FLUSH TABLES WITH READ LOCK option¶

The FLUSH TABLES WITH READ LOCK option does the following with a global read lock:

Closes all open tables
Locks all tables for all databases

Release the lock with UNLOCK TABLES.

Note

FLUSH TABLES WITH READ LOCK does not prevent inserting rows into the log tables.

To ensure consistent backups, use the FLUSH TABLES WITH READ LOCK option before taking a non-InnoDB file backup. The option does not affect long-running queries.

Long-running queries with FLUSH TABLES WITH READ LOCK enabled can leave the server in a read-only mode until the queries finish. Killing the FLUSH TABLES WITH READ LOCK does not help if the database is in either the Waiting for table flush or Waiting for master to send event state. To return to normal operation, you must kill any long-running queries.

Note

All described in this section has no effect when backup locks are used. Percona XtraBackup will use Backup locks where available as a lightweight alternative to FLUSH TABLES WITH READ LOCK. This feature is available in Percona Server for MySQL 5.6+. Percona XtraBackup uses this automatically to copy non-InnoDB data to avoid blocking DML queries that modify InnoDB tables.

In order to prevent this from happening two things have been implemented:

xtrabackup waits for a good moment to issue the global lock
xtrabackup kills all queries or only the SELECT queries which prevent the global lock from being acquired

Wait for queries to finish¶

You should issue a global lock when no long queries are running. Waiting to issue the global lock for extended period of time is not a good method. The wait can extend the time needed for backup to take place. The –ftwrl-wait-timeout option can limit the waiting time. If it cannot issue the lock during this time, xtrabackup stops the option, exits with an error message, and backup is not be taken.

The default value for this option is zero (0) value which turns off the option.

Another possibility is to specify the type of query to wait on. In this case --ftwrl-wait-query-type. Possible values are all and update. When all is used xtrabackup will wait for all long running queries (execution time longer than allowed by --ftwrl-wait-threshold) to finish before running the FLUSH TABLES WITH READ LOCK. When update is used xtrabackup will wait on UPDATE/ALTER/REPLACE/INSERT queries to finish.

The time needed for a specific query to complete is hard to predict. We assume that the long-running queries will not finish in a timely manner. Other queries which run for a short time finish quickly. xtrabackup uses the value of –ftwrl-wait-threshold option to specify the long-running queries and will block a global lock. In order to use this option xtrabackup user should have PROCESS and SUPER privileges.

Kill the blocking queries¶

The second option is to kill all the queries which prevent from acquiring the global lock. In this case, all queries which run longer than FLUSH TABLES WITH READ LOCK are potential blockers. Although all queries can be killed, additional time can be specified for the short running queries to finish using the --kill-long-queries-timeout option. This option specifies the time for queries to complete, after the value is reached, all the running queries will be killed. The default value is zero, which turns this feature off.

The --kill-long-query-type option can be used to specify all or only SELECT queries that are preventing global lock from being acquired. In order to use this option xtrabackup user should have PROCESS and SUPER privileges.

Options summary¶

--ftwrl-wait-timeout (seconds) - how long to wait for a good moment. Default is 0, not to wait.
--ftwrl-wait-query-type - which long queries should be finished before FLUSH TABLES WITH READ LOCK is run. Default is all.
--ftwrl-wait-threshold (seconds) - how long query should be running before we consider it long running and potential blocker of global lock.
--kill-long-queries-timeout (seconds) - how many time we give for queries to complete after FLUSH TABLES WITH READ LOCK is issued before start to kill. Default if 0, not to kill.
--kill-long-query-type - which queries should be killed once kill-long-queries-timeout has expired.

Example¶

Running the xtrabackup with the following options will cause xtrabackup to spend no longer than 3 minutes waiting for all queries older than 40 seconds to complete.

$  xtrabackup --backup --ftwrl-wait-threshold=40 \
--ftwrl-wait-query-type=all --ftwrl-wait-timeout=180 \
--kill-long-queries-timeout=20 --kill-long-query-type=all \
--target-dir=/data/backups/

After FLUSH TABLES WITH READ LOCK is issued, xtrabackup will wait for 20 seconds for lock to be acquired. If lock is still not acquired after 20 seconds, it will kill all queries which are running longer that the FLUSH TABLES WITH READ LOCK.

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Last update: 2022-12-06