14.6.3.3 Undo Tablespaces

Undo tablespaces contain undo logs, which are collections of undo log records that contain information about how to undo the latest change by a transaction to a clustered index record. Undo logs exist within undo log segments, which are contained within rollback segments. The innodb_rollback_segments variable defines the number of rollback segments allocated to each undo tablespace.

Undo logs can be stored in one or more undo tablespaces instead of the system tablespace. This layout differs from the default configuration in which undo logs reside in the system tablespace. The I/O patterns for undo logs make undo tablespaces good candidates for SSD storage, while keeping the system tablespace on hard disk storage.

The number of undo tablespaces used by InnoDB is controlled by the innodb_undo_tablespaces configuration option. This option can only be configured when initializing the MySQL instance. It cannot be changed afterward.

Undo tablespaces and individual segments inside those tablespaces cannot be dropped.

Configuring Undo Tablespaces

To configure undo tablespaces for a MySQL instance, perform the following steps. It is assumed that you are performing the procedure on a test instance prior to deploying the configuration to a production system.

Important

The number of undo tablespaces can only be configured when initializing a MySQL instance and is fixed for the life of the instance.

1. Specify a directory location for undo tablespaces using the innodb_undo_directory configuration option. If a directory location is not specified, undo tablespaces are created in the data directory.

2. Define the number of rollback segments using the innodb_rollback_segments configuration option. Start with a relatively low value and increase it incrementally over time to examine the effect on performance. The default setting for innodb_rollback_segments is 128, which is also the maximum value.

   One rollback segment is always assigned to the system tablespace. Therefore, to allocate rollback segments to undo tablespaces, set innodb_rollback_segments to a value greater than 1. For example, if you have two undo tablespaces, set innodb_rollback_segments to 3 to assign one rollback segment to each of the two undo tablespaces. Rollback segments are distributed among undo tablespaces in a circular fashion.

   When you configure separate undo tablespaces, the rollback segment in the system tablespace is rendered inactive.

3. Define the number of undo tablespaces using the innodb_undo_tablespaces option. The specified number of undo tablespaces is fixed for the life of the MySQL instance, so if you are uncertain about an optimal value, estimate on the high side.

4. Create a new MySQL test instance using the option values you have chosen.

5. Use a realistic workload on your test instance with data volume similar to your production servers to test the configuration.

6. Benchmark the performance of I/O intensive workloads.

7. Periodically increase the value of innodb_rollback_segments and rerun performance tests until there are no further improvements in I/O performance.