEY index, InnoDB must do some extra work. For UNIQUE indexes, InnoDB checks that the table contains no duplicate values for the key. For a PRIMARY KEY index, InnoDB also checks that none of the PRIMARY KEY columns contains a NULL. It is best to define the primary key when you create a table, so you need not rebuild the table later.

13.4.2.3. Implementation Details of Fast Index Creation

InnoDB has two types of indexes: the clustered index and secondary indexes. Since the clustered index contains the data values in its B-tree nodes, adding or dropping a clustered index does involve copying the data, and creating a new copy of the table. A secondary index, however, contains only the index key and the value of the primary key. This type of index can be created or dropped without copying the data in the clustered index. Because each secondary index contains copies of the primary key values (used to access the clustered index when needed), when you change the definition of the primary key, all secondary indexes are recreated as well.

Dropping a secondary index is simple. Only the internal InnoDB system tables and the MySQL data dictionary tables are updated to reflect the fact that the index no longer exists. InnoDB returns the storage used for the index to the tablespace that contained it, so that new indexes or additional table rows can use the space.

To add a secondary index to an existing table, InnoDB scans the table, and sorts the rows using memory buffers and temporary files in order by the values of the secondary index key columns. The B-tree is then built in key-value order, which is more efficient than inserting rows into an index in random order. Because the B-tree nodes are split when they fill, building the index in this way results in a higher fill-factor for the index, making it more efficient for subsequent access.

13.4.2.4. Concurrency Considerations for Fast Index Creation

While an InnoDB secondary index is being created or dropped, the table is locked in shared mode. Any writes to the table are blocked, but the data in the table can be read. When you alter the clustered index of a table, the table is locked in exclusive mode, because the data must be copied. Thus, during the creation of a new clustered index, all operations on the table are blocked.

A CREATE INDEX or ALTER TABLE statement for an InnoDB table always waits for currently executing transactions that are accessing the table to commit or roll back. ALTER TABLE statements that redefine an InnoDB primary key wait for all SELECT statements that access the table to complete, or their containing transactions to commit. No transactions whose execution spans the creation of the index can be accessing the table, because the original table is dropped when the clustered index is restructured.

Once a CREATE INDEX or ALTER TABLE statement that creates an InnoDB secondary index begins executing, queries can access the table for read access, but cannot update the table. If an ALTER TABLE statement is changing the clustered index for an InnoDB table, all queries wait until the operation completes.

A newly-created InnoDB secondary index contains only the committed data in the table at the time the CREATE INDEX or ALTER TABLE statement begins to execute. It does not contain any uncommitted values, old versions of values, or values marked for deletion but not yet removed from the old index.

Because a newly-created index contains only information about data current at the time the index was created, queries that need to see data that was deleted or changed before the index was created cannot use the index. The only queries that could be affected by this limitation are those executing in transactions that began before the creation of the index was begun. For such queries, unpredictable results could occur. Newer queries can use the index.

13.4.2.5. How Crash Recovery Works with Fast Index Creation

Although no data is lost if the server crashes while an ALTER TABLE statement is executing, the crash recovery process is different for clustered indexes and secondary indexes.

If the server crashes while creating an InnoDB secondary index, upon recovery, MySQL drops any partially created indexes. You must re-run the ALTER TABLE or CREATE INDEX statement.

When a crash occurs during the creation of an InnoDB clustered index, recovery is more complicated, because the data in the table must be copied to an entirely new clustered index. Remember that all InnoDB tables are stored as clustered indexes. In the following discussion, we use the word table and clustered index interchangeably.

MySQL creates the new clustered index by copying the existing data from the original InnoDB table to a temporary table that has the desired index structure. Once the data is completely copied to this temporary table, the original table is renamed with a different temporary table name. The temporary table comprising the new clustered index is renamed with the name of the original table, and the original table is dropped from the database.

If a system crash occurs while creating a new clustered index, no data is lost, but you must complete the recovery process using the temporary tables that exist during the process. Since it is rare to re-create a clustered index or re-define primary keys on large tables, or to encounter a system crash during this operation, this manual does not provide information on recovering from this scenario. Instead, please see the InnoDB web site: http://www.innodb.com/support/tips.

13.4.2.6. Limitations of Fast Index Creation

Take the following considerations into account when creating or dropping InnoDB indexes:

  • During index creation, files are written to the temporary directory ($TMPDIR on Unix, %TEMP% on Windows, or the value of the --tmpdir configuration variable). Each temporary file is large enough to hold one column that makes up the new index, and each one is removed as soon as it is merged into the final index.

  • The table is copied, rather than using Fast Index Creation when you create an index on a TEMPORARY TABLE. This has been reported as MySQL Bug #39833.

  • To avoid consistency issues between the InnoDB data dictionary and the MySQL data dictionary, the table is copied, rather than using Fast Index Creation when you use the ALTER TABLE ... RENAME COLUMN syntax.

  • The statement ALTER IGNORE TABLE t ADD UNIQUE INDEX does not delete duplicate rows. This has been reported as MySQL Bug #40344. The IGNORE keyword is ignored. If any duplicate rows exist, the operation fails with the following error message:

    ERROR 23000: Duplicate entry '347' for key 'pl'
  • As noted above, a newly-created index contains only information about data current at the time the index was created. Therefore, you should not run queries in a transaction that might use a secondary index that did not exist at the beginning of the transaction. There is no way for InnoDB to access “old” data that is consistent with the rest of the data read by the transaction. See the discussion of locking in Section 13.4.2.4, “Concurrency Considerations for Fast Index Creation”.

    Prior to InnoDB storage engine 1.0.4, unexpected results could occur if a query attempts to use an index created after the start of the transaction containing the query. If an old transaction attempts to access a “too new” index, InnoDB storage engine 1.0.4 and later reports an error:

    ERROR HY000: Table definition has changed, please retry transaction

    As the error message suggests, committing (or rolling back) the transaction, and restarting it, cures the problem.

  • InnoDB storage engine 1.0.2 introduces some improvements in error handling when users attempt to drop indexes. See section Section 13.4.8.6, “Better Error Handling when Dropping Indexes” for details.

  • MySQL 5.5 does not support efficient creation or dropping of FOREIGN KEY constraints. Therefore, if you use ALTER TABLE to add or remove a REFERENCES constraint, the child table is copied, rather than using Fast Index Creation.

13.4.3. InnoDB Data Compression

By setting InnoDB configuration options, you can create tables where the data is stored in compressed form. The compression means less data is transferred between disk and memory, and takes up less space in memory. The benefits are amplified for tables with secondary indexes, because index data is compressed also.

13.4.3.1. Overview of Table Compression

Because processors and cache memories have increased in speed more than disk storage devices, many workloads are I/O-bound. Data compression enables smaller database size, reduced I/O, and improved throughput, at the small cost of increased CPU utilization. Compression is especially valuable for read-intensive applications, on systems with enough RAM to keep frequently-used data in memory.

An InnoDB table created with ROW_FORMAT=COMPRESSED can use a smaller page size on disk than the usual 16KB default. Smaller pages require less I/O to read from and write to disk, which is especially valuable for SSD devices.

The page size is specified through the KEY_BLOCK_SIZE parameter. The different page size means the table must be in its own .ibd file rather than the system tablespace, which requires enabling the innodb_file_per_table option. The level of compression is the same regardless of the KEY_BLOCK_SIZE value. As you specify smaller values for KEY_BLOCK_SIZE, you get the I/O benefits of increasingly smaller pages. But if you specify a value that is too small, there is additional overhead to reorganize the pages when data values cannot be compressed enough to fit multiple rows in each page. There is a hard limit on how small KEY_BLOCK_SIZE can be for a table, based on the lengths of the key columns for each of its indexes. Specify a value that is too small, and the CREATE TABLE or ALTER TABLE statement fails.

In the buffer pool, the compressed data is held in small pages, with a page size based on the KEY_BLOCK_SIZE value. For extracting or updating the column values, InnoDB also creates a 16KB page in the buffer pool with the uncompressed data. Within the buffer pool, any updates to the uncompressed page are also re-written back to the equivalent compressed page. You might need to size your buffer pool to accommodate the additional data of both compressed and uncompressed pages, although the uncompressed pages are evicted from the buffer pool when space is needed, and then uncompressed again on the next access.

13.4.3.2. Enabling Compression for a Table

The default uncompressed size of InnoDB data pages is 16KB. You can use the attributes ROW_FORMAT=COMPRESSED, KEY_BLOCK_SIZE, or both in the CREATE TABLE and ALTER TABLE statements to enable table compression. Depending on the combination of option values, InnoDB uses a page size of 1KB, 2KB, 4KB, 8KB, or 16KB for the .ibd file of the table. (The actual compression algorithm is not affected by the KEY_BLOCK_SIZE value.)

Замечание

Compression is applicable to tables, not to individual rows, despite the option name ROW_FORMAT.

To create a compressed table, you might use a statement like this:

CREATE TABLE name
 (column1 INT PRIMARY KEY) 
 ENGINE=InnoDB
 ROW_FORMAT=COMPRESSED 
 KEY_BLOCK_SIZE=4;

If you specify ROW_FORMAT=COMPRESSED but not KEY_BLOCK_SIZE, the default compressed page size of 8KB is used. If KEY_BLOCK_SIZE is specified, you can omit the attribute ROW_FORMAT=COMPRESSED.

Setting KEY_BLOCK_SIZE=16 typically does not result in much compression, since the normal InnoDB page size is 16KB. This setting may still be useful for tables with many long BLOB, VARCHAR or TEXT columns, because such values often do compress well, and might therefore require fewer “overflow” pages as described in Section 13.4.3.4, “ Compressing BLOB, VARCHAR and TEXT Columns ”.

All indexes of a table (including the clustered index) are compressed using the same page size, as specified in the CREATE TABLE or ALTER TABLE statement. Table attributes such as ROW_FORMAT and KEY_BLOCK_SIZE are not part of the CREATE INDEX syntax, and are ignored if they are specified (although you see them in the output of the SHOW CREATE TABLE statement).

13.4.3.2.1. Configuration Parameters for Compression

Compressed tables are stored in a format that previous versions of InnoDB cannot process. To preserve downward compatibility of database files, compression can be specified only when the Barracuda data file format is enabled using the configuration parameter innodb_file_format.

Table compression is also not available for the InnoDB system tablespace. The system tablespace (space 0, the ibdata* files) may contain user data, but it also contains internal InnoDB system information, and therefore is never compressed. Thus, compression applies only to tables (and indexes) stored in their own tablespaces.

To use compression, enable the file-per-table mode using the configuration parameter innodb_file_per_table and enable the Barracuda disk file format using the parameter innodb_file_format. If necessary, you can set these parameters in the MySQL option file my.cnf or my.ini, or with the SET statement without shutting down the MySQL server.

Specifying ROW_FORMAT=COMPRESSED or KEY_BLOCK_SIZE in CREATE TABLE or ALTER TABLE statements produces these warnings if the Barracuda file format is not enabled. You can view them with the SHOW WARNINGS statement.

LevelCodeMessage
Warning1478InnoDB: KEY_BLOCK_SIZE requires innodb_file_per_table.
Warning1478InnoDB: KEY_BLOCK_SIZE requires innodb_file_format=1
Warning1478InnoDB: ignoring KEY_BLOCK_SIZE=4.
Warning1478InnoDB: ROW_FORMAT=COMPRESSED requires innodb_file_per_table.
Warning1478InnoDB: assuming ROW_FORMAT=COMPACT.
Замечание

These messages are only warnings, not errors, and the table is created as if the options were not specified. When InnoDB “strict mode” (see Section 13.4.8.4, “InnoDB Strict Mode”) is enabled, InnoDB generates an error, not a warning, for these cases. In strict mode, the table is not created if the current configuration does not permit using compressed tables.

The “non-strict” behavior is intended to permit you to import a mysqldump file into a database that does not support compressed tables, even if the source database contained compressed tables. In that case, MySQL creates the table in ROW_FORMAT=COMPACT instead of preventing the operation.

When you import the dump file into a new database, if you want to have the tables re-created as they exist in the original database, ensure the server is running the InnoDB storage engine with the proper settings for the configuration parameters innodb_file_format and innodb_file_per_table,

13.4.3.2.2. SQL Compression Синтаксис Warnings and Ошибки

The attribute KEY_BLOCK_SIZE is permitted only when ROW_FORMAT is specified as COMPRESSED or is omitted. Specifying a KEY_BLOCK_SIZE with any other ROW_FORMAT generates a warning that you can view with SHOW WARNINGS. However, the table is non-compressed; the specified KEY_BLOCK_SIZE is ignored).

LevelCodeMessage
Warning1478 InnoDB: ignoring KEY_BLOCK_SIZE=n unless ROW_FORMAT=COMPRESSED.

If you are running in InnoDB strict mode, the combination of a KEY_BLOCK_SIZE with any ROW_FORMAT other than COMPRESSED generates an error, not a warning, and the table is not created.

Table 13.4, “Meaning of CREATE TABLE and ALTER TABLE options” summarizes how the various options on CREATE TABLE and ALTER TABLE are handled.

Table 13.4. Meaning of CREATE TABLE and ALTER TABLE options

OptionUsageОписание
ROW_FORMAT=​REDUNDANTStorage format used prior to MySQL 5.0.3Less efficient than ROW_FORMAT=COMPACT; for backward compatibility
ROW_FORMAT=​COMPACTDefault storage format since MySQL 5.0.3Stores a prefix of 768 bytes of long column values in the clustered index page, with the remaining bytes stored in an overflow page
ROW_FORMAT=​DYNAMICAvailable only with innodb_file​_format=BarracudaStore values within the clustered index page if they fit; if not, stores only a 20-byte pointer to an overflow page (no prefix)
ROW_FORMAT=​COMPRESSEDAvailable only with innodb_file​_format=BarracudaCompresses the table and indexes using zlib to default compressed page size of 8K bytes; implies ROW_FORMAT=DYNAMIC
KEY_BLOCK_​SIZE=nAvailable only with innodb_file​_format=BarracudaSpecifies compressed page size of 1, 2, 4, 8 or 16K bytes; implies ROW_FORMAT=DYNAMIC and ROW_FORMAT=COMPRESSED

Table 13.5, “CREATE/ALTER TABLE Warnings and Ошибки when InnoDB Strict Mode is OFF” summarizes error conditions that occur with certain combinations of configuration parameters and options on the CREATE TABLE or ALTER TABLE statements, and how the options appear in the output of SHOW TABLE STATUS.

When InnoDB strict mode is OFF, InnoDB creates or alters the table, but may ignore certain settings, as shown below. You can see the warning messages in the MySQL error log. When InnoDB strict mode is ON, these specified combinations of options generate errors, and the table is not created or altered. You can see the full description of the error condition with SHOW ERRORS. For example:

mysql> CREATE TABLE x (id INT PRIMARY KEY, c INT)

-> ENGINE=INNODB KEY_BLOCK_SIZE=33333;

ERROR 1005 (HY000): Can't create table 'test.x' (errno: 1478)

mysql> SHOW ERRORS;
+-------+------+-------------------------------------------+ 
| Level | Code | Message                                   | 
+-------+------+-------------------------------------------+ 
| Error | 1478 | InnoDB: invalid KEY_BLOCK_SIZE=33333.     | 
| Error | 1005 | Can't create table 'test.x' (errno: 1478) | 
+-------+------+-------------------------------------------+ 
2 rows in set (0.00 sec)

Table 13.5. CREATE/ALTER TABLE Warnings and Ошибки when InnoDB Strict Mode is OFF

SyntaxWarning or Error ConditionResulting ROW_FORMAT, as shown in SHOW TABLE STATUS
ROW_FORMAT=REDUNDANTNoneREDUNDANT
ROW_FORMAT=COMPACTNoneCOMPACT
ROW_FORMAT=COMPRESSED or ROW_FORMAT=DYNAMIC or KEY_BLOCK_SIZE is specifiedIgnored unless both innodb_file_format=Barracuda and innodb_file_per_table are enabledCOMPACT
Invalid KEY_BLOCK_SIZE is specified (not 1, 2, 4, 8 or 16)KEY_BLOCK_SIZE is ignoredthe requested one, or COMPACT by default
ROW_FORMAT=COMPRESSED and valid KEY_BLOCK_SIZE are specifiedNone; KEY_BLOCK_SIZE specified is used, not the 8K defaultCOMPRESSED
KEY_BLOCK_SIZE is specified with REDUNDANT, COMPACT or DYNAMIC row formatKEY_BLOCK_SIZE is ignoredREDUNDANT, COMPACT or DYNAMIC
ROW_FORMAT is not one of REDUNDANT, COMPACT, DYNAMIC or COMPRESSEDIgnored if recognized by the MySQL parser. Otherwise, an error is issued.COMPACT or N/A

When InnoDB strict mode is ON (innodb_strict_mode=1), the InnoDB storage engine rejects invalid ROW_FORMAT or KEY_BLOCK_SIZE parameters. For compatibility with earlier versions of InnoDB, strict mode is not enabled by default; instead, InnoDB issues warnings (not errors) for ignored invalid parameters.

Note that it is not possible to see the chosen KEY_BLOCK_SIZE using SHOW TABLE STATUS. The statement SHOW CREATE TABLE displays the KEY_BLOCK_SIZE (even if it was ignored by InnoDB). The real compressed page size inside InnoDB cannot be displayed by MySQL.

13.4.3.3. Tuning InnoDB Compression

Most often, the internal optimizations in InnoDB described in Section 13.4.3.4, “ InnoDB Data Storage and Compression ” ensure that the system runs well with compressed data. However, because the efficiency of compression depends on the nature of your data, there are some factors you should consider to get best performance. You need to choose which tables to compress, and what compressed page size to use. You might also adjust the size of the buffer pool based on run-time performance characteristics, such as the amount of time the system spends compressing and uncompressing data.

