15.6.3.4 Undo Tablespaces

Because undo logs can become large during long-running transactions, creating additional undo tablespaces can help prevent individual undo tablespaces from becoming too large. As of MySQL 8.0.14, additional undo tablespaces can be created at runtime using CREATE UNDO TABLESPACE syntax.

The undo tablespace file name must have an .ibu extension. It is not permitted to specify a relative path when defining the undo tablespace file name. A fully qualified path is permitted, but the path must be known to InnoDB. Known paths are those defined by the innodb_directories variable. Unique undo tablespace file names are recommended to avoid potential file name conflicts when moving or cloning data.

At startup, directories defined by the innodb_directories variable are scanned for undo tablespace files. (The scan also traverses subdirectories.) Directories defined by the innodb_data_home_dir, innodb_undo_directory, and datadir variables are automatically appended to the innodb_directories value, regardless of whether the innodb_directories variable is defined explicitly. An undo tablespace can therefore reside in paths defined by any of those variables.

If the undo tablespace file name does not include a path, the undo tablespace is created in the directory defined by the innodb_undo_directory variable. If that variable is undefined, the undo tablespace is created in the data directory.

To create undo tablespaces in a path relative to the data directory, set the innodb_undo_directory variable to the relative path, and specify the file name only when creating an undo tablespace.

To view undo tablespace names and paths, query INFORMATION_SCHEMA.FILES:

A MySQL instance supports up to 127 undo tablespaces including the two default undo tablespaces created when the MySQL instance is initialized.

As of MySQL 8.0.14, undo tablespaces created using CREATE UNDO TABLESPACE syntax can be dropped at runtime using DROP UNDO TABALESPACE syntax.

An undo tablespace must be empty before it can be dropped. To empty an undo tablespace, the undo tablespace must first be marked as inactive using ALTER UNDO TABLESPACE syntax so that the tablespace is no longer used for assigning rollback segments to new transactions.

After an undo tablespace is marked as inactive, transactions currently using rollback segments in the undo tablespace are permitted to finish, as are any transactions started before those transactions are completed. After transactions are completed, the purge system frees the rollback segments in the undo tablespace, and the undo tablespace is truncated to its initial size. (The same process is used when truncating undo tablespaces. See Truncating Undo Tablespaces.) When the undo tablespace is empty, it can be dropped.

The state of an undo tablespace can be monitored by querying the INFORMATION_SCHEMA.INNODB_TABLESPACES table.

An inactive state indicates that rollback segments in an undo tablespace are no longer used by new transactions. An empty state indicates that an undo tablespace is empty and ready to be dropped, or made active again using an ALTER UNDO TABLESPACE tablespace_name SET ACTIVE statement. Attempting to drop an undo tablespace that is not empty returns an error.

The default undo tablespaces (innodb_undo_001 and innodb_undo_002) created when the MySQL instance is initialized cannot be dropped. They can, however, be made inactive using an ALTER UNDO TABLESPACE tablespace_name SET INACTIVE statement. Before a default undo tablespace can be made inactive, there must be an undo tablespace to take its place. A minimum of two active undo tablespaces are required at all times to support automated truncation of undo tablespaces.

Undo tablespaces created with CREATE UNDO TABLESPACE syntax can be moved while the server is offline to any known directory. Known directories are those defined by the innodb_directories variable. Directories defined by innodb_data_home_dir, innodb_undo_directory, and datadir are automatically appended to the innodb_directories value regardless of whether the innodb_directories variable is defined explicitly. Those directories and their subdirectories are scanned at startup for undo tablespaces files. An undo tablespace file moved to any of those directories is discovered at startup and assumed to be the undo tablespace that was moved.

The default undo tablespaces (innodb_undo_001 and innodb_undo_002) created when the MySQL instance is initialized must always reside in the directory defined by the innodb_undo_directory variable. If the innodb_undo_directory variable is undefined, default undo tablespaces reside in the data directory. If default undo tablespaces are moved while the server is offline, the server must be started with the innodb_undo_directory variable configured to the new directory.

