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 whenInnoDB
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 theInnoDB
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 theinnodb_purge_threads
弗吉尼亚州riable. 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 theinnodb_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 theinnodb_purge_threads
setting accordingly. Theinnodb_max_purge_lag
弗吉尼亚州riable 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 theinnodb_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 higherinnodb_purge_threads
setting. The maximum number of purge threads is 32.
Theinnodb_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
Theinnodb_purge_batch_size
弗吉尼亚州riable 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 dividesinnodb_purge_batch_size
byinnodb_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, theinnodb_purge_batch_size
弗吉尼亚州riable defines the number of undo log pages that purge frees every 128 iterations through the undo logs.
Theinnodb_purge_batch_size
弗吉尼亚州riable is intended for advanced performance tuning and experimentation. Most users need not changeinnodb_purge_batch_size
from its default value.
Configuring the Maximum Purge Lag
Theinnodb_max_purge_lag
弗吉尼亚州riable defines the desired maximum purge lag. When the purge lag exceeds theinnodb_max_purge_lag
threshold, a delay is imposed onINSERT
,UPDATE
, andDELETE
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.
TheInnoDB
transaction system maintains a list of transactions that have index records delete-marked byUPDATE
orDELETE
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 typicalinnodb_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 theHistory list length
弗吉尼亚州lue in theTRANSACTIONS
section ofSHOW 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
TheHistory 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 theHistory list length
to increase is that under a consistent read transaction isolation level such asREPEATABLE READ
, a transaction must return the same result as when the read view for that transaction was created. Consequently, theInnoDB
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 theHistory list length
to increase:
Amysqldumpoperation that uses the
--single-transaction
option while there is a significant amount of concurrent DML.Running a
SELECT
query after disablingautocommit
, and forgetting to issue an explicitCOMMIT
orROLLBACK
.
To prevent excessive delays in extreme situations where the purge lag becomes huge, you can limit the delay by setting theinnodb_max_purge_lag_delay
弗吉尼亚州riable. Theinnodb_max_purge_lag_delay
弗吉尼亚州riable specifies the maximum delay in microseconds for the delay imposed when theinnodb_max_purge_lag
threshold is exceeded. The specifiedinnodb_max_purge_lag_delay
弗吉尼亚州lue is an upper limit on the delay period calculated by theinnodb_max_purge_lag
formula.
Purge and Undo Tablespace Truncation
als净化系统o responsible for truncating undo tablespaces. You can configure theinnodb_purge_rseg_truncate_frequency
弗吉尼亚州riable to control the frequency with which the purge system looks for undo tablespaces to truncate. For more information, seeTruncating Undo Tablespaces.