When to Use Compression

In general, compression works best on tables that include a reasonable number of character string columns and where the data is read far more often than it is written. Because there are no guaranteed ways to predict whether or not compression benefits a particular situation, always test with a specific workload and data set running on a representative configuration. Consider the following factors when deciding which tables to compress.

Data Characteristics and Compression

A key determinant of the efficiency of compression in reducing the size of data files is the nature of the data itself. Recall that compression works by identifying repeated strings of bytes in a block of data. Completely randomized data is the worst case. Typical data often has repeated values, and so compresses effectively. Character strings often compress well, whether defined in CHAR, VARCHAR, TEXT or BLOB columns. On the other hand, tables containing mostly binary data (integers or floating point numbers) or data that is previously compressed (for example JPEG or PNG images) may not generally compress well, significantly or at all.

You choose whether to turn on compression for each InnoDB tables. A table and all of its indexes use the same (compressed) page size. It might be that the primary key (clustered) index, which contains the data for all columns of a table, compresses more effectively than the secondary indexes. For those cases where there are long rows, the use of compression might result in long column values being stored “off-page”, as discussed in Section 13.4.5.3, “Barracuda File Format: DYNAMIC and COMPRESSED Row Formats”. Those overflow pages may compress well. Given these considerations, for many applications, some tables compress more effectively than others, and you might find that your workload performs best only with a subset of tables compressed.

Experimenting is the only way to determine whether or not to compress a particular table. InnoDB compresses data in 16K chunks corresponding to the uncompressed page size, and in addition to user data, the page format includes some internal system data that is not compressed. Compression utilities compress an entire stream of data, and so may find more repeated strings across the entire input stream than InnoDB would find in a table compressed in 16K chunks. But you can get a sense of how compression efficiency by using a utility that implements LZ77 compression (such as gzip or WinZip) on your data file.

Another way to test compression on a specific table is to copy some data from your uncompressed table to a similar, compressed table (having all the same indexes) and look at the size of the resulting file. When you do so (if nothing else using compression is running), you can examine the ratio of successful compression operations to overall compression operations. (In the INNODB_CMP table, compare COMPRESS_OPS to COMPRESS_OPS_OK. See INNODB_CMP for more information.) If a high percentage of compression operations complete successfully, the table might be a good candidate for compression.

Compression and Application and Schema Design

Decide whether to compress data in your application or in the InnoDB table. It is usually not sensible to store data that is compressed by an application in an InnoDB compressed table. Further compression is extremely unlikely, and the attempt to compress just wastes CPU cycles.

Compressing in the Database

The InnoDB table compression is automatic and applies to all columns and index values. The columns can still be tested with operators such as LIKE, and sort operations can still use indexes even when the index values are compressed. Because indexes are often a significant fraction of the total size of a database, compression could result in significant savings in storage, I/O or processor time. The compression and decompression operations happen on the database server, which likely is a powerful system that is sized to handle the expected load.

Compressing in the Application

If you compress data such as text in your application, before it is inserted into the database, You might save overhead for data that does not compress well by compressing some columns and not others. This approach uses CPU cycles for compression and uncompression on the client machine rather than the database server, which might be appropriate for a distributed application with many clients, or where the client machine has spare CPU cycles.

Hybrid Approach

Of course, it is possible to combine these approaches. For some applications, it may be appropriate to use some compressed tables and some uncompressed tables. It may be best to externally compress some data (and store it in uncompressed InnoDB tables) and allow InnoDB to compress (some of) the other tables in the application. As always, up-front design and real-life testing are valuable in reaching the right decision.

Workload Characteristics and Compression

In addition to choosing which tables to compress (and the page size), the workload is another key determinant of performance. If the application is dominated by reads, rather than updates, fewer pages need to be reorganized and recompressed after the index page runs out of room for the per-page “modification log” that InnoDB maintains for compressed data. If the updates predominantly change non-indexed columns or those containing BLOBs or large strings that happen to be stored “off-page”, the overhead of compression may be acceptable. If the only changes to a table are INSERTs that use a monotonically increasing primary key, and there are few secondary indexes, there is little need to reorganize and recompress index pages. Since InnoDB can “delete-mark” and delete rows on compressed pages “in place” by modifying uncompressed data, DELETE operations on a table are relatively efficient.

For some environments, the time it takes to load data can be as important as run-time retrieval. Especially in data warehouse environments, many tables may be read-only or read-mostly. In those cases, it might or might not be acceptable to pay the price of compression in terms of increased load time, unless the resulting savings in fewer disk reads or in storage cost is significant.

Fundamentally, compression works best when the CPU time is available for compressing and uncompressing data. Thus, if your workload is I/O bound, rather than CPU-bound, you might find that compression can improve overall performance. When you test your application performance with different compression configurations, test on a platform similar to the planned configuration of the production system.

Configuration Characteristics and Compression

Reading and writing database pages from and to disk is the slowest aspect of system performance. Compression attempts to reduce I/O by using CPU time to compress and uncompress data, and is most effective when I/O is a relatively scarce resource compared to processor cycles.

This is often especially the case when running in a multi-user environment with fast, multi-core CPUs. When a page of a compressed table is in memory, InnoDB often uses an additional 16K in the buffer pool for an uncompressed copy of the page. The adaptive LRU algorithm in the InnoDB storage engine attempts to balance the use of memory between compressed and uncompressed pages to take into account whether the workload is running in an I/O-bound or CPU-bound manner. Still, a configuration with more memory dedicated to the InnoDB buffer pool tends to run better when using compressed tables than a configuration where memory is highly constrained.

Choosing the Compressed Page Size

The optimal setting of the compressed page size depends on the type and distribution of data that the table and its indexes contain. The compressed page size should always be bigger than the maximum record size, or operations may fail as noted in Section 13.4.3.4, “ Compression of B-Tree Pages ”.

Setting the compressed page size too large wastes some space, but the pages do not have to be compressed as often. If the compressed page size is set too small, inserts or updates may require time-consuming recompression, and the B-tree nodes may have to be split more frequently, leading to bigger data files and less efficient indexing.

Typically, you set the compressed page size to 8K or 4K bytes. Given that the maximum InnoDB record size is around 8K, KEY_BLOCK_SIZE=8 is usually a safe choice.

Monitoring Compression at Runtime

Overall application performance, CPU and I/O utilization and the size of disk files are good indicators of how effective compression is for your application.

To dig deeper into performance considerations for compressed tables, you can monitor compression performance at run time. using the Information Schema tables described in Пример 13.1, “Using the Compression Information Schema Tables”. These tables reflect the internal use of memory and the rates of compression used overall.

The INNODB_CMP tables report information about compression activity for each compressed page size (KEY_BLOCK_SIZE) in use. The information in these tables is system-wide, and includes summary data across all compressed tables in your database. You can use this data to help decide whether or not to compress a table by examining these tables when no other compressed tables are being accessed.

The key statistics to consider are the number of, and amount of time spent performing, compression and uncompression operations. Since InnoDB must split B-tree nodes when they are too full to contain the compressed data following a modification, compare the number of “successful” compression operations with the number of such operations overall. Based on the information in the INNODB_CMP tables and overall application performance and hardware resource utilization, you might make changes in your hardware configuration, adjust the size of the InnoDB buffer pool, choose a different page size, or select a different set of tables to compress.

If the amount of CPU time required for compressing and uncompressing is high, changing to faster CPUs, or those with more cores, can help improve performance with the same data, application workload and set of compressed tables. Increasing the size of the InnoDB buffer pool might also help performance, so that more uncompressed pages can stay in memory, reducing the need to uncompress pages that exist in memory only in compressed form.

A large number of compression operations overall (compared to the number of INSERT, UPDATE and DELETE operations in your application and the size of the database) could indicate that some of your compressed tables are being updated too heavily for effective compression. If so, choose a larger page size, or be more selective about which tables you compress.

If the number of “successful” compression operations (COMPRESS_OPS_OK) is a high percentage of the total number of compression operations (COMPRESS_OPS), then the system is likely performing well. If the ratio is low, then InnoDB is reorganizing, recompressing, and splitting B-tree nodes more often than is desirable. In this case, avoid compressing some tables, or increase KEY_BLOCK_SIZE for some of the compressed tables. You might turn off compression for tables that cause the number of “compression failures” in your application to be more than 1% or 2% of the total. (Such a failure ratio might be acceptable during a temporary operation such as a data load).

13.4.3.4. How Compression Works in InnoDB

This section describes some internal implementation details about compression in InnoDB. The information presented here may be helpful in tuning for performance, but is not necessary to know for basic use of compression.

Compression Algorithms

Some operating systems implement compression at the file system level. Files are typically divided into fixed-size blocks that are compressed into variable-size blocks, which easily leads into fragmentation. Every time something inside a block is modified, the whole block is recompressed before it is written to disk. These properties make this compression technique unsuitable for use in an update-intensive database system.

InnoDB implements compression with the help of the well-known zlib library, which implements the LZ77 compression algorithm. This compression algorithm is mature, robust, and efficient in both CPU utilization and in reduction of data size. The algorithm is “lossless”, so that the original uncompressed data can always be reconstructed from the compressed form. LZ77 compression works by finding sequences of data that are repeated within the data to be compressed. The patterns of values in your data determine how well it compresses, but typical user data often compresses by 50% or more.

Unlike compression performed by an application, or compression features of some other database management systems, InnoDB compression applies both to user data and to indexes. In many cases, indexes can constitute 40-50% or more of the total database size, so this difference is significant. When compression is working well for a data set, the size of the InnoDB data files (the .idb files) is 25% to 50% of the uncompressed size or possibly smaller. Depending on the workload, this smaller database can in turn lead to a reduction in I/O, and an increase in throughput, at a modest cost in terms of increased CPU utilization.

InnoDB Data Storage and Compression

All user data in InnoDB is stored in pages comprising a B-tree index (the clustered index). In some other database systems, this type of index is called an “index-organized table”. Each row in the index node contains the values of the (user-specified or system-generated) primary key and all the other columns of the table.

Secondary indexes in InnoDB are also B-trees, containing pairs of values: the index key and a pointer to a row in the clustered index. The pointer is in fact the value of the primary key of the table, which is used to access the clustered index if columns other than the index key and primary key are required. Secondary index records must always fit on a single B-tree page.

The compression of B-tree nodes (of both clustered and secondary indexes) is handled differently from compression of overflow pages used to store long VARCHAR, BLOB, or TEXT columns, as explained in the following sections.

Compression of B-Tree Pages

Because they are frequently updated, B-tree pages require special treatment. It is important to minimize the number of times B-tree nodes are split, as well as to minimize the need to uncompress and recompress their content.

One technique InnoDB uses is to maintain some system information in the B-tree node in uncompressed form, thus facilitating certain in-place updates. For example, this allows rows to be delete-marked and deleted without any compression operation.

In addition, InnoDB attempts to avoid unnecessary uncompression and recompression of index pages when they are changed. Within each B-tree page, the system keeps an uncompressed “modification log” to record changes made to the page. Updates and inserts of small records may be written to this modification log without requiring the entire page to be completely reconstructed.

When the space for the modification log runs out, InnoDB uncompresses the page, applies the changes and recompresses the page. If recompression fails, the B-tree nodes are split and the process is repeated until the update or insert succeeds.

Generally, InnoDB requires that each B-tree page can accommodate at least two records. For compressed tables, this requirement has been relaxed. Leaf pages of B-tree nodes (whether of the primary key or secondary indexes) only need to accommodate one record, but that record must fit in uncompressed form, in the per-page modification log. Starting with InnoDB storage engine version 1.0.2, and if InnoDB strict mode is ON, the InnoDB storage engine checks the maximum row size during CREATE TABLE or CREATE INDEX. If the row does not fit, the following error message is issued: ERROR HY000: Too big row.

If you create a table when InnoDB strict mode is OFF, and a subsequent INSERT or UPDATE statement attempts to create an index entry that does not fit in the size of the compressed page, the operation fails with ERROR 42000: Row size too large. (This error message does not name the index for which the record is too large, or mention the length of the index record or the maximum record size on that particular index page.) To solve this problem, rebuild the table with ALTER TABLE and select a larger compressed page size (KEY_BLOCK_SIZE), shorten any column prefix indexes, or disable compression entirely with ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPACT.

Compressing BLOB, VARCHAR and TEXT Columns

In a clustered index, BLOB, VARCHAR and TEXT columns that are not part of the primary key may be stored on separately allocated (“overflow”) pages. We call these off-page columns whose values are stored on singly-linked lists of overflow pages.

For tables created in ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED, the values of BLOB, TEXT or VARCHAR columns may be stored fully off-page, depending on their length and the length of the entire row. For columns that are stored off-page, the clustered index record only contains 20-byte pointers to the overflow pages, one per column. Whether any columns are stored off-page depends on the page size and the total size of the row. When the row is too long to fit entirely within the page of the clustered index, InnoDB chooses the longest columns for off-page storage until the row fits on the clustered index page. As noted above, if a row does not fit by itself on a compressed page, an error occurs.

Tables created in previous versions of InnoDB use the Antelope file format, which supports only ROW_FORMAT=REDUNDANT and ROW_FORMAT=COMPACT. In these formats, InnoDB stores the first 768 bytes of BLOB, VARCHAR and TEXT columns in the clustered index record along with the primary key. The 768-byte prefix is followed by a 20-byte pointer to the overflow pages that contain the rest of the column value.

When a table is in COMPRESSED format, all data written to overflow pages is compressed “as is”; that is, InnoDB applies the zlib compression algorithm to the entire data item. Other than the data, compressed overflow pages contain an uncompressed header and trailer comprising a page checksum and a link to the next overflow page, among other things. Therefore, very significant storage savings can be obtained for longer BLOB, TEXT or VARCHAR columns if the data is highly compressible, as is often the case with text data (but not previously compressed images).

The overflow pages are of the same size as other pages. A row containing ten columns stored off-page occupies ten overflow pages, even if the total length of the columns is only 8K bytes. In an uncompressed table, ten uncompressed overflow pages occupy 160K bytes. In a compressed table with an 8K page size, they occupy only 80K bytes. Thus, it is often more efficient to use compressed table format for tables with long column values.

Using a 16K compressed page size can reduce storage and I/O costs for BLOB, VARCHAR or TEXT columns, because such data often compress well, and might therefore require fewer “overflow” pages, even though the B-tree nodes themselves take as many pages as in the uncompressed form.

Compression and the InnoDB Buffer Pool

In a compressed InnoDB table, every compressed page (whether 1K, 2K, 4K or 8K) corresponds to an uncompressed page of 16K bytes. To access the data in a page, InnoDB reads the compressed page from disk if it is not already in the buffer pool, then uncompresses the page to its original 16K byte form. This section describes how InnoDB manages the buffer pool with respect to pages of compressed tables.

To minimize I/O and to reduce the need to uncompress a page, at times the buffer pool contains both the compressed and uncompressed form of a database page. To make room for other required database pages, InnoDB may “evict” from the buffer pool an uncompressed page, while leaving the compressed page in memory. Or, if a page has not been accessed in a while, the compressed form of the page may be written to disk, to free space for other data. Thus, at any given time, the buffer pool may contain both the compressed and uncompressed forms of the page, or only the compressed form of the page, or neither.

InnoDB keeps track of which pages to keep in memory and which to evict using a least-recently-used (LRU) list, so that “hot” or frequently accessed data tends to stay in memory. When compressed tables are accessed, InnoDB uses an adaptive LRU algorithm to achieve an appropriate balance of compressed and uncompressed pages in memory. This adaptive algorithm is sensitive to whether the system is running in an I/O-bound or CPU-bound manner. The goal is to avoid spending too much processing time uncompressing pages when the CPU is busy, and to avoid doing excess I/O when the CPU has spare cycles that can be used for uncompressing compressed pages (that may already be in memory). When the system is I/O-bound, the algorithm prefers to evict the uncompressed copy of a page rather than both copies, to make more room for other disk pages to become memory resident. When the system is CPU-bound, InnoDB prefers to evict both the compressed and uncompressed page, so that more memory can be used for “hot” pages and reducing the need to uncompress data in memory only in compressed form.

Compression and the InnoDB Log Files

Before a compressed page is written to a database file, InnoDB writes a copy of the page to the redo log (if it has been recompressed since the last time it was written to the database). This is done to ensure that redo logs will always be usable, even if a future version of InnoDB uses a slightly different compression algorithm. Therefore, some increase in the size of log files, or a need for more frequent checkpoints, can be expected when using compression. The amount of increase in the log file size or checkpoint frequency depends on the number of times compressed pages are modified in a way that requires reorganization and recompression.

Note that the redo log file format (and the database file format) are different from previous releases when using compression. The MySQL Enterprise Backup product does support this latest Barracuda file format for compressed InnoDB tables. The older InnoDB Hot Backup product can only back up tables using the file format Antelope, and thus does not support InnoDB tables that use compression.

13.4.4. InnoDB File-Format Management

As InnoDB evolves, new on-disk data structures are sometimes required to support new features. Features such as compressed tables (see Section 13.4.3, “InnoDB Data Compression”), and long variable-length columns stored off-page (see Section 13.4.5, “How InnoDB Stores Variable-Length Columns”) require data file formats that are not compatible with prior versions of InnoDB. These features both require use of the new Barracuda file format.

Замечание

All other new features are compatible with the original Antelope file format and do not require the Barracuda file format.

This section discusses how to specify the file format for new InnoDB tables, compatibility of different file formats between MySQL releases,

Named File Formats.  InnoDB 1.1 has the idea of a named file format and a configuration parameter to enable the use of features that require use of that format. The new file format is the Barracuda format, and the original InnoDB file format is called Antelope. Compressed tables and the new row format that stores long columns “off-page” require the use of the Barracuda file format or newer. Future versions of InnoDB may introduce a series of file formats, identified with the names of animals, in ascending alphabetic order.

13.4.4.1. Enabling File Formats

The configuration parameter innodb_file_format controls whether such statements as CREATE TABLE and ALTER TABLE can be used to create tables that depend on support for the Barracuda file format.

