17.8.9 Purge Configuration

InnoDB does not physically remove a row from the database immediately when you delete it with an SQL statement. A row and its index records are only physically removed when InnoDB discards the undo log record written for the deletion. This removal operation, which only occurs after the row is no longer required for multi-version concurrency control (MVCC) or rollback, is called a purge.

Purge runs on a periodic schedule. It parses and processes undo log pages from the history list, which is a list of undo log pages for committed transactions that is maintained by the InnoDB transaction system. Purge frees the undo log pages from the history list after processing them.

Configuring Purge Threads

Purge operations are performed in the background by one or more purge threads. The number of purge threads is controlled by the innodb_purge_threads variable. The default value is 4.

If DML action is concentrated on a single table, purge operations for the table are performed by a single purge thread, which can result in slowed purge operations, increased purge lag, and increased tablespace file size if the DML operations involve large object values. From MySQL 8.0.26, if the innodb_max_purge_lag setting is exceeded, purge work is automatically redistributed among available purge threads. Too many active purge threads in this scenario can cause contention with user threads, so manage the innodb_purge_threads setting accordingly. The innodb_max_purge_lag variable is set to 0 by default, which means that there is no maximum purge lag by default.

If DML action is concentrated on few tables, keep the innodb_purge_threads setting low so that the threads do not contend with each other for access to the busy tables. If DML operations are spread across many tables, consider a higher innodb_purge_threads setting. The maximum number of purge threads is 32.

The innodb_purge_threads setting is the maximum number of purge threads permitted. The purge system automatically adjusts the number of purge threads that are used.

Configuring Purge Batch Size

The innodb_purge_batch_size variable defines the number of undo log pages that purge parses and processes in one batch from the history list. The default value is 300. In a multithreaded purge configuration, the coordinator purge thread divides innodb_purge_batch_size by innodb_purge_threads and assigns that number of pages to each purge thread.

The purge system also frees the undo log pages that are no longer required. It does so every 128 iterations through the undo logs. In addition to defining the number of undo log pages parsed and processed in a batch, the innodb_purge_batch_size variable defines the number of undo log pages that purge frees every 128 iterations through the undo logs.

The innodb_purge_batch_size variable is intended for advanced performance tuning and experimentation. Most users need not change innodb_purge_batch_size from its default value.

Configuring the Maximum Purge Lag

The innodb_max_purge_lag variable defines the desired maximum purge lag. When the purge lag exceeds the innodb_max_purge_lag threshold, a delay is imposed on INSERT, UPDATE, and DELETE operations to allow time for purge operations to catch up. The default value is 0, which means there is no maximum purge lag and no delay.

The InnoDB transaction system maintains a list of transactions that have index records delete-marked by UPDATE or DELETE operations. The length of the list is the purge lag. Prior to MySQL 8.0.14, the purge lag delay is calculated by the following formula, which results in a minimum delay of 5000 microseconds:

(purge lag/innodb_max_purge_lag - 0.5) * 10000

As of MySQL 8.0.14, the purge lag delay is calculated by the following revised formula, which reduces the minimum delay to 5 microseconds. A delay of 5 microseconds is more appropriate for modern systems.

(purge_lag/innodb_max_purge_lag - 0.9995) * 10000

The delay is calculated at the beginning of a purge batch.

A typical innodb_max_purge_lag setting for a problematic workload might be 1000000 (1 million), assuming that transactions are small, only 100 bytes in size, and it is permissible to have 100MB of unpurged table rows.

The purge lag is presented as the History list length value in the TRANSACTIONS section of SHOW ENGINE INNODB STATUS output.

mysql> SHOW ENGINE INNODB STATUS;
...
------------
TRANSACTIONS
------------
Trx id counter 0 290328385
Purge done for trx's n:o < 0 290315608 undo n:o < 0 17
History list length 20

The History list length is typically a low value, usually less than a few thousand, but a write-heavy workload or long running transactions can cause it to increase, even for transactions that are read only. The reason that a long running transaction can cause the History list length to increase is that under a consistent read transaction isolation level such as REPEATABLE READ, a transaction must return the same result as when the read view for that transaction was created. Consequently, the InnoDB multi-version concurrency control (MVCC) system must keep a copy of the data in the undo log until all transactions that depend on that data have completed. The following are examples of long running transactions that could cause the History list length to increase:

• A mysqldump operation that uses the --single-transaction option while there is a significant amount of concurrent DML.

• Running a SELECT query after disabling autocommit, and forgetting to issue an explicit COMMIT or ROLLBACK.

To prevent excessive delays in extreme situations where the purge lag becomes huge, you can limit the delay by setting the innodb_max_purge_lag_delay variable. The innodb_max_purge_lag_delay variable specifies the maximum delay in microseconds for the delay imposed when the innodb_max_purge_lag threshold is exceeded. The specified innodb_max_purge_lag_delay value is an upper limit on the delay period calculated by the innodb_max_purge_lag formula.

Purge and Undo Tablespace Truncation

The purge system is also responsible for truncating undo tablespaces. You can configure the innodb_purge_rseg_truncate_frequency variable to control the frequency with which the purge system looks for undo tablespaces to truncate. For more information, see Truncating Undo Tablespaces.