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.
Purge operations are performed in the background by one or more
purge threads. The number of purge threads is controlled by the
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_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.
is the maximum number of purge threads permitted. The purge system
automatically adjusts the number of purge threads that are used.
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_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
defines the number of undo log pages that purge frees every 128
iterations through the undo logs.
defines the desired maximum purge lag. When the purge lag exceeds
threshold, a delay is imposed on
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.
InnoDB transaction system maintains a list
of transactions that have index records delete-marked by
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.
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
length value in the
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
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
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
History list length to increase:
To prevent excessive delays in extreme situations where the purge
lag becomes huge, you can limit the delay by setting the
variable specifies the maximum delay in microseconds for the delay
imposed when the
innodb_max_purge_lag threshold is
exceeded. The specified
is an upper limit on the delay period calculated by the
The purge system is also responsible for truncating undo
tablespaces. You can configure the
variable to control the frequency with which the purge system
looks for undo tablespaces to truncate. For more information, see
Truncating Undo Tablespaces.