Although Oracle recommends using the Barracuda format for new tables where practical, in MySQL 5.5 the default file format is still Antelope, for maximum compatibility with replication configurations containing different MySQL releases.

The file format is a dynamic, global parameter that can be specified in the MySQL option file (my.cnf or my.ini) or changed with the SET GLOBAL command.

13.4.4.2. Verifying File Format Compatibility

InnoDB 1.1 incorporates several checks to guard against the possible crashes and data corruptions that might occur if you run an older release of the MySQL server on InnoDB data files using a newer file format. These checks take place when the server is started, and when you first access a table. This section describes these checks, how you can control them, and error and warning conditions that might arise.

Backward Compatibility

Considerations of backward compatibility only apply when using a recent version of InnoDB (the InnoDB Plugin, or MySQL 5.5 and higher with InnoDB 1.1) alongside an older one (MySQL 5.1 or earlier, with the built-in InnoDB rather than the InnoDB Plugin). To minimize the chance of compatibility issues, you can standardize on the InnoDB Plugin for all your MySQL 5.1 and earlier database servers.

In general, a newer version of InnoDB may create a table or index that cannot safely be read or written with a prior version of InnoDB without risk of crashes, hangs, wrong results or corruptions. InnoDB 1.1 includes a mechanism to guard against these conditions, and to help preserve compatibility among database files and versions of InnoDB. This mechanism lets you take advantage of some new features of an InnoDB release (such as performance improvements and bug fixes), and still preserve the option of using your database with a prior version of InnoDB, by preventing accidental use of new features that create downward-incompatible disk files.

If a version of InnoDB supports a particular file format (whether or not that format is the default), you can query and update any table that requires that format or an earlier format. Only the creation of new tables using new features is limited based on the particular file format enabled. Conversely, if a tablespace contains a table or index that uses a file format that is not supported by the currently running software, it cannot be accessed at all, even for read access.

The only way to “downgrade” an InnoDB tablespace to an earlier file format is to copy the data to a new table, in a tablespace that uses the earlier format. This can be done with the ALTER TABLE statement, as described in Section 13.4.4.4, “Downgrading the File Format”.

The easiest way to determine the file format of an existing InnoDB tablespace is to examine the properties of the table it contains, using the SHOW TABLE STATUS command or querying the table INFORMATION_SCHEMA.TABLES. If the Row_format of the table is reported as 'Compressed' or 'Dynamic', the tablespace containing the table uses the Barracuda format. Otherwise, it uses the prior InnoDB file format, Antelope.

Internal Details

Every InnoDB per-table tablespace (represented by a *.ibd file) file is labeled with a file format identifier. The system tablespace (represented by the ibdata files) is tagged with the “highest” file format in use in a group of InnoDB database files, and this tag is checked when the files are opened.

Creating a compressed table, or a table with ROW_FORMAT=DYNAMIC, updates the file header for the corresponding .ibd file and the table type in the InnoDB data dictionary with the identifier for the Barracuda file format. From that point forward, the table cannot be used with a version of InnoDB that does not support this new file format. To protect against anomalous behavior, InnoDB version 5.0.21 and later performs a compatibility check when the table is opened. (In many cases, the ALTER TABLE statement recreates a table and thus changes its properties. The special case of adding or dropping indexes without rebuilding the table is described in Section 13.4.2, “Fast Index Creation in the InnoDB Storage Engine”.)

Definition of ib-file set

To avoid confusion, for the purposes of this discussion we define the term “ib-file set” to mean the set of operating system files that InnoDB manages as a unit. The ib-file set includes the following files:

  • The system tablespace (one or more ibdata files) that contain internal system information (including internal catalogs and undo information) and may include user data and indexes.

  • Zero or more single-table tablespaces (also called “file per table” files, named *.ibd files).

  • InnoDB log files; usually two, ib_logfile0 and ib_logfile1. Used for crash recovery and in backups.

An “ib-file set” does not include the corresponding .frm files that contain metadata about InnoDB tables. The .frm files are created and managed by MySQL, and can sometimes get out of sync with the internal metadata in InnoDB.

Multiple tables, even from more than one database, can be stored in a single “ib-file set”. (In MySQL, a “database” is a logical collection of tables, what other systems refer to as a “schema” or “catalog”.)

13.4.4.2.1. Compatibility Check When InnoDB Is Started

To prevent possible crashes or data corruptions when InnoDB opens an ib-file set, it checks that it can fully support the file formats in use within the ib-file set. If the system is restarted following a crash, or a “fast shutdown” (i.e., innodb_fast_shutdown is greater than zero), there may be on-disk data structures (such as redo or undo entries, or doublewrite pages) that are in a “too-new” format for the current software. During the recovery process, serious damage can be done to your data files if these data structures are accessed. The startup check of the file format occurs before any recovery process begins, thereby preventing consistency issues with the new tables or startup problems for the MySQL server.

Beginning with version InnoDB 1.0.1, the system tablespace records an identifier or tag for the “highest” file format used by any table in any of the tablespaces that is part of the ib-file set. Checks against this file format tag are controlled by the configuration parameter innodb_file_format_check, which is ON by default.

If the file format tag in the system tablespace is newer or higher than the highest version supported by the particular currently executing software and if innodb_file_format_check is ON, the following error is issued when the server is started:

InnoDB: Error: the system tablespace is in a
file format that this version doesn't support

You can also set innodb_file_format to a file format name. Doing so prevents InnoDB from starting if the current software does not support the file format specified. It also sets the “high water mark” to the value you specify. The ability to set innodb_file_format_check will be useful (with future releases of InnoDB) if you manually “downgrade” all of the tables in an ib-file set (as described in Section 13.4.11, “Downgrading the InnoDB Storage Engine”). You can then rely on the file format check at startup if you subsequently use an older version of InnoDB to access the ib-file set.

In some limited circumstances, you might want to start the server and use an ib-file set that is in a “too new” format (one that is not supported by the software you are using). If you set the configuration parameter innodb_file_format_check to OFF, InnoDB opens the database, but issues this warning message in the error log:

InnoDB: Warning: the system tablespace is in a
file format that this version doesn't support
Замечание

This is a very dangerous setting, as it permits the recovery process to run, possibly corrupting your database if the previous shutdown was a crash or “fast shutdown”. You should only set innodb_file_format_check to OFF if you are sure that the previous shutdown was done with innodb_fast_shutdown=0, so that essentially no recovery process occurs. In a future release, this parameter setting may be renamed from OFF to UNSAFE. (However, until there are newer releases of InnoDB that support additional file formats, even disabling the startup checking is in fact “safe”.)

The parameter innodb_file_format_check affects only what happens when a database is opened, not subsequently. Conversely, the parameter innodb_file_format (which enables a specific format) only determines whether or not a new table can be created in the enabled format and has no effect on whether or not a database can be opened.

The file format tag is a “high water mark”, and as such it is increased after the server is started, if a table in a “higher” format is created or an existing table is accessed for read or write (assuming its format is supported). If you access an existing table in a format higher than the format the running software supports, the system tablespace tag is not updated, but table-level compatibility checking applies (and an error is issued), as described in Section 13.4.4.2.2, “Compatibility Check When a Table Is Opened”. Any time the high water mark is updated, the value of innodb_file_format_check is updated as well, so the command SELECT @@innodb_file_format_check; displays the name of the newest file format known to be used by tables in the currently open ib-file set and supported by the currently executing software.

To best illustrate this behavior, consider the scenario described in Table 13.6, “InnoDB Data File Compatibility and Related InnoDB Parameters”. Imagine that some future version of InnoDB supports the Cheetah format and that an ib-file set has been used with that version.

Table 13.6. InnoDB Data File Compatibility and Related InnoDB Parameters

innodb file format checkinnodb file formatHighest file format used in ib-file setHighest file format supported by InnoDBResult
OFFAntelope or BarracudaBarracudaBarracudaDatabase can be opened; tables can be created which require Antelope or Barracuda file format
OFFAntelope or BarracudaCheetahBarracudaDatabase can be opened with a warning, since the database contains files in a “too new” format; tables can be created in Antelope or Barracuda file format; tables in Cheetah format cannot be accessed
OFFCheetahBarracudaBarracudaDatabase cannot be opened; innodb_file_format cannot be set to Cheetah
ONAntelope or BarracudaBarracudaBarracudaDatabase can be opened; tables can be created in Antelope or Barracuda file format
ONAntelope or BarracudaCheetahBarracudaDatabase cannot be opened, since the database contains files in a “too new” format (Cheetah)
ONCheetahBarracudaBarracudaDatabase cannot be opened; innodb_file_format cannot be set to Cheetah
13.4.4.2.2. Compatibility Check When a Table Is Opened

When a table is first accessed, InnoDB (including some releases prior to InnoDB 1.0) checks that the file format of the tablespace in which the table is stored is fully supported. This check prevents crashes or corruptions that would otherwise occur when tables using a “too new” data structure are encountered.

All tables using any file format supported by a release can be read or written (assuming the user has sufficient privileges). The setting of the system configuration parameter innodb_file_format can prevent creating a new table that uses specific file formats, even if they are supported by a given release. Such a setting might be used to preserve backward compatibility, but it does not prevent accessing any table that uses any supported format.

As noted in Named File Formats, versions of MySQL older than 5.0.21 cannot reliably use database files created by newer versions if a new file format was used when a table was created. To prevent various error conditions or corruptions, InnoDB checks file format compatibility when it opens a file (for example, upon first access to a table). If the currently running version of InnoDB does not support the file format identified by the table type in the InnoDB data dictionary, MySQL reports the following error:

ERROR 1146 (42S02): Table 'test.t1' doesn't exist

InnoDB also writes a message to the error log:

InnoDB: table test/t1: unknown table type 33

The table type should be equal to the tablespace flags, which contains the file format version as discussed in Section 13.4.4.3, “Identifying the File Format in Use”.

Versions of InnoDB prior to MySQL 4.1 did not include table format identifiers in the database files, and versions prior to MySQL 5.0.21 did not include a table format compatibility check. Therefore, there is no way to ensure proper operations if a table in a “too new” format is used with versions of InnoDB prior to 5.0.21.

The file format management capability in InnoDB 1.0 and higher (tablespace tagging and run-time checks) allows InnoDB to verify as soon as possible that the running version of software can properly process the tables existing in the database.

If you permit InnoDB to open a database containing files in a format it does not support (by setting the parameter innodb_file_format_check to OFF), the table-level checking described in this section still applies.

Users are strongly urged not to use database files that contain Barracuda file format tables with releases of InnoDB older than the MySQL 5.1 with the InnoDB Plugin. It is possible to “downgrade” such tables to the Antelope format with the procedure described in Section 13.4.4.4, “Downgrading the File Format”.

13.4.4.3. Identifying the File Format in Use

After you enable a given innodb_file_format, this change applies only to newly created tables rather than existing ones. If you do create a new table, the tablespace containing the table is tagged with the “earliest” or “simplest” file format that is required for the table's features. For example, if you enable file format Barracuda, and create a new table that is not compressed and does not use ROW_FORMAT=DYNAMIC, the new tablespace that contains the table is tagged as using file format Antelope.

It is easy to identify the file format used by a given tablespace or table. The table uses the Barracuda format if the Row_format reported by SHOW CREATE TABLE or INFORMATION_SCHEMA.TABLES is one of 'Compressed' or 'Dynamic'. (The Row_format is a separate column; ignore the contents of the Create_options column, which may contain the string ROW_FORMAT.) If the table in a tablespace uses neither of those features, the file uses the format supported by prior releases of InnoDB, now called file format Antelope. Then, the Row_format is one of 'Redundant' or 'Compact'.

Internal Details

The file format identifier is written as part of the tablespace flags (a 32-bit number) in the *.ibd file in the 4 bytes starting at position 54 of the file, most significant byte first. (The first byte of the file is byte zero.) On some systems, you can display these bytes in hexadecimal with the command od -t x1 -j 54 -N 4 tablename.ibd. If all bytes are zero, the tablespace uses the Antelope file format (which is the format used by the standard InnoDB storage engine up to version 5.1). Otherwise, the least significant bit should be set in the tablespace flags, and the file format identifier is written in the bits 5 through 11. (Divide the tablespace flags by 32 and take the remainder after dividing the integer part of the result by 128.)

13.4.4.4. Downgrading the File Format

Each InnoDB tablespace file (with a name matching *.ibd) is tagged with the file format used to create its table and indexes. The way to downgrade the tablespace is to re-create the table and its indexes. The easiest way to recreate a table and its indexes is to use the command:

ALTER TABLE t ROW_FORMAT=COMPACT;

on each table that you want to downgrade. The COMPACT row format uses the file format Antelope. It was introduced in MySQL 5.0.3.

13.4.4.5. Future InnoDB File Formats

The file format used by the standard built-in InnoDB in MySQL 5.1 is the Antelope format. The file format introduced with InnoDB Plugin 1.0 is the Barracuda format. Although no new features have been announced that would require additional new file formats, the InnoDB file format mechanism allows for future enhancements.

For the sake of completeness, these are the file format names that might be used for future file formats: Antelope, Barracuda, Cheetah, Dragon, Elk, Fox, Gazelle, Hornet, Impala, Jaguar, Kangaroo, Leopard, Moose, Nautilus, Ocelot, Porpoise, Quail, Rabbit, Shark, Tiger, Urchin, Viper, Whale, Xenops, Yak and Zebra. These file formats correspond to the internal identifiers 0..25.

13.4.5. How InnoDB Stores Variable-Length Columns

This section discusses how certain InnoDB features, such as table compression and off-page storage of long columns, are controlled by the ROW_FORMAT clause of the CREATE TABLE statement. It discusses considerations for choosing the right row format and compatibility of row formats between MySQL releases.

13.4.5.1. Overview of InnoDB Row Storage

The storage for rows and associated columns affects performance for queries and DML operations. As more rows fit into a single disk page, queries and index lookups can work faster, less cache memory is required in the InnoDB buffer pool, and less I/O is required to write out updated values for the numeric and short string columns.

All data in InnoDB is stored in database pages that make up a B-tree index (the clustered index organized according to the primary key columns). Table data and indexes both use this type of structure. The nodes of the index data structure contain the values of all the columns in that row (for the clustered index) or the index columns and the primary key columns (for secondary indexes).

Variable-length columns are an exception to this rule. Columns such as BLOB and VARCHAR that are too long to fit on a B-tree page are stored on separately allocated disk pages called overflow pages. We call such columns off-page column. The values of these columns are stored on singly-linked lists of overflow pages, and each such column has its own list of one or more overflow pages. In some cases, all or a prefix of the long column value is stored in the B-tree, to avoid wasting storage and eliminating the need to read a separate page.

The Barracuda file format provides a new option (KEY_BLOCK_SIZE) to control how much column data is stored in the clustered index, and how much is placed on overflow pages.

13.4.5.2. Specifying the Row Format for a Table

You specify the row format for a table with the ROW_FORMAT clause of the CREATE TABLE and ALTER TABLE statements.

13.4.5.3. Barracuda File Format: DYNAMIC and COMPRESSED Row Formats

When innodb_file_format is set to Barracuda and a table is created with ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED, long column values are stored fully off-page, and the clustered index record contains only a 20-byte pointer to the overflow page.

Whether any columns are stored off-page depends on the page size and the total size of the row. When the row is too long, InnoDB chooses the longest columns for off-page storage until the clustered index record fits on the B-tree page.

The DYNAMIC row format maintains the efficiency of storing the entire row in the index node if it fits (as do the COMPACT and REDUNDANT formats), but this new format avoids the problem of filling B-tree nodes with a large number of data bytes of long columns. The DYNAMIC format is based on the idea that if a portion of a long data value is stored off-page, it is usually most efficient to store all of the value off-page. With DYNAMIC format, shorter columns are likely to remain in the B-tree node, minimizing the number of overflow pages needed for any given row.

The COMPRESSED row format uses similar internal details for off-page storage as the DYNAMIC row format, with additional storage and performance considerations from the table and index data being compressed and using smaller page sizes. For full details about the COMPRESSED row format, see Section 13.4.3, “InnoDB Data Compression”.

13.4.5.4. Antelope File Format: COMPACT and REDUNDANT Row Formats

Early versions of InnoDB used an unnamed file format (now called Antelope) for database files. With that format, tables were defined with ROW_FORMAT=COMPACT (or ROW_FORMAT=REDUNDANT) and InnoDB stored up to the first 768 bytes of variable-length columns (such as BLOB and VARCHAR) in the index record within the B-tree node, with the remainder stored on the overflow pages.

To preserve compatibility with those prior versions, tables created with the newest InnoDB use the prefix format, unless one of ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED is specified (or implied) on the CREATE TABLE statement.

With the Antelope file format, if the value of a column is 768 bytes or less, no overflow page is needed, and some savings in I/O may result, since the value is in the B-tree node. This works well for relatively short BLOBs, but may cause B-tree nodes to fill with data rather than key values, reducing their efficiency. Tables with many BLOB columns could cause B-tree nodes to become too full of data, and contain too few rows, making the entire index less efficient than if the rows were shorter or if the column values were stored off-page.

13.4.6. InnoDB INFORMATION_SCHEMA tables

The INFORMATION_SCHEMA is a MySQL feature that helps you monitor server activity to diagnose capacity and performance issues. Several InnoDB-related INFORMATION_SCHEMA tables (INNODB_CMP, INNODB_CMP_RESET, INNODB_CMPMEM, INNODB_CMPMEM_RESET, INNODB_TRX, INNODB_LOCKS and INNODB_LOCK_WAITS) contain live information about compressed InnoDB tables, the compressed InnoDB buffer pool, all transactions currently executing inside InnoDB, the locks that transactions hold and those that are blocking transactions waiting for access to a resource (a table or row).

The Information Schema tables are themselves plugins to the MySQL server, and must be activated by INSTALL statements. If they are installed, but the InnoDB storage engine plugin is not installed, these tables appear to be empty.

This section describes the InnoDB-related Information Schema tables and shows some examples of their use.