The I/O patterns for undo logs make undo tablespaces good candidates for SSD storage.

The innodb_rollback_segments variable defines the number of rollback segments allocated to each undo tablespace and to the global temporary tablespace. The innodb_rollback_segments variable can be configured at startup or while the server is running.

The default setting for innodb_rollback_segments is 128, which is also the maximum value. For information about the number of transactions that a rollback segment supports, see Section 15.6.6, “Undo Logs”.

There are two methods of truncating undo tablespaces, which can be used individually or in combination to manage undo tablespace size. One method is automated, enabled using configuration variables. The other method is manual, performed using SQL statements.

The automated method does not require monitoring undo tablespace size and, once enabled, it performs deactivation, truncation, and reactivation of undo tablespaces without manual intervention. The manual truncation method may be preferable if you want to control when undo tablespaces are taken offline for truncation. For example, you may want to avoid truncating undo tablespaces during peak workload times.

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Automated Truncation

Automated truncation of undo tablespaces requires a minimum of two active undo tablespaces, which ensures that one undo tablespace remains active while the other is taken offline to be truncated. By default, two undo tablespaces are created when the MySQL instance is initialized.

To have undo tablespaces automatically truncated, enable the innodb_undo_log_truncate variable. For example:

When the innodb_undo_log_truncate variable is enabled, undo tablespaces that exceed the size limit defined by the innodb_max_undo_log_size variable are subject to truncation. The innodb_max_undo_log_size variable is dynamic and has a default value of 1073741824 bytes (1024 MiB).

When the innodb_undo_log_truncate variable is enabled:

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Manual Truncation

Manual truncation of undo tablespaces requires a minimum of three active undo tablespaces. Two active undo tablespaces are required at all times to support the possibility that automated truncation is enabled. A minimum of three undo tablespaces satisfies this requirement while permitting an undo tablespace to be taken offline manually.

To manually initiate truncation of an undo tablespace, deactivate the undo tablespace by issuing the following statement:

After the undo tablespace is marked as inactive, transactions currently using rollback segments in the undo tablespace are permitted to finish, as are any transactions started before those transactions are completed. After transactions are completed, the purge system frees the rollback segments in the undo tablespace, the undo tablespace is truncated to its initial size, and the undo tablespace state changes from inactive to empty.

To check the state of an undo tablespace, query the INFORMATION_SCHEMA.INNODB_TABLESPACES table.

Once the undo tablespace is in an empty state, it can be reactivated by issuing the following statement:

An undo tablespace in an empty state can also be dropped. See Dropping Undo Tablespaces.

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Expediting Automated Truncation of Undo Tablespaces

The purge thread is responsible for emptying and truncating undo tablespaces. By default, the purge thread looks for undo tablespaces to truncate once every 128 times that purge is invoked. The frequency with which the purge thread looks for undo tablespaces to truncate is controlled by the innodb_purge_rseg_truncate_frequency variable, which has a default setting of 128.

To increase that frequency, decrease the innodb_purge_rseg_truncate_frequency setting. For example, to have the purge thread look for undo tabespaces once every 32 timees that purge is invoked, set innodb_purge_rseg_truncate_frequency to 32.

When the purge thread finds an undo tablespace that requires truncation, the purge thread returns with increased frequency to quickly empty and truncate the undo tablespace.

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Performance Impact of Truncating Undo Tablespace Files

When an undo tablespace is truncated, the rollback segments in the undo tablespace are deactivated. The active rollback segments in other undo tablespaces assume responsibility for the entire system load, which may result in a slight performance degradation. The amount of performance degradation depends on a number of factors:

The easiest way to avoid impacting performance when truncating undo tablespaces is to increase the number of undo tablespaces.

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Monitoring Undo Tablespace Truncation

As of MySQL 8.0.16, undo and purge susbsystem counters are provided for monitoring background activities associated with undo log truncation. For counter names and descriptions, query the INFORMATION_SCHEMA.INNODB_METRICS table.

For information about enabling counters and querying counter data, see Section 15.14.6, “InnoDB INFORMATION_SCHEMA Metrics Table”.