FLUSH TABLES WITH READ LOCK option¶
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
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.
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
Waiting 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 is used xtrabackup will wait for all long running
queries (execution time longer than allowed by
to finish before running the
FLUSH TABLES WITH READ LOCK. When
used xtrabackup will wait on
UPDATE/ALTER/REPLACE/INSERT queries to
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
Killing 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
--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
--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
--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 LOCKis 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 LOCKis issued before start to kill. Default if
0, not to kill.
--kill-long-query-type- which queries should be killed once
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/
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