13.4.6.1. Information Schema Tables about Compression

Two new pairs of Information Schema tables provided by the InnoDB storage engine can give you some insight into how well compression is working overall. One pair of tables contains information about the number of compression operations and the amount of time spent performing compression. Another pair of tables contains information on the way memory is allocated for compression.

13.4.6.1.1. INNODB_CMP and INNODB_CMP_RESET

The tables INNODB_CMP and INNODB_CMP_RESET contain status information on the operations related to compressed tables, which are covered in Section 13.4.3, “InnoDB Data Compression”. The compressed page size is in the column PAGE_SIZE.

These two tables have identical contents, but reading from INNODB_CMP_RESET resets the statistics on compression and uncompression operations. For example, if you archive the output of INNODB_CMP_RESET every 60 minutes, you see the statistics for each hourly period. If you monitor the output of INNODB_CMP (making sure never to read INNODB_CMP_RESET), you see the cumulated statistics since InnoDB was started.

For the table definition, see Table 19.1, “Columns of INNODB_CMP and INNODB_CMP_RESET.

13.4.6.1.2. INNODB_CMPMEM and INNODB_CMPMEM_RESET

The tables INNODB_CMPMEM and INNODB_CMPMEM_RESET contain status information on the compressed pages that reside in the buffer pool. Please consult Section 13.4.3, “InnoDB Data Compression” for further information on compressed tables and the use of the buffer pool. The tables INNODB_CMP and INNODB_CMP_RESET should provide more useful statistics on compression.

Internal Details

The InnoDB storage engine uses a so-called “buddy allocator” system to manage memory allocated to pages of various sizes, from 1KB to 16KB. Each row of the two tables described here corresponds to a single page size.

These two tables have identical contents, but reading from INNODB_CMPMEM_RESET resets the statistics on relocation operations. For example, if every 60 minutes you archived the output of INNODB_CMPMEM_RESET, it would show the hourly statistics. If you never read INNODB_CMPMEM_RESET and monitored the output of INNODB_CMPMEM instead, it would show the cumulated statistics since InnoDB was started.

For the table definition, see Table 19.2, “Columns of INNODB_CMPMEM and INNODB_CMPMEM_RESET”.

13.4.6.1.3. Using the Compression Information Schema Tables

Пример 13.1. Using the Compression Information Schema Tables

The following is sample output from a database that contains compressed tables (see Section 13.4.3, “InnoDB Data Compression”, INNODB_CMP, and INNODB_CMPMEM).

The following table shows the contents of INFORMATION_SCHEMA.INNODB_CMP under light load. The only compressed page size that the buffer pool contains is 8K. Compressing or uncompressing pages has consumed less than a second since the time the statistics were reset, because the columns COMPRESS_TIME and UNCOMPRESS_TIME are zero.

page sizecompress opscompress ops okcompress timeuncompress opsuncompress time
102400000
204800000
409600000
819210489210610
1638400000

According to INNODB_CMPMEM, there are 6169 compressed 8KB pages in the buffer pool.

The following table shows the contents of INFORMATION_SCHEMA.INNODB_CMPMEM under light load. We can see that some memory is unusable due to fragmentation of the InnoDB memory allocator for compressed pages: SUM(PAGE_SIZE*PAGES_FREE)=6784. This is because small memory allocation requests are fulfilled by splitting bigger blocks, starting from the 16K blocks that are allocated from the main buffer pool, using the buddy allocation system. The fragmentation is this low, because some allocated blocks have been relocated (copied) to form bigger adjacent free blocks. This copying of SUM(PAGE_SIZE*RELOCATION_OPS) bytes has consumed less than a second (SUM(RELOCATION_TIME)=0).

page sizepages usedpages freerelocation opsrelocation time
10240000
20480100
40960100
81926169050
163840000

13.4.6.2. Information Schema Tables about Transactions

Three InnoDB-related Information Schema tables make it easy to monitor transactions and diagnose possible locking problems. The three tables are INNODB_TRX, INNODB_LOCKS and INNODB_LOCK_WAITS.

13.4.6.2.1. INNODB_TRX

Contains information about every transaction currently executing inside InnoDB, including whether the transaction is waiting for a lock, when the transaction started, and the particular SQL statement the transaction is executing.

For the table definition, see Table 19.3, “INNODB_TRX Columns”.

13.4.6.2.2. INNODB_LOCKS

Each transaction in InnoDB that is waiting for another transaction to release a lock (INNODB_TRX.TRX_STATE='LOCK WAIT') is blocked by exactly one “blocking lock request”. That blocking lock request is for a row or table lock held by another transaction in an incompatible mode. The waiting or blocked transaction cannot proceed until the other transaction commits or rolls back, thereby releasing the requested lock. For every blocked transaction, INNODB_LOCKS contains one row that describes each lock the transaction has requested, and for which it is waiting. INNODB_LOCKS also contains one row for each lock that is blocking another transaction, whatever the state of the transaction that holds the lock ('RUNNING', 'LOCK WAIT', 'ROLLING BACK' or 'COMMITTING'). The lock that is blocking a transaction is always held in a mode (read vs. write, shared vs. exclusive) incompatible with the mode of requested lock.

For the table definition, see Table 19.4, “INNODB_LOCKS Columns”.

13.4.6.2.3. INNODB_LOCK_WAITS

Using this table, you can tell which transactions are waiting for a given lock, or for which lock a given transaction is waiting. This table contains one or more rows for each blocked transaction, indicating the lock it has requested and the lock(s) that is (are) blocking that request. The REQUESTED_LOCK_ID refers to the lock that a transaction is requesting, and the BLOCKING_LOCK_ID refers to the lock (held by another transaction) that is preventing the first transaction from proceeding. For any given blocked transaction, all rows in INNODB_LOCK_WAITS have the same value for REQUESTED_LOCK_ID and different values for BLOCKING_LOCK_ID.

For the table definition, see Table 19.5, “INNODB_LOCK_WAITS Columns”.

13.4.6.2.4. Using the Transaction Information Schema Tables

Пример 13.2. Identifying Blocking Transactions

It is sometimes helpful to be able to identify which transaction is blocking another. You can use the Information Schema tables to find out which transaction is waiting for another, and which resource is being requested.

Suppose you have the following scenario, with three users running concurrently. Each user (or session) corresponds to a MySQL thread, and executes one transaction after another. Consider the state of the system when these users have issued the following commands, but none has yet committed its transaction:

  • User A:

    BEGIN;
    SELECT a FROM t FOR UPDATE;
    SELECT SLEEP(100);
  • User B:

    SELECT b FROM t FOR UPDATE;
  • User C:

    SELECT c FROM t FOR UPDATE;

In this scenario, you may use this query to see who is waiting for whom:

SELECT r.trx_id waiting_trx_id,  
       r.trx_mysql_thread_id waiting_thread,
       r.trx_query waiting_query,
       b.trx_id blocking_trx_id, 
       b.trx_mysql_thread_id blocking_thread,
       b.trx_query blocking_query
   FROM       information_schema.innodb_lock_waits w
   INNER JOIN information_schema.innodb_trx b  ON  
    b.trx_id = w.blocking_trx_id
  INNER JOIN information_schema.innodb_trx r  ON  
    r.trx_id = w.requesting_trx_id;
waiting trx idwaiting threadwaiting queryblocking trx idblocking threadblocking query
A46SELECT b FROM t FOR UPDATEA35SELECT SLEEP(100)
A57SELECT c FROM t FOR UPDATEA35SELECT SLEEP(100)
A57SELECT c FROM t FOR UPDATEA46SELECT b FROM t FOR UPDATE

In the above result, you can identify users by the “waiting query” or “blocking query”. As you can see:

  • User B (trx id 'A4', thread 6) and User C (trx id 'A5', thread 7) are both waiting for User A (trx id 'A3', thread 5).

  • User C is waiting for User B as well as User A.

You can see the underlying data in the tables INNODB_TRX, INNODB_LOCKS, and INNODB_LOCK_WAITS.

The following table shows some sample Contents of INFORMATION_SCHEMA.INNODB_TRX.

trx idtrx statetrx startedtrx requested lock idtrx wait startedtrx weighttrx mysql thread idtrx query
A3RUN­NING2008-01-15 16:44:54NULLNULL25SELECT SLEEP(100)
A4LOCK WAIT2008-01-15 16:45:09A4:1:3:22008-01-15 16:45:0926SELECT b FROM t FOR UPDATE
A5LOCK WAIT2008-01-15 16:45:14A5:1:3:22008-01-15 16:45:1427SELECT c FROM t FOR UPDATE

The following table shows some sample contents of INFORMATION_SCHEMA.INNODB_LOCKS.

lock idlock trx idlock modelock typelock tablelock indexlock spacelock pagelock reclock data
A3:1:3:2A3XRECORD`test`.`t``PRIMARY`1320x0200
A4:1:3:2A4XRECORD`test`.`t``PRIMARY`1320x0200
A5:1:3:2A5XRECORD`test`.`t``PRIMARY`1320x0200

The following table shows some sample contents of INFORMATION_SCHEMA.INNODB_LOCK_WAITS.

requesting trx idrequested lock idblocking trx idblocking lock id
A4A4:1:3:2A3A3:1:3:2
A5A5:1:3:2A3A3:1:3:2
A5A5:1:3:2A4A4:1:3:2

Пример 13.3. More Complex Пример of Transaction Data in Information Schema Tables

Sometimes you would like to correlate the internal InnoDB locking information with session-level information maintained by MySQL. For example, you might like to know, for a given InnoDB transaction ID, the corresponding MySQL session ID and name of the user that may be holding a lock, and thus blocking another transaction.

The following output from the INFORMATION_SCHEMA tables is taken from a somewhat loaded system.

As can be seen in the following tables, there are several transactions running.

The following INNODB_LOCKS and INNODB_LOCK_WAITS tables shows that:

  • Transaction 77F (executing an INSERT) is waiting for transactions 77E, 77D and 77B to commit.

  • Transaction 77E (executing an INSERT) is waiting for transactions 77D and 77B to commit.

  • Transaction 77D (executing an INSERT) is waiting for transaction 77B to commit.

  • Transaction 77B (executing an INSERT) is waiting for transaction 77A to commit.

  • Transaction 77A is running, currently executing SELECT.

  • Transaction E56 (executing an INSERT) is waiting for transaction E55 to commit.

  • Transaction E55 (executing an INSERT) is waiting for transaction 19C to commit.

  • Transaction 19C is running, currently executing an INSERT.

Note that there may be an inconsistency between queries shown in the two tables INNODB_TRX.TRX_QUERY and PROCESSLIST.INFO. The current transaction ID for a thread, and the query being executed in that transaction, may be different in these two tables for any given thread. See Section 13.4.6.3.3, “Possible Inconsistency with PROCESSLIST for an explanation.

The following table shows the contents of INFORMATION_SCHEMA.PROCESSLIST in a loaded system.

IDUSERHOSTDBCOMMANDTIMESTATEINFO
384rootlocalhosttestQuery10updateinsert into t2 values …
257rootlocalhosttestQuery3updateinsert into t2 values …
130rootlocalhosttestQuery0updateinsert into t2 values …
61rootlocalhosttestQuery1updateinsert into t2 values …
8rootlocalhosttestQuery1updateinsert into t2 values …
4rootlocalhosttestQuery0preparingSELECT * FROM processlist
2rootlocalhosttestSleep566NULL

The following table shows the contents of INFORMATION_SCHEMA.INNODB_TRX in a loaded system.

trx idtrx statetrx startedtrx requested lock idtrx wait startedtrx weighttrx mysql thread idtrx query
77FLOCK WAIT2008-01-15 13:10:1677F:8062008-01-15 13:10:161876insert into t09 (D, B, C) values …
77ELOCK WAIT2008-01-15 13:10:1677E:8062008-01-15 13:10:161875insert into t09 (D, B, C) values …
77DLOCK WAIT2008-01-15 13:10:1677D:8062008-01-15 13:10:161874insert into t09 (D, B, C) values …
77BLOCK WAIT2008-01-15 13:10:1677B:733​:12:12008-01-15 13:10:164873insert into t09 (D, B, C) values …
77ARUN­NING2008-01-15 13:10:16NULLNULL4872select b, c from t09 where …
E56LOCK WAIT2008-01-15 13:10:06E56:743​:6:22008-01-15 13:10:065384insert into t2 values …
E55LOCK WAIT2008-01-15 13:10:06E55:743​:38:22008-01-15 13:10:13965257insert into t2 values …
19CRUN­NING2008-01-15 13:09:10NULLNULL2900130insert into t2 values …
E15RUN­NING2008-01-15 13:08:59NULLNULL539561insert into t2 values …
51DRUN­NING2008-01-15 13:08:47NULLNULL98078insert into t2 values …

The following table shows the contents of INFORMATION_SCHEMA.INNODB_LOCK_WAITS in a loaded system

requesting trx idrequested lock idblocking trx idblocking lock id
77F77F:80677E77E:806
77F77F:80677D77D:806
77F77F:80677B77B:806
77E77E:80677D77D:806
77E77E:80677B77B:806
77D77D:80677B77B:806
77B77B:733:12:177A77A:733:12:1
E56E56:743:6:2E55E55:743:6:2
E55E55:743:38:219C19C:743:38:2

The following table shows the contents of INFORMATION_SCHEMA.INNODB_LOCKS in a loaded system.

lock idlock trx idlock modelock typelock tablelock indexlock spacelock pagelock reclock data
77F:80677FAUTO​_INCTABLE`test`​.`t09`NULLNULLNULLNULLNULL
77E:80677EAUTO​_INCTABLE`test`​.`t09`NULLNULLNULLNULLNULL
77D:80677DAUTO​_INCTABLE`test`​.`t09`NULLNULLNULLNULLNULL
77B:80677BAUTO​_INCTABLE`test`​.`t09`NULLNULLNULLNULLNULL
77B:733​:12:177BXRECORD`test`​.`t09``PRIMARY`733121supremum pseudo-record
77A:733​:12:177AXRECORD`test`​.`t09``PRIMARY`733121supremum pseudo-record
E56:743​:6:2E56SRECORD`test`​.`t2``PRIMARY`743620, 0
E55:743​:6:2E55XRECORD`test`​.`t2``PRIMARY`743620, 0
E55:743​:38:2E55SRECORD`test`​.`t2``PRIMARY`7433821922, 1922
19C:743​:38:219CXRECORD`test`​.`t2``PRIMARY`7433821922, 1922

13.4.6.3. Special Locking Considerations for InnoDB INFORMATION_SCHEMA Tables

13.4.6.3.1. Understanding InnoDB Locking

When a transaction updates a row in a table, or locks it with SELECT FOR UPDATE, InnoDB establishes a list or queue of locks on that row. Similarly, InnoDB maintains a list of locks on a table for table-level locks transactions hold. If a second transaction wants to update a row or lock a table already locked by a prior transaction in an incompatible mode, InnoDB adds a lock request for the row to the corresponding queue. For a lock to be acquired by a transaction, all incompatible lock requests previously entered into the lock queue for that row or table must be removed (the transactions holding or requesting those locks either commit or roll back).

A transaction may have any number of lock requests for different rows or tables. At any given time, a transaction may be requesting a lock that is held by another transaction, in which case it is blocked by that other transaction. The requesting transaction must wait for the transaction that holds the blocking lock to commit or rollback. If a transaction is not waiting for a a lock, it is in the 'RUNNING' state. If a transaction is waiting for a lock, it is in the 'LOCK WAIT' state.

The table INNODB_LOCKS holds one or more row for each 'LOCK WAIT' transaction, indicating the lock request(s) that is (are) preventing its progress. This table also contains one row describing each lock in a queue of locks pending for a given row or table. The table INNODB_LOCK_WAITS shows which locks already held by a transaction are blocking locks requested by other transactions.

13.4.6.3.2. Granularity of INFORMATION_SCHEMA Data

The data exposed by the transaction and locking tables represent a glimpse into fast-changing data. This is not like other (user) tables, where the data only changes when application-initiated updates occur. The underlying data is internal system-managed data, and can change very quickly.

For performance reasons, and to minimize the chance of misleading JOINs between the INFORMATION_SCHEMA tables, InnoDB collects the required transaction and locking information into an intermediate buffer whenever a SELECT on any of the tables is issued. This buffer is refreshed only if more than 0.1 seconds has elapsed since the last time the buffer was read. The data needed to fill the three tables is fetched atomically and consistently and is saved in this global internal buffer, forming a point-in-time “snapshot”. If multiple table accesses occur within 0.1 seconds (as they almost certainly do when MySQL processes a join among these tables), then the same snapshot is used to satisfy the query.

A correct result is returned when you JOIN any of these tables together in a single query, because the data for the three tables comes from the same snapshot. Because the buffer is not refreshed with every query of any of these tables, if you issue separate queries against these tables within a tenth of a second, the results are the same from query to query. On the other hand, two separate queries of the same or different tables issued more than a tenth of a second apart may see different results, since the data come from different snapshots.

Because InnoDB must temporarily stall while the transaction and locking data is collected, too frequent queries of these tables can negatively impact performance as seen by other users.

As these tables contain sensitive information (at least INNODB_LOCKS.LOCK_DATA and INNODB_TRX.TRX_QUERY), for security reasons, only the users with the PROCESS privilege are allowed to SELECT from them.

13.4.6.3.3. Possible Inconsistency with PROCESSLIST

As just described, while the transaction and locking data is correct and consistent when these INFORMATION_SCHEMA tables are populated, the underlying data changes so fast that similar glimpses at other, similarly fast-changing data, may not be in sync. Thus, you should be careful in comparing the data in the InnoDB transaction and locking tables with that in the MySQL table PROCESSLIST. The data from the PROCESSLIST table does not come from the same snapshot as the data about locking and transactions. Even if you issue a single SELECT (JOINing INNODB_TRX and PROCESSLIST, for example), the content of those tables is generally not consistent. INNODB_TRX may reference rows that are not present in PROCESSLIST or the currently executing SQL query of a transaction, shown in INNODB_TRX.TRX_QUERY may be different from the one in PROCESSLIST.INFO. The query in INNODB_TRX is always consistent with the rest of INNODB_TRX, INNODB_LOCKS and INNODB_LOCK_WAITS when the data comes from the same snapshot.

13.4.7. InnoDB Performance and Scalability Enhancements

This section discusses recent InnoDB enhancements to performance and scalability, covering the performance features in InnoDB 1.1 with MySQL 5.5, and the features in the InnoDB Plugin for MySQL 5.1. This information is useful to any DBA or developer who is concerned with performance and scalability. Although some of the enhancements do not require any action on your part, knowing this information can still help you diagnose performance issues more quickly and modernize systems and applications that rely on older, inefficient behavior.

13.4.7.1. Overview of InnoDB Performance

InnoDB has always been highly efficient, and includes several unique architectural elements to assure high performance and scalability. The latest InnoDB storage engine includes new features that take advantage of advances in operating systems and hardware platforms, such as multi-core processors and improved memory allocation systems. In addition, new configuration options let you better control some InnoDB internal subsystems to achieve the best performance with your workload.

Starting with MySQL 5.5 and InnoDB 1.1, the built-in InnoDB storage engine within MySQL is upgraded to the full feature set and performance of the former InnoDB Plugin. This change makes these performance and scalability enhancements available to a much wider audience than before, and eliminates the separate installation step of the InnoDB Plugin. After learning about the InnoDB performance features in this section, continue with Глава 7, Optimization to learn the best practices for overall MySQL performance, and Section 7.5, “Optimizing for InnoDB Tables” in particular for InnoDB tips and guidelines.

13.4.7.2. Faster Locking for Improved Scalability

In MySQL and InnoDB, multiple threads of execution access shared data structures. InnoDB synchronizes these accesses with its own implementation of mutexes and read/write locks. InnoDB has historically protected the internal state of a read/write lock with an InnoDB mutex. On Unix and Linux platforms, the internal state of an InnoDB mutex is protected by a Pthreads mutex, as in IEEE Std 1003.1c (POSIX.1c).

On many platforms, there is a more efficient way to implement mutexes and read/write locks. Atomic operations can often be used to synchronize the actions of multiple threads more efficiently than Pthreads. Each operation to acquire or release a lock can be done in fewer CPU instructions, and thus result in less wasted time when threads are contending for access to shared data structures. This in turn means greater scalability on multi-core platforms.

InnoDB implements mutexes and read/write locks with the built-in functions provided by the GNU Compiler Collection (GCC) for atomic memory access instead of using the Pthreads approach previously used. More specifically, an InnoDB that is compiled with GCC version 4.1.2 or later uses the atomic builtins instead of a pthread_mutex_t to implement InnoDB mutexes and read/write locks.

On 32-bit Microsoft Windows, InnoDB has implemented mutexes (but not read/write locks) with hand-written assembler instructions. Beginning with Microsoft Windows 2000, functions for Interlocked Variable Access are available that are similar to the built-in functions provided by GCC. On Windows 2000 and higher, InnoDB makes use of the Interlocked functions. Unlike the old hand-written assembler code, the new implementation supports read/write locks and 64-bit platforms.

Solaris 10 introduced library functions for atomic operations, and InnoDB uses these functions by default. When MySQL is compiled on Solaris 10 with a compiler that does not support the built-in functions provided by the GNU Compiler Collection (GCC) for atomic memory access, InnoDB uses the library functions.

This change improves the scalability of InnoDB on multi-core systems. This feature is enabled out-of-the-box on the platforms where it is supported. You do not have to set any parameter or option to take advantage of the improved performance. On platforms where the GCC, Windows, or Solaris functions for atomic memory access are not available, InnoDB uses the traditional Pthreads method of implementing mutexes and read/write locks.

When MySQL starts, InnoDB writes a message to the log file indicating whether atomic memory access is used for mutexes, for mutexes and read/write locks, or neither. If suitable tools are used to build InnoDB and the target CPU supports the atomic operations required, InnoDB uses the built-in functions for mutexing. If, in addition, the compare-and-swap operation can be used on thread identifiers (pthread_t), then InnoDB uses the instructions for read-write locks as well.

Note: If you are building from source, ensure that the build process properly takes advantage of your platform capabilities.

For more information about the performance implications of locking, see Section 7.10, “Optimizing Locking Operations”.

13.4.7.3. Using Operating System Memory Allocators

When InnoDB was developed, the memory allocators supplied with operating systems and run-time libraries were often lacking in performance and scalability. At that time, there were no memory allocator libraries tuned for multi-core CPUs. Therefore, InnoDB implemented its own memory allocator in the mem subsystem. This allocator is guarded by a single mutex, which may become a bottleneck. InnoDB also implements a wrapper interface around the system allocator (malloc and free) that is likewise guarded by a single mutex.

Today, as multi-core systems have become more widely available, and as operating systems have matured, significant improvements have been made in the memory allocators provided with operating systems. New memory allocators perform better and are more scalable than they were in the past. The leading high-performance memory allocators include Hoard, libumem, mtmalloc, ptmalloc, tbbmalloc, and TCMalloc. Most workloads, especially those where memory is frequently allocated and released (such as multi-table joins), benefit from using a more highly tuned memory allocator as opposed to the internal, InnoDB-specific memory allocator.

You can control whether InnoDB uses its own memory allocator or an allocator of the operating system, by setting the value of the system configuration parameter innodb_use_sys_malloc in the MySQL option file (my.cnf or my.ini). If set to ON or 1 (the default), InnoDB uses the malloc and free functions of the underlying system rather than manage memory pools itself. This parameter is not dynamic, and takes effect only when the system is started. To continue to use the InnoDB memory allocator, set innodb_use_sys_malloc to 0.

Замечание

When the InnoDB memory allocator is disabled, InnoDB ignores the value of the parameter innodb_additional_mem_pool_size. The InnoDB memory allocator uses an additional memory pool for satisfying allocation requests without having to fall back to the system memory allocator. When the InnoDB memory allocator is disabled, all such allocation requests are fulfilled by the system memory allocator.

On Unix-like systems that use dynamic linking, replacing the memory allocator may be as easy as making the environment variable LD_PRELOAD or LD_LIBRARY_PATH point to the dynamic library that implements the allocator. On other systems, some relinking may be necessary. Please refer to the documentation of the memory allocator library of your choice.

Since InnoDB cannot track all memory use when the system memory allocator is used (innodb_use_sys_malloc is ON), the section “BUFFER POOL AND MEMORY” in the output of the SHOW ENGINE INNODB STATUS command only includes the buffer pool statistics in the “Total memory allocated”. Any memory allocated using the mem subsystem or using ut_malloc is excluded.

For more information about the performance implications of InnoDB memory usage, see Section 7.9, “Buffering and Caching”.

13.4.7.4. Controlling InnoDB Change Buffering

When INSERT, UPDATE, and DELETE operations are done to a table, often the values of indexed columns (particularly the values of secondary keys) are not in sorted order, requiring substantial I/O to bring secondary indexes up to date. InnoDB has an insert buffer that caches changes to secondary index entries when the relevant page is not in the buffer pool, thus avoiding I/O operations by not reading in the page from the disk. The buffered changes are merged when the page is loaded to the buffer pool, and the updated page is later flushed to disk using the normal mechanism. The InnoDB main thread merges buffered changes when the server is nearly idle, and during a slow shutdown.

Because it can result in fewer disk reads and writes, this feature is most valuable for workloads that are I/O-bound, for example applications with a high volume of DML operations such as bulk inserts.

However, the insert buffer occupies a part of the buffer pool, reducing the memory available to cache data pages. If the working set almost fits in the buffer pool, or if your tables have relatively few secondary indexes, it may be useful to disable insert buffering. If the working set entirely fits in the buffer pool, insert buffering does not impose any extra overhead, because it only applies to pages that are not in the buffer pool.

You can control the extent to which InnoDB performs insert buffering with the system configuration parameter innodb_change_buffering. You can turn on and off buffering for inserts, delete operations (when index records are initially marked for deletion) and purge operations (when index records are physically deleted). An update operation is represented as a combination of an insert and a delete. In MySQL 5.5 and higher, the default value is changed from inserts to all.

The allowed values of innodb_change_buffering are:

  • all

    The default value: buffer inserts, delete-marking operations, and purges.

  • none

    Do not buffer any operations.

  • inserts

    Buffer insert operations.

  • deletes

    Buffer delete-marking operations.

  • changes

    Buffer both inserts and delete-marking.

  • purges

    Buffer the physical deletion operations that happen in the background.

You can set the value of this parameter in the MySQL option file (my.cnf or my.ini) or change it dynamically with the SET GLOBAL command, which requires the SUPER privilege. Changing the setting affects the buffering of new operations; the merging of already buffered entries is not affected.

For more information about speeding up INSERT, UPDATE, and DELETE statements, see Section 7.2.2, “Optimizing DML Statements”.

13.4.7.5. Controlling Adaptive Hash Indexing

If a table fits almost entirely in main memory, the fastest way to perform queries on it is to use hash indexes rather than B-tree lookups. MySQL monitors searches on each index defined for an InnoDB table. If it notices that certain index values are being accessed frequently, it automatically builds an in-memory hash table for that index. Based on the observed pattern of searches, it builds a hash index using a prefix of the index key. The prefix of the key can be any length, and it may be that only some of the values in the B-tree appear in the hash index. Hash indexes are built on demand for those pages of the index that are often accessed.

This adaptive hash index mechanism allows InnoDB to take advantage of large amounts of memory, something typically done only by database systems specifically designed for databases that reside entirely in memory. Normally, the automatic building and use of adaptive hash indexes improves performance. However, sometimes, the read/write lock that guards access to the adaptive hash index may become a source of contention under heavy workloads, such as multiple concurrent joins.

You can monitor the use of the adaptive hash index and the contention for its use in the “SEMAPHORES” section of the output of the SHOW ENGINE INNODB STATUS command. If you see many threads waiting on an RW-latch created in btr0sea.c, then it might be useful to disable adaptive hash indexing.

The configuration parameter innodb_adaptive_hash_index can be set to disable or enable the adaptive hash index. See Section 13.4.8.2.4, “Dynamically Changing innodb_adaptive_hash_index for details.

For more information about the performance characteristics of hash indexes, see Section 7.3.8, “Comparison of B-Tree and Hash Indexes”.

13.4.7.6. Changes Regarding Thread Concurrency

InnoDB uses operating system threads to process requests from user transactions. (Transactions may issue many requests to InnoDB before they commit or roll back.) On modern operating systems and servers with multi-core processors, where context switching is efficient, most workloads run well without any limit on the number of concurrent threads. Scalability improvements in MySQL 5.5 and up reduce the need to limit the number of concurrently executing threads inside InnoDB.

In situations where it is helpful to minimize context switching between threads, InnoDB can use a number of techniques to limit the number of concurrently executing operating system threads (and thus the number of requests that are processed at any one time). When InnoDB receives a new request from a user session, if the number of threads concurrently executing is at a pre-defined limit, the new request sleeps for a short time before it tries again. A request that cannot be rescheduled after the sleep is put in a first-in/first-out queue and eventually is processed. Threads waiting for locks are not counted in the number of concurrently executing threads.

You can limit the number of concurrent threads by setting the configuration parameter innodb_thread_concurrency. Once the number of executing threads reaches this limit, additional threads sleep for a number of microseconds, set by the configuration parameter innodb_thread_sleep_delay, before being placed into the queue.

The default value for innodb_thread_concurrency and the implied default limit on the number of concurrent threads has been changed in various releases of MySQL and InnoDB. Currently, the default value of innodb_thread_concurrency is 0, so that by default there is no limit on the number of concurrently executing threads, as shown in Table 13.7, “Changes to innodb_thread_concurrency.

Table 13.7. Changes to innodb_thread_concurrency

InnoDB VersionMySQL VersionDefault valueDefault limit of concurrent threadsValue to allow unlimited threads
Built-inEarlier than 5.1.1120No limit20 or higher
Built-in5.1.11 and newer880
InnoDB before 1.0.3(corresponding to Plugin)880
InnoDB 1.0.3 and newer(corresponding to Plugin)0No limit0

Note that InnoDB causes threads to sleep only when the number of concurrent threads is limited. When there is no limit on the number of threads, all contend equally to be scheduled. That is, if innodb_thread_concurrency is 0, the value of innodb_thread_sleep_delay is ignored.

When there is a limit on the number of threads, InnoDB reduces context switching overhead by permitting multiple requests made during the execution of a single SQL statement to enter InnoDB without observing the limit set by innodb_thread_concurrency. Since an SQL statement (such as a join) may comprise multiple row operations within InnoDB, InnoDB assigns “tickets” that allow a thread to be scheduled repeatedly with minimal overhead.

When a new SQL statement starts, a thread has no tickets, and it must observe innodb_thread_concurrency. Once the thread is entitled to enter InnoDB, it is assigned a number of tickets that it can use for subsequently entering InnoDB. If the tickets run out, innodb_thread_concurrency is observed again and further tickets are assigned. The number of tickets to assign is specified by the global option innodb_concurrency_tickets, which is 500 by default. A thread that is waiting for a lock is given one ticket once the lock becomes available.

The correct values of these variables depend on your environment and workload. Try a range of different values to determine what value works for your applications. Before limiting the number of concurrently executing threads, review configuration options that may improve the performance of InnoDB on multi-core and multi-processor computers, such as innodb_use_sys_malloc and innodb_adaptive_hash_index.

For general performance information about MySQL thread handling, see Section 7.11.5.1, “How MySQL Uses Threads for Client Connections”.

13.4.7.7. Changes in the Read-Ahead Algorithm

A read-ahead request is an I/O request to prefetch multiple pages in the buffer pool asynchronously, in anticipation that these pages will be needed soon. InnoDB uses or has used two read-ahead algorithms to improve I/O performance:

Linear read-ahead is based on the access pattern of the pages in the buffer pool, not just their number. You can control when InnoDB performs a read-ahead operation by adjusting the number of sequential page accesses required to trigger an asynchronous read request, using the configuration parameter innodb_read_ahead_threshold. Before this parameter was added, InnoDB would only calculate whether to issue an asynchronous prefetch request for the entire next extent when it read in the last page of the current extent.

Random read-ahead is a former technique that has now been removed as of MySQL 5.5. If a certain number of pages from the same extent (64 consecutive pages) were found in the buffer pool, InnoDB asynchronously issued a request to prefetch the remaining pages of the extent. Random read-ahead added unnecessary complexity to the InnoDB code and often resulted in performance degradation rather than improvement. This feature is no longer part of InnoDB, and users should generally see equivalent or improved performance.

If the number of pages read from an extent of 64 pages is greater or equal to innodb_read_ahead_threshold, InnoDB initiates an asynchronous read-ahead operation of the entire following extent. Thus, this parameter controls how sensitive InnoDB is to the pattern of page accesses within an extent in deciding whether to read the following extent asynchronously. The higher the value, the more strict the access pattern check. For example, if you set the value to 48, InnoDB triggers a linear read-ahead request only when 48 pages in the current extent have been accessed sequentially. If the value is 8, InnoDB would trigger an asynchronous read-ahead even if as few as 8 pages in the extent were accessed sequentially.

The new configuration parameter innodb_read_ahead_threshold can be set to any value from 0-64. The default value is 56, meaning that an asynchronous read-ahead is performed only when 56 of the 64 pages in the extent are accessed sequentially. You can set the value of this parameter in the MySQL option file (my.cnf or my.ini), or change it dynamically with the SET GLOBAL command, which requires the SUPER privilege.

The SHOW ENGINE INNODB STATUS command displays statistics to help you evaluate the effectiveness of the read-ahead algorithm. See Section 13.4.8.8, “More Read-Ahead Statistics” for more information.

For more information about I/O performance, see Section 7.5.7, “Optimizing InnoDB Disk I/O” and Section 7.11.3, “Optimizing Disk I/O”.

13.4.7.8. Multiple Background I/O Threads

InnoDB uses background threads to service various types of I/O requests. You can configure the number of background threads that service read and write I/O on data pages, using the configuration parameters innodb_read_io_threads and innodb_write_io_threads. These parameters signify the number of background threads used for read and write requests respectively. They are effective on all supported platforms. You can set the value of these parameters in the MySQL option file (my.cnf or my.ini); you cannot change them dynamically. The default value for these parameters is 4 and the permissible values range from 1-64.

These parameters replace innodb_file_io_threads from earlier versions of MySQL. If you try to set a value for this obsolete parameter, a warning is written to the log file and the value is ignored. This parameter only applied to Windows platforms. (On non-Windows platforms, there was only one thread each for read and write.)

The purpose of this change is to make InnoDB more scalable on high end systems. Each background thread can handle up to 256 pending I/O requests. A major source of background I/O is theread-ahead requests. InnoDB tries to balance the load of incoming requests in such way that most of the background threads share work equally. InnoDB also attempts to allocate read requests from the same extent to the same thread to increase the chances of coalescing the requests together. If you have a high end I/O subsystem and you see more than 64 × innodb_read_io_threads pending read requests in SHOW ENGINE INNODB STATUS, you might gain by increasing the value of innodb_read_io_threads.

For more information about InnoDB I/O performance, see Section 7.5.7, “Optimizing InnoDB Disk I/O”.

13.4.7.9. Asynchronous I/O on Linux

Starting in InnoDB 1.1 with MySQL 5.5, the asynchronous I/O capability that InnoDB has had on Windows systems is now available on Linux systems. (Other Unix-like systems continue to use synchronous I/O calls.) This feature improves the scalability of heavily I/O-bound systems, which typically show many pending reads/writes in the output of the command SHOW ENGINE INNODB STATUS\G.

Running with a large number of InnoDB I/O threads, and especially running multiple such instances on the same server machine, can exceed capacity limits on Linux systems. In this case, you can fix the error:

EAGAIN: The specified maxevents exceeds the user's limit of available events. 

by writing a higher limit to /proc/sys/fs/aio-max-nr.

In general, if a problem with the asynchronous I/O subsystem in the OS prevents InnoDB from starting, set the option innodb_use_native_aio=0 in the configuration file. This new configuration option applies to Linux systems only, and cannot be changed once the server is running.

For more information about InnoDB I/O performance, see Section 7.5.7, “Optimizing InnoDB Disk I/O”.

13.4.7.10. Group Commit

InnoDB, like any other ACID-compliant database engine, flushes the redo log of a transaction before it is committed. Historically, InnoDB used group commit functionality to group multiple such flush requests together to avoid one flush for each commit. With group commit, InnoDB issues a single write to the log file to perform the commit action for multiple user transactions that commit at about the same time, significantly improving throughput.

Group commit in InnoDB worked until MySQL 4.x, and works once again with MySQL 5.1 with the InnoDB Plugin, and MySQL 5.5 and higher. The introduction of support for the distributed transactions and Two Phase Commit (2PC) in MySQL 5.0 interfered with the InnoDB group commit functionality. This issue is now resolved.

The group commit functionality inside InnoDB works with the Two Phase Commit protocol in MySQL. Re-enabling of the group commit functionality fully ensures that the ordering of commit in the MySQL binlog and the InnoDB logfile is the same as it was before. It means it is totally safe to use MySQL Enterprise Backup with InnoDB 1.0.4 (that is, the InnoDB Plugin with MySQL 5.1) and above. When the binlog is enabled, you typically also set the configuration option sync_binlog=0, because group commit for the binary log is only supported if it is set to 0.

Group commit is transparent; you do not need to do anything to take advantage of this significant performance improvement.

For more information about performance of COMMIT and other transactional operations, see Section 7.5.2, “Optimizing InnoDB Transaction Management”.

13.4.7.11. Controlling the Master Thread I/O Rate

The master thread in InnoDB is a thread that performs various tasks in the background. Most of these tasks are I/O related, such as flushing dirty pages from the buffer pool or writing changes from the insert buffer to the appropriate secondary indexes. The master thread attempts to perform these tasks in a way that does not adversely affect the normal working of the server. It tries to estimate the free I/O bandwidth available and tune its activities to take advantage of this free capacity. Historically, InnoDB has used a hard coded value of 100 IOPs (input/output operations per second) as the total I/O capacity of the server.

The parameter innodb_io_capacity indicates the overall I/O capacity available to InnoDB. This parameter should be set to approximately the number of I/O operations that the system can perform per second. The value depends on your system configuration. When innodb_io_capacity is set, the master threads estimates the I/O bandwidth available for background tasks based on the set value. Setting the value to 100 reverts to the old behavior.

You can set the value of innodb_io_capacity to any number 100 or greater. The default value is 200, reflecting that the performance of typical modern I/O devices is higher than in the early days of MySQL. Typically, values around the previous default of 100 are appropriate for consumer-level storage devices, such as hard drives up to 7200 RPMs. Faster hard drives, RAID configurations, and SSDs benefit from higher values.

You can set the value of this parameter in the MySQL option file (my.cnf or my.ini) or change it dynamically with the SET GLOBAL command, which requires the SUPER privilege.

For more information about InnoDB I/O performance, see Section 7.5.7, “Optimizing InnoDB Disk I/O”.

13.4.7.12. Controlling the Flushing Rate of Dirty Pages

InnoDB performs certain tasks in the background, including flushing of dirty pages (those pages that have been changed but are not yet written to the database files) from the buffer pool, a task performed by the master thread. Currently, InnoDB aggressively flushes buffer pool pages if the percentage of dirty pages in the buffer pool exceeds innodb_max_dirty_pages_pct.

InnoDB uses a new algorithm to estimate the required rate of flushing, based on the speed of redo log generation and the current rate of flushing. The intent is to smooth overall performance by ensuring that buffer flush activity keeps up with the need to keep the buffer pool “clean”. Automatically adjusting the rate of flushing can help to avoid steep dips in throughput, when excessive buffer pool flushing limits the I/O capacity available for ordinary read and write activity.

InnoDB uses its log files in a circular fashion. Before reusing a portion of a log file, InnoDB flushes to disk all dirty buffer pool pages whose redo entries are contained in that portion of the log file, a process known as a sharp checkpoint. If a workload is write-intensive, it generates a lot of redo information, all written to the log file. If all available space in the log files is used up, a sharp checkpoint occurs, causing a temporary reduction in throughput. This situation can happen even though innodb_max_dirty_pages_pct is not reached.

InnoDB uses a heuristic-based algorithm to avoid such a scenario, by measuring the number of dirty pages in the buffer pool and the rate at which redo is being generated. Based on these numbers, InnoDB decides how many dirty pages to flush from the buffer pool each second. This self-adapting algorithm is able to deal with sudden changes in the workload.

Internal benchmarking has also shown that this algorithm not only maintains throughput over time, but can also improve overall throughput significantly.

Because adaptive flushing is a new feature that can significantly affect the I/O pattern of a workload, a new configuration parameter lets you turn off this feature. The default value of the boolean parameter innodb_adaptive_flushing is TRUE, enabling the new algorithm. You can set the value of this parameter in the MySQL option file (my.cnf or my.ini) or change it dynamically with the SET GLOBAL command, which requires the SUPER privilege.

For more information about InnoDB I/O performance, see Section 7.5.7, “Optimizing InnoDB Disk I/O”.

13.4.7.13. Using the PAUSE Instruction in InnoDB Spin Loops

Synchronization inside InnoDB frequently involves the use of spin loops: while waiting, InnoDB executes a tight loop of instructions repeatedly to avoid having the InnoDB process and threads be rescheduled by the operating system. If the spin loops are executed too quickly, system resources are wasted, imposing a performance penalty on transaction throughput. Most modern processors implement the PAUSE instruction for use in spin loops, so the processor can be more efficient.

InnoDB uses a PAUSE instruction in its spin loops on all platforms where such an instruction is available. This technique increases overall performance with CPU-bound workloads, and has the added benefit of minimizing power consumption during the execution of the spin loops.

You do not have to do anything to take advantage of this performance improvement.

For performance considerations for InnoDB locking operations, see Section 7.10, “Optimizing Locking Operations”.

13.4.7.14. Control of Spin Lock Polling

Many InnoDB mutexes and rw-locks are reserved for a short time. On a multi-core system, it can be more efficient for a thread to continuously check if it can acquire a mutex or rw-lock for a while before sleeping. If the mutex or rw-lock becomes available during this polling period, the thread can continue immediately, in the same time slice. However, too-frequent polling by multiple threads of a shared object can cause “cache ping pong”, different processors invalidating portions of each others' cache. InnoDB minimizes this issue by waiting a random time between subsequent polls. The delay is implemented as a busy loop.

You can control the maximum delay between testing a mutex or rw-lock using the parameter innodb_spin_wait_delay. The duration of the delay loop depends on the C compiler and the target processor. (In the 100MHz Pentium era, the unit of delay was one microsecond.) On a system where all processor cores share a fast cache memory, you might reduce the maximum delay or disable the busy loop altogether by setting innodb_spin_wait_delay=0. On a system with multiple processor chips, the effect of cache invalidation can be more significant and you might increase the maximum delay.

The default value of innodb_spin_wait_delay is 6. The spin wait delay is a dynamic global parameter that you can specify in the MySQL option file (my.cnf or my.ini) or change at runtime with the command SET GLOBAL innodb_spin_wait_delay=delay, where delay is the desired maximum delay. Changing the setting requires the SUPER privilege.

For performance considerations for InnoDB locking operations, see Section 7.10, “Optimizing Locking Operations”.

13.4.7.15. Making Buffer Pool Scan Resistant

Rather than using a strictly LRU algorithm, InnoDB uses a technique to minimize the amount of data that is brought into the buffer pool and never accessed again. The goal is to make sure that frequently accessed (“hot”) pages remain in the buffer pool, even as read-ahead and full table scans bring in new blocks that might or might not be accessed afterward.

Newly read blocks are inserted into the middle of the list representing the buffer pool. of the LRU list. All newly read pages are inserted at a location that by default is 3/8 from the tail of the LRU list. The pages are moved to the front of the list (the most-recently used end) when they are accessed in the buffer pool for the first time. Thus pages that are never accessed never make it to the front portion of the LRU list, and “age out” sooner than with a strict LRU approach. This arrangement divides the LRU list into two segments, where the pages downstream of the insertion point are considered “old” and are desirable victims for LRU eviction.

For an explanation of the inner workings of the InnoDB buffer pool and the specifics of its LRU replacement algorithm, see Section 7.9.1, “The InnoDB Buffer Pool”.

You can control the insertion point in the LRU list, and choose whether InnoDB applies the same optimization to blocks brought into the buffer pool by table or index scans. The configuration parameter innodb_old_blocks_pct controls the percentage of “old” blocks in the LRU list. The default value of innodb_old_blocks_pct is 37, corresponding to the original fixed ratio of 3/8. The value range is 5 (new pages in the buffer pool age out very quickly) to 95 (only 5% of the buffer pool is reserved for hot pages, making the algorithm close to the familiar LRU strategy).

The optimization that keeps the buffer pool from being churned by read-ahead can avoid similar problems due to table or index scans. In these scans, a data page is typically accessed a few times in quick succession and is never touched again. The configuration parameter innodb_old_blocks_time specifies the time window (in milliseconds) after the first access to a page during which it can be accessed without being moved to the front (most-recently used end) of the LRU list. The default value of innodb_old_blocks_time is 0, corresponding to the original behavior of moving a page to the most-recently used end of the buffer pool list when it is first accessed in the buffer pool. Increasing this value makes more and more blocks likely to age out faster from the buffer pool.

Both the new parameters innodb_old_blocks_pct and innodb_old_blocks_time are dynamic, global and can be specified in the MySQL option file (my.cnf or my.ini) or changed at runtime with the SET GLOBAL command. Changing the setting requires the SUPER privilege.

To help you gauge the effect of setting these parameters, the SHOW ENGINE INNODB STATUS command reports additional statistics. The BUFFER POOL AND MEMORY section now looks like:

Total memory allocated 1107296256; in additional pool allocated 0
Dictionary memory allocated 80360
Buffer pool size   65535
Free buffers       0
Database pages     63920
Old database pages 23600
Modified db pages  34969
Pending reads 32
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 414946, not young 2930673
1274.75 youngs/s, 16521.90 non-youngs/s
Pages read 486005, created 3178, written 160585
2132.37 reads/s, 3.40 creates/s, 323.74 writes/s
Buffer pool hit rate 950 / 1000, young-making rate 30 / 1000 not 392 / 1000
Pages read ahead 1510.10/s, evicted without access 0.00/s
LRU len: 63920, unzip_LRU len: 0
I/O sum[43690]:cur[221], unzip sum[0]:cur[0]
  • Old database pages is the number of pages in the “old” segment of the LRU list.

  • Pages made young and not young is the total number of “old” pages that have been made young or not respectively.

  • youngs/s and non-young/s is the rate at which page accesses to the “old” pages have resulted in making such pages young or otherwise respectively since the last invocation of the command.

  • young-making rate and not provides the same rate but in terms of overall buffer pool accesses instead of accesses just to the “old” pages.

Because the effects of these parameters can vary widely based on your hardware configuration, your data, and the details of your workload, always benchmark to verify the effectiveness before changing these settings in any performance-critical or production environment.

In mixed workloads where most of the activity is OLTP type with periodic batch reporting queries which result in large scans, setting the value of innodb_old_blocks_time during the batch runs can help keep the working set of the normal workload in the buffer pool.

When scanning large tables that cannot fit entirely in the buffer pool, setting innodb_old_blocks_pct to a small value keeps the data that is only read once from consuming a significant portion of the buffer pool. For example, setting innodb_old_blocks_pct=5 restricts this data that is only read once to 5% of the buffer pool.

When scanning small tables that do fit into memory, there is less overhead for moving pages around within the buffer pool, so you can leave innodb_old_blocks_pct at its default value, or even higher, such as innodb_old_blocks_pct=50.

The effect of the innodb_old_blocks_time parameter is harder to predict than the innodb_old_blocks_pct parameter, is relatively small, and varies more with the workload. To arrive at an optimal value, conduct your own benchmarks if the performance improvement from adjusting innodb_old_blocks_pct is not sufficient.

For more information about the InnoDB buffer pool, see Section 7.9.1, “The InnoDB Buffer Pool”.

13.4.7.16. Improvements to Crash Recovery Performance

A number of optimizations speed up certain steps of the recovery that happens on the next startup after a crash. In particular, scanning the redo log and applying the redo log are faster than in MySQL 5.1 and earlier, due to improved algorithms for memory management. You do not need to take any actions to take advantage of this performance enhancement. If you kept the size of your redo log files artificially low because recovery took a long time, you can consider increasing the file size.

For more information about InnoDB recovery, see Section 13.3.7.1, “The InnoDB Recovery Process”.

13.4.7.17. Integration with MySQL PERFORMANCE_SCHEMA

Starting with InnoDB 1.1 with MySQL 5.5, you can profile certain internal InnoDB operations using the MySQL Performance Schema feature. This type of tuning is primarily for expert users, those who push the limits of MySQL performance, read the MySQL source code, and evaluate optimization strategies to overcome performance bottlenecks. DBAs can also use this feature for capacity planning, to see whether their typical workload encounters any performance bottlenecks with a particular combination of CPU, RAM, and disk storage; and if so, to judge whether performance can be improved by increasing the capacity of some part of the system.

To use this feature to examine InnoDB performance:

  • You must be running MySQL 5.5 or higher. You must build the database server from source, enabling the Performance Schema feature by building with the --with-perfschema option. Since the Performance Schema feature introduces some performance overhead, you should use it on a test or development system rather than on a production system.

  • You must be running InnoDB 1.1 or higher.

  • You must be generally familiar with how to use the Performance Schema feature, for example to query tables in the performance_schema database.

  • Examine the following kinds of InnoDB objects by querying the appropriate performance_schema tables. The items associated with InnoDB all contain the substring innodb in the NAME column.

    For the definitions of the *_instances tables, see Section 20.7.2, “Performance Schema Instance Tables”. For the definitions of the *_summary_* tables, see Section 20.7.4, “Performance Schema Summary Tables”. For the definition of the thread table, see Section 20.7.5, “Performance Schema Miscellaneous Tables”. For the definition of the *_current_* and *_history_* tables, see Section 20.7.3, “Performance Schema Wait Event Tables”.

    • Mutexes in the mutex_instances table. (Mutexes and RW-locks related to the InnoDB buffer pool are not included in this coverage; the same applies to the output of the SHOW ENGINE INNODB MUTEX command.)

    • RW-locks in the rwlock_instances table.

    • RW-locks in the rwlock_instances table.

    • File I/O operations in the file_instances, file_summary_by_event_name, and file_summary_by_instance tables.

    • Threads in the PROCESSLIST table.

  • During performance testing, examine the performance data in the events_waits_current and events_waits_history_long tables. If you are interested especially in InnoDB-related objects, use the clause where name like "%innodb%" to see just those entries; otherwise, examine the performance statistics for the overall MySQL server.

  • You must be running MySQL 5.5, with the Performance Schema enabled by building with the --with-perfschema build option.

For more information about the MySQL Performance Schema, see Глава 20, MySQL Performance Schema.

13.4.7.18. Improvements to Performance from Multiple Buffer Pools

This performance enhancement is primarily useful for people with a large buffer pool size, typically in the multi-gigabyte range. To take advantage of this speedup, you must set the new innodb_buffer_pool_instances configuration option, and you might also adjust the innodb_buffer_pool_size value.

When the InnoDB buffer pool is large, many data requests can be satisfied by retrieving from memory. You might encounter bottlenecks from multiple threads trying to access the buffer pool at once. Starting in InnoDB 1.1 and MySQL 5.5, you can enable multiple buffer pools to minimize this contention. Each page that is stored in or read from the buffer pool is assigned to one of the buffer pools randomly, using a hashing function. Each buffer pool manages its own free lists, flush lists, LRUs, and all other data structures connected to a buffer pool, and is protected by its own buffer pool mutex.

To enable this feature, set the innodb_buffer_pool_instances configuration option to a value greater than 1 (the default) up to 64 (the maximum). This option only takes effect when you set the innodb_buffer_pool_size to a size of 1 gigabyte or more. The total size you specify is divided up among all the buffer pools. We recommend specifying a combination of innodb_buffer_pool_instances and innodb_buffer_pool_size so that each buffer pool instance is at least 1 gigabyte.

For more information about the InnoDB buffer pool, see Section 7.9.1, “The InnoDB Buffer Pool”.

13.4.7.19. Better Scalability with Multiple Rollback Segments

Starting in InnoDB 1.1 with MySQL 5.5, the limit on concurrent transactions is greatly expanded, removing a bottleneck with the InnoDB rollback segment that affected high-capacity systems. The limit applies to concurrent transactions that change any data; read-only transactions do not count against that maximum.

The single rollback segment is now divided into 128 segments, each of which can support up to 1023 transactions that perform writes, for a total of approximately 128K concurrent transactions. The original transaction limit was 1023.

Each transaction is assigned to one of the rollback segments, and remains tied to that rollback segment for the duration. This enhancement improves both scalability (higher number of concurrent transactions) and performance (less contention when different transactions access the rollback segments).

To take advantage of this feature, you do not need to create any new database or tables, or reconfigure anything. You must do a slow shutdown before upgrading from MySQL 5.1 or earlier, or some time afterward. InnoDB makes the required changes inside the system tablespace automatically, the first time you restart after performing a slow shutdown.

For more information about performance of InnoDB under high transactional load, see Section 7.5.2, “Optimizing InnoDB Transaction Management”.

13.4.7.20. Better Scalability with Improved Purge Scheduling

Starting in InnoDB 1.1 with MySQL 5.5, the purge operations (a type of garbage collection) that InnoDB performs automatically can be done in a separate thread, rather than as part of the master thread. This change improves scalability, because the main database operations run independently from maintenance work happening in the background.

To enable this feature, set the configuration option innodb_purge_threads=1, as opposed to the default of 0, which combines the purge operation into the master thread.

You might not notice a significant speedup, because the purge thread might encounter new types of contention; the single purge thread really lays the groundwork for further tuning and possibly multiple purge threads in the future. There is another new configuration option, innodb_purge_batch_size with a default of 20 and maximum of 5000. This option is mainly intended for experimentation and tuning of purge operations, and should not be interesting to typical users.

For more information about InnoDB I/O performance, see Section 7.5.7, “Optimizing InnoDB Disk I/O”.

13.4.7.21. Improved Log Sys Mutex

This is another performance improvement that comes for free, with no user action or configuration needed. The details here are intended for performance experts who delve into the InnoDB source code, or interpret reports with keywords such as “mutex” and “log_sys”.

The mutex known as the log sys mutex has historically done double duty, controlling access to internal data structures related to log records and the LSN, as well as pages in the buffer pool that are changed when a mini-transaction is committed. Starting in InnoDB 1.1 with MySQL 5.5, these two kinds of operations are protected by separate mutexes, with a new log_buf mutex controlling writes to buffer pool pages due to mini-transactions.

For performance considerations for InnoDB locking operations, see Section 7.10, “Optimizing Locking Operations”.

13.4.7.22. Separate Flush List Mutex

Starting with InnoDB 1.1 with MySQL 5.5, concurrent access to the buffer pool is faster. Operations involving the flush list, a data structure related to the buffer pool, are now controlled by a separate mutex and do not block access to the buffer pool. You do not need to configure anything to take advantage of this speedup; it is fully automatic.

For more information about the InnoDB buffer pool, see Section 7.9.1, “The InnoDB Buffer Pool”.

13.4.8. Changes for Flexibility, Ease of Use and Reliability

This chapter describes several recently added InnoDB features that offer new flexibility and improve ease of use, reliability and performance. The Barracuda file format improves efficiency for storing large variable-length columns, and enables table compression. Configuration options that once were unchangeable after startup, are now flexible and can be changed dynamically. Some improvements are automatic, such as faster and more efficient TRUNCATE TABLE. Others allow you the flexibility to control InnoDB behavior; for example, you can control whether certain problems cause errors or just warnings. And informational messages and error reporting continue to be made more user-friendly.

13.4.8.1. The Barracuda File Format

InnoDB has started using named file formats to improve compatibility in upgrade and downgrade situations, or heterogeneous systems running different levels of MySQL. Many important InnoDB features, such as table compression and the DYNAMIC row format for more efficient BLOB storage, require creating tables in the Barracuda file format. The original file format, which previously didn't have a name, is known now as Antelope.

To create new tables that take advantage of the Barracuda features, enable that file format using the configuration parameter innodb_file_format. The value of this parameter determines whether a newly created table or index can use compression or the new DYNAMIC row format.

To preclude the use of new features that would make your database inaccessible to the built-in InnoDB in MySQL 5.1 and prior releases, omit innodb_file_format or set it to Antelope.

You can set the value of innodb_file_format on the command line when you start mysqld, or in the option file my.cnf (Unix operating systems) or my.ini (Windows). You can also change it dynamically with the SET GLOBAL statement.

For more information about managing file formats, see Section 13.4.4, “InnoDB File-Format Management”.

13.4.8.2. Dynamic Control of System Configuration Parameters

In MySQL 5.5 and higher, you can change certain system configuration parameters without shutting down and restarting the server, as was necessary in MySQL 5.1 and lower. This increases uptime, and makes it easier to test and prototype new SQL and application code. The following sections explain these parameters.

13.4.8.2.1. Dynamically Changing innodb_file_per_table

Since MySQL version 4.1, InnoDB has provided two alternatives for how tables are stored on disk. You can create a new table and its indexes in the shared system tablespace, physically stored in the ibdata files. Or, you can store a new table and its indexes in a separate tablespace (a .ibd file). The storage layout for each InnoDB table is determined by the configuration parameter innodb_file_per_table at the time the table is created.

In MySQL 5.5 and higher, the configuration parameter innodb_file_per_table is dynamic, and can be set ON or OFF using the SET GLOBAL. Previously, the only way to set this parameter was in the MySQL option file (my.cnf or my.ini), and changing it required shutting down and restarting the server.

The default setting is OFF, so new tables and indexes are created in the system tablespace. Dynamically changing the value of this parameter requires the SUPER privilege and immediately affects the operation of all connections.

Tables created when innodb_file_per_table is enabled can use the Barracuda file format, and TRUNCATE returns the disk space for those tables to the operating system. The Barracuda file format in turn enables features such as table compression and the DYNAMIC row format. Tables created when innodb_file_per_table is off cannot use these features. To take advantage of those features for an existing table, you can turn on the file-per-table setting and run ALTER TABLE t ENGINE=INNODB for that table.

When you redefine the primary key for an InnoDB table, the table is re-created using the current settings for innodb_file_per_table and innodb_file_format. This behavior does not apply when adding or dropping InnoDB secondary indexes, as explained in Section 13.4.2, “Fast Index Creation in the InnoDB Storage Engine”. When a secondary index is created without rebuilding the table, the index is stored in the same file as the table data, regardless of the current innodb_file_per_table setting.

13.4.8.2.2. Dynamically Changing innodb_stats_on_metadata

In MySQL 5.5 and higher, you can change the setting of innodb_stats_on_metadata dynamically at runtime, to control whether or not InnoDB performs statistics gathering when metadata statements are executed. To change the setting, issue the statement SET GLOBAL innodb_stats_on_metadata=mode, where mode is either ON or OFF (or 1 or 0). Changing this setting requires the SUPER privilege and immediately affects the operation of all connections.

This setting is related to the feature described in Section 13.4.8.5, “Controlling Optimizer Statistics Estimation”.

13.4.8.2.3. Dynamically Changing innodb_lock_wait_timeout

The length of time a transaction waits for a resource, before giving up and rolling back the statement, is determined by the value of the configuration parameter innodb_lock_wait_timeout. (In MySQL 5.0.12 and earlier, the entire transaction was rolled back, not just the statement.) Your application can try the statement again (usually after waiting for a while), or roll back the entire transaction and restart.

The error returned when the timeout period is exceeded is:

ERROR HY000: Lock wait timeout exceeded; try restarting transaction

In MySQL 5.5 and higher, the configuration parameter innodb_lock_wait_timeout can be set at runtime with the SET GLOBAL or SET SESSION statement. Changing the GLOBAL setting requires the SUPER privilege and affects the operation of all clients that subsequently connect. Any client can change the SESSION setting for innodb_lock_wait_timeout, which affects only that client.

In MySQL 5.1 and earlier, the only way to set this parameter was in the MySQL option file (my.cnf or my.ini), and changing it required shutting down and restarting the server.

13.4.8.2.4. Dynamically Changing innodb_adaptive_hash_index

As described in Section 13.4.7.5, “Controlling Adaptive Hash Indexing”, it may be desirable, depending on your workload, to dynamically enable or disable the adaptive hash indexing scheme InnoDB uses to improve query performance.

The start-up option innodb_adaptive_hash_index allows the adaptive hash index to be disabled. It is enabled by default. You can modify this parameter through the SET GLOBAL statement, without restarting the server. Changing the setting requires the SUPER privilege.

Disabling the adaptive hash index empties the hash table immediately. Normal operations can continue while the hash table is emptied, and executing queries that were using the hash table access the index B-trees directly instead. When the adaptive hash index is re-enabled, the hash table is populated again during normal operation.

13.4.8.3. TRUNCATE TABLE Reclaims Space

When you truncate a table that is stored in a .ibd file of its own (because innodb_file_per_table was enabled when the table was created), and if the table is not referenced in a FOREIGN KEY constraint, the table is dropped and re-created in a new .ibd file. This operation is much faster than deleting the rows one by one. The operating system can reuse the disk space, in contrast to tables within the InnoDB system tablespace, where only InnoDB can use the space after they are truncated. Physical backups can also be smaller, without big blocks of unused space in the middle of the system tablespace.

Previous versions of InnoDB would re-use the existing .ibd file, thus releasing the space only to InnoDB for storage management, but not to the operating system. Note that when the table is truncated, the count of rows affected by the TRUNCATE TABLE statement is an arbitrary number.

Замечание

If there is a referential constraint between two columns in the same table, that table can still be truncated using this fast technique.

If there are referential constraints between the table being truncated and other tables, the truncate operation fails. This is a change to the previous behavior, which would transform the TRUNCATE operation to a DELETE operation that removed all the rows and triggered ON DELETE operations on child tables.

13.4.8.4. InnoDB Strict Mode

To guard against ignored typos and syntax errors in SQL, or other unintended consequences of various combinations of operational modes and SQL statements, InnoDB provides a strict mode of operations. In this mode, InnoDB raises error conditions in certain cases, rather than issuing a warning and processing the specified statement (perhaps with unintended behavior). This is analogous to sql_mode in MySQL, which controls what SQL syntax MySQL accepts, and determines whether it silently ignores errors, or validates input syntax and data values. Since strict mode is relatively new, some statements that execute without errors with earlier versions of MySQL might generate errors unless you disable strict mode.

The setting of InnoDB strict mode affects the handling of syntax errors on the CREATE TABLE, ALTER TABLE and CREATE INDEX statements. The strict mode also enables a record size check, so that an INSERT or UPDATE never fails due to the record being too large for the selected page size.

We recommend running in strict mode when using the ROW_FORMAT and KEY_BLOCK_SIZE clauses on CREATE TABLE, ALTER TABLE, and CREATE INDEX statements. Without strict mode, InnoDB ignores conflicting clauses and creates the table or index, with only a warning in the message log. The resulting table might have different behavior than you intended, such as having no compression when you tried to create a compressed table. When InnoDB strict mode is on, such problems generate an immediate error and the table or index is not created, avoiding a troubleshooting session later.

InnoDB strict mode is set with the configuration parameter innodb_strict_mode, which can be specified as on or off. You can set the value on the command line when you start mysqld, or in the configuration file my.cnf or my.ini. You can also enable or disable InnoDB strict mode at run time with the statement SET [GLOBAL|SESSION] innodb_strict_mode=mode, where mode is either ON or OFF. Changing the GLOBAL setting requires the SUPER privilege and affects the operation of all clients that subsequently connect. Any client can change the SESSION setting for innodb_strict_mode, and the setting affects only that client.

13.4.8.5. Controlling Optimizer Statistics Estimation

The MySQL query optimizer uses estimated statistics about key distributions to choose the indexes for an execution plan, based on the relative selectivity of the index. Certain operations cause InnoDB to sample random pages from each index on a table to estimate the cardinality of the index. (This technique is known as random dives.) These operations include the ANALYZE TABLE statement, the SHOW TABLE STATUS statement, and accessing the table for the first time after a restart.

To give you control over the quality of the statistics estimate (and thus better information for the query optimizer), you can now change the number of sampled pages using the parameter innodb_stats_sample_pages. Previously, the number of sampled pages was always 8, which could be insufficient to produce an accurate estimate, leading to poor index choices by the query optimizer. This technique is especially important for large tables and tables used in joins. Unnecessary full table scans for such tables can be a substantial performance issue.

You can set the global parameter innodb_stats_sample_pages, at run time. The default value for this parameter is 8, preserving the same behavior as in past releases.

Замечание

The value of innodb_stats_sample_pages affects the index sampling for all tables and indexes. There are the following potentially significant impacts when you change the index sample size:

  • Small values like 1 or 2 can result in very inaccurate estimates of cardinality.

  • Increasing the innodb_stats_sample_pages value might require more disk reads. Values much larger than 8 (say, 100), can cause a big slowdown in the time it takes to open a table or execute SHOW TABLE STATUS.

  • The optimizer might choose very different query plans based on different estimates of index selectivity.

To disable the cardinality estimation for metadata statements such as SHOW TABLE STATUS, execute the statement SET GLOBAL innodb_stats_on_metadata=OFF (or 0). The ability to set this option dynamically is also relatively new.

All InnoDB tables are opened, and the statistics are re-estimated for all associated indexes, when the mysql client starts if the auto-rehash setting is set on (the default). To improve the start up time of the mysql client, you can turn auto-rehash off. The auto-rehash feature enables automatic name completion of database, table, and column names for interactive users.

Whatever value of innodb_stats_sample_pages works best for a system, set the option and leave it at that value. Choose a value that results in reasonably accurate estimates for all tables in your database without requiring excessive I/O. Because the statistics are automatically recalculated at various times other than on execution of ANALYZE TABLE, it does not make sense to increase the index sample size, run ANALYZE TABLE, then decrease sample size again. The more accurate statistics calculated by ANALYZE running with a high value of innodb_stats_sample_pages can be wiped away later.

Although it is not possible to specify the sample size on a per-table basis, smaller tables generally require fewer index samples than larger tables do. If your database has many large tables, consider using a higher value for innodb_stats_sample_pages than if you have mostly smaller tables.

13.4.8.6. Better Error Handling when Dropping Indexes

For optimal performance with DML statements, InnoDB requires an index to exist on foreign key columns, so that UPDATE and DELETE operations on a parent table can easily check whether corresponding rows exist in the child table. MySQL creates or drops such indexes automatically when needed, as a side-effect of CREATE TABLE, CREATE INDEX, and ALTER TABLE statements.

When you drop an index, InnoDB checks whether the index is not used for checking a foreign key constraint. It is still OK to drop the index if there is another index that can be used to enforce the same constraint. InnoDB prevents you from dropping the last index that can enforce a particular referential constraint.

The message that reports this error condition is:

ERROR 1553 (HY000): Cannot drop index 'fooIdx':
needed in a foreign key constraint

This message is friendlier than the earlier message it replaces:

ERROR 1025 (HY000): Error on rename of './db2/#sql-18eb_3'
to './db2/foo'(errno: 150)

A similar change in error reporting applies to an attempt to drop the primary key index. For tables without an explicit PRIMARY KEY, InnoDB creates an implicit clustered index using the first columns of the table that are declared UNIQUE and NOT NULL. When you drop such an index, InnoDB automatically copies the table and rebuilds the index using a different UNIQUE NOT NULL group of columns or a system-generated key. Since this operation changes the primary key, it uses the slow method of copying the table and re-creating the index, rather than the Fast Index Creation technique from Section 13.4.2.3, “Implementation Details of Fast Index Creation”.

Previously, an attempt to drop an implicit clustered index (the first UNIQUE NOT NULL index) failed if the table did not contain a PRIMARY KEY:

ERROR 42000: This table type requires a primary key

13.4.8.7. More Compact Output of SHOW ENGINE INNODB MUTEX

The statement SHOW ENGINE INNODB MUTEX displays information about InnoDB mutexes and rw-locks. Although this information is useful for tuning on multi-core systems, the amount of output can be overwhelming on systems with a big buffer pool. There is one mutex and one rw-lock in each 16K buffer pool block, and there are 65,536 blocks per gigabyte. It is unlikely that a single block mutex or rw-lock from the buffer pool could become a performance bottleneck.

SHOW ENGINE INNODB MUTEX now skips the mutexes and rw-locks of buffer pool blocks. It also does not list any mutexes or rw-locks that have never been waited on (os_waits=0). Thus, SHOW ENGINE INNODB MUTEX only displays information about mutexes and rw-locks outside of the buffer pool that have caused at least one OS-level wait.

13.4.8.8. More Read-Ahead Statistics

As described in Section 13.4.7.7, “Changes in the Read-Ahead Algorithm”, a read-ahead request is an asynchronous I/O request issued in anticipation that a page will be used in the near future. Knowing how many pages are read through this read-ahead mechanism, and how many of them are evicted from the buffer pool without ever being accessed, can be useful to help fine-tune the parameter innodb_read_ahead_threshold.

SHOW ENGINE INNODB STATUS output displays the global status variables Innodb_buffer_pool_read_ahead and Innodb_buffer_pool_read_ahead_evicted. These variables indicate the number of pages brought into the buffer pool by read-ahead requests, and the number of such pages evicted from the buffer pool without ever being accessed respectively. These counters provide global values since the last server restart.

SHOW ENGINE INNODB INNODB STATUS also shows the rate at which the read-ahead pages are read in and the rate at which such pages are evicted without being accessed. The per-second averages are based on the statistics collected since the last invocation of SHOW ENGINE INNODB INNODB STATUS and are displayed in the BUFFER POOL AND MEMORY section of the output.

Since the InnoDB read-ahead mechanism has been simplified to remove random read-ahead, the status variables Innodb_buffer_pool_read_ahead_rnd and Innodb_buffer_pool_read_ahead_seq are no longer part of the SHOW ENGINE INNODB STATUS output.

13.4.9. Installing the InnoDB Storage Engine

When you use the InnoDB storage engine 1.1 and above, with MySQL 5.5 and above, you do not need to do anything special to install: everything comes configured as part of the MySQL source and binary distributions. This is a change from earlier releases of the InnoDB Plugin, where you were required to match up MySQL and InnoDB version numbers and update your build and configuration processes.

The InnoDB storage engine is included in the MySQL distribution, starting from MySQL 5.1.38. From MySQL 5.1.46 and up, this is the only download location for the InnoDB storage engine; it is not available from the InnoDB web site.

If you used any scripts or configuration files with the earlier InnoDB storage engine from the InnoDB web site, be aware that the filename of the shared library as supplied by MySQL is ha_innodb_plugin.so or ha_innodb_plugin.dll, as opposed to ha_innodb.so or ha_innodb.dll in the older Plugin downloaded from the InnoDB web site. You might need to change the applicable file names in your startup or configuration scripts.

Because the InnoDB storage engine has now replaced the built-in InnoDB, you no longer need to specify options like --ignore-builtin-innodb and --plugin-load during startup.

To take best advantage of current InnoDB features, we recommend specifying the following options in your configuration file:

innodb_file_per_table=1
innodb_file_format=barracuda
innodb_strict_mode=1

For information about these new features, see Section 13.4.8.2.1, “Dynamically Changing innodb_file_per_table, Section 13.4.4, “InnoDB File-Format Management”, and Section 13.4.8.4, “InnoDB Strict Mode”. You might need to continue to use the previous values for these parameters in some replication and similar configurations involving both new and older versions of MySQL.

13.4.10. Upgrading the InnoDB Storage Engine

Prior to MySQL 5.5, some upgrade scenarios involved upgrading the separate instance of InnoDB known as the InnoDB Plugin. In MySQL 5.5 and higher, the features of the InnoDB Plugin have been folded back into built-in InnoDB, so the upgrade procedure for InnoDB is the same as the one for the MySQL server. For details, see Section 2.11.1, “Upgrading MySQL”.

13.4.11. Downgrading the InnoDB Storage Engine

13.4.11.1. Overview

Prior to MySQL 5.5, some downgrade scenarios involved switching the separate instance of InnoDB known as the InnoDB Plugin back to the built-in InnoDB storage engine. In MySQL 5.5 and higher, the features of the InnoDB Plugin have been folded back into built-in InnoDB, so the downgrade procedure for InnoDB is the same as the one for the MySQL server. For details, see Section 2.11.2, “Downgrading MySQL”.

13.4.12. InnoDB Storage Engine Change History

13.4.12.1. Changes in InnoDB Storage Engine 1.x

Since InnoDB 1.1 is tightly integrated with MySQL 5.5, for changes after the initial InnoDB 1.1 release, see the MySQL 5.5 Reference Manual, Section D.1, “Changes in Release 5.5.x (Production)”.

13.4.12.2. Changes in InnoDB Storage Engine 1.1 (April 13, 2010)

For an overview of the changes, see this introduction article for MySQL 5.5 with InnoDB 1.1. The following is a condensed version of the change log.

Fix for bug #52580: Crash in ha_innobase::open on executing INSERT with concurrent ALTER TABLE.

Change in MySQL bug #51557 releases the mutex LOCK_open before ha_innobase::open(), causing racing condition for index translation table creation. Fix it by adding dict_sys mutex for the operation.

Add support for multiple buffer pools.

Fix Bug #26590: MySQL does not allow more than 1023 open transactions. Create additional rollback segments on startup. Reduce the upper limit of total rollback segments from 256 to 128. This is because we can't use the sign bit. It has not caused problems in the past because we only created one segment. InnoDB has always had the capability to use the additional rollback segments, therefore this patch is backward compatible. The only requirement to maintain backward compatibility has been to ensure that the additional segments are created after the double write buffer. This is to avoid breaking assumptions in the existing code.

Implement Performance Schema in InnoDB. Objects in four different modules in InnoDB have been performance instrumented, these modules are: mutexes, rwlocks, file I/O, and threads We mostly preserved the existing APIs, but APIs would point to instrumented function wrappers if performance schema is defined. There are 4 different defines that controls the instrumentation of each module. The feature is off by default, and will be compiled in with special build option, and requre configure option to turn it on when server boots.

Implement the buf_pool_watch for DeleteBuffering in the page hash table. This serves two purposes. It allows multiple watches to be set at the same time (by multiple purge threads) and it removes a race condition when the read of a block completes about the time the buffer pool watch is being set.

Introduce a new mutex to protect flush_list. Redesign mtr_commit() in a way that log_sys mutex is not held while all mtr_memos are popped and is released just after the modified blocks are inserted into the flush_list. This should reduce contention on log_sys mutex.

Implement the global variable innodb_change_buffering, with the following values:

  • none: buffer nothing

  • inserts: buffer inserts (like InnoDB so far)

  • deletes: buffer delete-marks

  • changes: buffer inserts and delete-marks

  • purges: buffer delete-marks and deletes

  • all: buffer all operations (insert, delete-mark, delete)

The default is all. All values except none and inserts will make InnoDB write new-format records to the insert buffer, even for inserts.

Provide support for native AIO on Linux.

13.4.12.3. Changes in InnoDB Plugin 1.0.x

The InnoDB 1.0.x releases that accompany MySQL 5.1 have their own change history. Changes up to InnoDB 1.0.8 are listed at http://dev.mysql.com/doc/innodb-plugin/1.0/en/innodb-changes.html. Changes from InnoDB 1.0.9 and up are listed in Changes in Release 5.1.x (Production), incorporated into the main MySQL change log.

13.4.13. Third-Party Software

Oracle acknowledges that certain Third Party and Open Source software has been used to develop or is incorporated in the InnoDB storage engine. This appendix includes required third-party license information.

13.4.13.1. Performance Patches from Google

Oracle gratefully acknowledges the following contributions from Google, Inc. to improve InnoDB performance:

  • Replacing InnoDB's use of Pthreads mutexes with calls to GCC atomic builtins, as discussed in Section 13.4.7.2, “Faster Locking for Improved Scalability”. This change means that InnoDB mutex and and rw-lock operations take less CPU time, and improves throughput on those platforms where the atomic operations are available.

  • Controlling master thread I/O rate, as discussed in Section 13.4.7.11, “Controlling the Master Thread I/O Rate”. The master thread in InnoDB is a thread that performs various tasks in the background. Historically, InnoDB has used a hard coded value as the total I/O capacity of the server. With this change, user can control the number of I/O operations that can be performed per second based on their own workload.

Changes from the Google contributions were incorporated in the following source code files: btr0cur.c, btr0sea.c, buf0buf.c, buf0buf.ic, ha_innodb.cc, log0log.c, log0log.h, os0sync.h, row0sel.c, srv0srv.c, srv0srv.h, srv0start.c, sync0arr.c, sync0rw.c, sync0rw.h, sync0rw.ic, sync0sync.c, sync0sync.h, sync0sync.ic, and univ.i.

These contributions are incorporated subject to the conditions contained in the file COPYING.Google, which are reproduced here.

Copyright (c) 2008, 2009, Google Inc.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above
      copyright notice, this list of conditions and the following
      disclaimer in the documentation and/or other materials
      provided with the distribution.
    * Neither the name of the Google Inc. nor the names of its
      contributors may be used to endorse or promote products
      derived from this software without specific prior written
      permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS“AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.

13.4.13.2. Multiple Background I/O Threads Patch from Percona

Oracle gratefully acknowledges the contribution of Percona, Inc. to improve InnoDB performance by implementing configurable background threads, as discussed in Section 13.4.7.8, “Multiple Background I/O Threads”. InnoDB uses background threads to service various types of I/O requests. The change provides another way to make InnoDB more scalable on high end systems.

Changes from the Percona, Inc. contribution were incorporated in the following source code files: ha_innodb.cc, os0file.c, os0file.h, srv0srv.c, srv0srv.h, and srv0start.c.

This contribution is incorporated subject to the conditions contained in the file COPYING.Percona, which are reproduced here.

Copyright (c) 2008, 2009, Percona Inc.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above
      copyright notice, this list of conditions and the following
      disclaimer in the documentation and/or other materials
      provided with the distribution.
    * Neither the name of the Percona Inc. nor the names of its
      contributors may be used to endorse or promote products
      derived from this software without specific prior written
      permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS“AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.

13.4.13.3. Performance Patches from Sun Microsystems

Oracle gratefully acknowledges the following contributions from Sun Microsystems, Inc. to improve InnoDB performance:

Changes from the Sun Microsystems, Inc. contribution were incorporated in the following source code files: univ.i, ut0ut.c, and ut0ut.h.

This contribution is incorporated subject to the conditions contained in the file COPYING.Sun_Microsystems, which are reproduced here.

Copyright (c) 2009, Sun Microsystems, Inc.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
    * Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above
      copyright notice, this list of conditions and the following
      disclaimer in the documentation and/or other materials
      provided with the distribution.
    * Neither the name of Sun Microsystems, Inc. nor the names of its
      contributors may be used to endorse or promote products
      derived from this software without specific prior written
      permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS“AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.

13.4.14. List of Parameters Changed in InnoDB 1.1 and InnoDB Plugin 1.0

13.4.14.1. New Parameters

Throughout the course of development, InnoDB 1.1 and its predecessor the InnoDB Plugin introduced new configuration parameters. The following table summarizes those parameters:

Table 13.8. InnoDB 1.1 New Parameter Summary

NameCmd-LineOption FileSystem VarScopeDynamicDefault
innodb_adaptive_flushingYESYESYESGLOBALYESTRUE
innodb_buffer_pool_instancesYESYESYESGLOBALYESTRUE
innodb_change_bufferingYESYESYESGLOBALYESinserts
innodb_file_formatYESYESYESGLOBALYESAntelope
innodb_file_format_checkYESYESYESGLOBALNO1
innodb_file_format_maxYESYESYESGLOBALYESAntelope for a new database; Barracuda if any tables using that file format exist in the database
innodb_io_capacityYESYESYESGLOBALYES200
innodb_old_blocks_pctYESYESYESGLOBALYES37
innodb_old_blocks_timeYESYESYESGLOBALYES0
innodb_purge_batch_sizeYESYESYESGLOBALYES0
innodb_purge_threadsYESYESYESGLOBALYES0
innodb_read_ahead_thresholdYESYESYESGLOBALYES56
innodb_read_io_threadsYESYESYESGLOBALNO4
innodb_spin_wait_delayYESYESYESGLOBALYES6
innodb_stats_sample_pagesYESYESYESGLOBALYES8
innodb_strict_modeYESYESYESGLOBAL|SESSIONYESFALSE
innodb_use_native_aioYESYESYESGLOBALNOTRUE
innodb_use_sys_mallocYESYESYESGLOBALNOTRUE
innodb_write_io_threadsYESYESYESGLOBALNO4

13.4.14.2. Deprecated Parameters

Beginning in InnoDB storage engine 1.0.4, the following configuration parameter has been removed:

13.4.14.3. Parameters with New Defaults

For better out-of-the-box performance, the following InnoDB configuration parameters have new default values since MySQL 5.1:

Table 13.9. InnoDB Parameters with New Defaults

NameOld DefaultNew Default
innodb_additional_mem_pool_size1MB8MB
innodb_buffer_pool_size8MB128MB
innodb_change_bufferinginsertsall
innodb_file_format_checkON1
innodb_log_buffer_size1MB8MB
innodb_max_dirty_pages_pct9075
innodb_sync_spin_loops2030
innodb_thread_concurrency80

13.5. The MyISAM Storage Engine

Before MySQL 5.5.5, MyISAM is the default storage engine. (The default was changed to InnoDB in MySQL 5.5.5.) MyISAM is based on the older (and no longer available) ISAM storage engine but has many useful extensions.

Table 13.10. MyISAM Storage Engine Features

Storage limits256TBTransactionsNoLocking granularityTable
MVCCNoGeospatial data type supportYesGeospatial indexing supportYes
B-tree indexesYesHash indexesNoFull-text search indexesYes
Clustered indexesNoData cachesNoIndex cachesYes
Compressed dataYes[a]Encrypted data[b]YesCluster database supportNo
Replication support[c]YesForeign key supportNoBackup / point-in-time recovery[d]Yes
Query cache supportYesUpdate statistics for data dictionaryYes  

[a] Compressed MyISAM tables are supported only when using the compressed row format. Tables using the compressed row format with MyISAM are read only.

[b] Implemented in the server (via encryption functions), rather than in the storage engine.

[c] Implemented in the server, rather than in the storage product.

[d] Implemented in the server, rather than in the storage product.

Each MyISAM table is stored on disk in three files. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format. The data file has an .MYD (MYData) extension. The index file has an .MYI (MYIndex) extension.

To specify explicitly that you want a MyISAM table, indicate that with an ENGINE table option:

CREATE TABLE t (i INT) ENGINE = MYISAM;

As of MySQL 5.5.5, it is normally necessary to use ENGINE to specify the MyISAM storage engine because InnoDB is the default engine. Before 5.5.5, this is unnecessary because MyISAM is the default engine unless the default has been changed. To ensure that MyISAM is used in situations where the default might have been changed, include the ENGINE option explicitly.

You can check or repair MyISAM tables with the mysqlcheck client or myisamchk utility. You can also compress MyISAM tables with myisampack to take up much less space. See Section 4.5.3, “mysqlcheck — A Table Maintenance Program”, Section 4.6.3, “myisamchk — MyISAM Table-Maintenance Utility”, and Section 4.6.5, “myisampack — Generate Compressed, Read-Only MyISAM Tables”.

MyISAM tables have the following characteristics:

  • All data values are stored with the low byte first. This makes the data machine and operating system independent. The only requirements for binary portability are that the machine uses two's-complement signed integers and IEEE floating-point format. These requirements are widely used among mainstream machines. Binary compatibility might not be applicable to embedded systems, which sometimes have peculiar processors.

    There is no significant speed penalty for storing data low byte first; the bytes in a table row normally are unaligned and it takes little more processing to read an unaligned byte in order than in reverse order. Also, the code in the server that fetches column values is not time critical compared to other code.

  • All numeric key values are stored with the high byte first to permit better index compression.

  • Large files (up to 63-bit file length) are supported on file systems and operating systems that support large files.

  • There is a limit of (232)2 (1.844E+19) rows in a MyISAM table.

  • The maximum number of indexes per MyISAM table is 64.

    The maximum number of columns per index is 16.

  • The maximum key length is 1000 bytes. This can also be changed by changing the source and recompiling. For the case of a key longer than 250 bytes, a larger key block size than the default of 1024 bytes is used.

  • When rows are inserted in sorted order (as when you are using an AUTO_INCREMENT column), the index tree is split so that the high node only contains one key. This improves space utilization in the index tree.

  • Internal handling of one AUTO_INCREMENT column per table is supported. MyISAM automatically updates this column for INSERT and UPDATE operations. This makes AUTO_INCREMENT columns faster (at least 10%). Values at the top of the sequence are not reused after being deleted. (When an AUTO_INCREMENT column is defined as the last column of a multiple-column index, reuse of values deleted from the top of a sequence does occur.) The AUTO_INCREMENT value can be reset with ALTER TABLE or myisamchk.

  • Dynamic-sized rows are much less fragmented when mixing deletes with updates and inserts. This is done by automatically combining adjacent deleted blocks and by extending blocks if the next block is deleted.

  • MyISAM supports concurrent inserts: If a table has no free blocks in the middle of the data file, you can INSERT new rows into it at the same time that other threads are reading from the table. A free block can occur as a result of deleting rows or an update of a dynamic length row with more data than its current contents. When all free blocks are used up (filled in), future inserts become concurrent again. See Section 7.10.3, “Concurrent Inserts”.

  • You can put the data file and index file in different directories on different physical devices to get more speed with the DATA DIRECTORY and INDEX DIRECTORY table options to CREATE TABLE. See Section 12.1.17, “CREATE TABLE Синтаксис”.

  • BLOB and TEXT columns can be indexed.

  • NULL values are permitted in indexed columns. This takes 0 to 1 bytes per key.

  • Each character column can have a different character set. See Section 9.1, “Character Set Support”.

  • There is a flag in the MyISAM index file that indicates whether the table was closed correctly. If mysqld is started with the --myisam-recover-options option, MyISAM tables are automatically checked when opened, and are repaired if the table wasn't closed properly.

  • myisamchk marks tables as checked if you run it with the --update-state option. myisamchk --fast checks only those tables that don't have this mark.

  • myisamchk --analyze stores statistics for portions of keys, as well as for entire keys.

  • myisampack can pack BLOB and VARCHAR columns.

MyISAM also supports the following features:

  • Support for a true VARCHAR type; a VARCHAR column starts with a length stored in one or two bytes.

  • Tables with VARCHAR columns may have fixed or dynamic row length.

  • The sum of the lengths of the VARCHAR and CHAR columns in a table may be up to 64KB.

  • Arbitrary length UNIQUE constraints.

Additional Resources

13.5.1. MyISAM Startup Options

The following options to mysqld can be used to change the behavior of MyISAM tables. For additional information, see Section 5.1.2, “Server Command Options”.

Table 13.11. MyISAM Option/Variable Reference

NameCmd-LineOption fileSystem VarStatus VarVar ScopeDynamic
bulk_insert_buffer_sizeYesYesYes BothYes
concurrent_insertYesYesYes GlobalYes
delay-key-writeYesYes  GlobalYes
- Variable: delay_key_write  Yes GlobalYes
have_rtree_keys  Yes GlobalNo
key_buffer_sizeYesYesYes GlobalYes
log-isamYesYes    
myisam-block-sizeYesYes    
myisam_data_pointer_sizeYesYesYes GlobalYes
myisam_max_sort_file_sizeYesYesYes GlobalYes
myisam_mmap_sizeYesYesYes GlobalNo
myisam-recoverYesYes    
- Variable: myisam_recover_options      
myisam-recover-optionsYesYes    
- Variable: myisam_recover_options      
myisam_recover_options  Yes GlobalNo
myisam_repair_threadsYesYesYes BothYes
myisam_sort_buffer_sizeYesYesYes BothYes
myisam_stats_methodYesYesYes BothYes
myisam_use_mmapYesYesYes GlobalYes
skip-concurrent-insertYesYes    
- Variable: concurrent_insert      
tmp_table_sizeYesYesYes BothYes
  • --myisam-recover-options=mode

    Set the mode for automatic recovery of crashed MyISAM tables.

  • --delay-key-write=ALL

    Don't flush key buffers between writes for any MyISAM table.

    Замечание

    If you do this, you should not access MyISAM tables from another program (such as from another MySQL server or with myisamchk) when the tables are in use. Doing so risks index corruption. Using --external-locking does not eliminate this risk.

The following system variables affect the behavior of MyISAM tables. For additional information, see Section 5.1.3, “Server System Variables”.

Automatic recovery is activated if you start mysqld with the --myisam-recover-options option. In this case, when the server opens a MyISAM table, it checks whether the table is marked as crashed or whether the open count variable for the table is not 0 and you are running the server w