Improved FLUSH TABLES WITH READ LOCK handling¶
When taking backups, FLUSH TABLES WITH READ LOCK is being used before the
non-InnoDB files are being backed up to ensure backup is being
consistent. FLUSH TABLES WITH READ LOCK can be run even though there may be
a running query that has been executing for hours. In this case everything will
be locked up in Waiting for table flush or Waiting for master to send event states. Killing the FLUSH TABLES WITH READ LOCK does not correct
this issue either. In this case the only way to get the server operating
normally again is to kill off the long running queries that blocked it to begin
with. This means that if there are long running queries FLUSH TABLES WITH READ
LOCK can get stuck, leaving server in read-only mode until waiting for these
queries to complete.
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:
-
innobackupex can wait for a good moment to issue the global lock.
-
innobackupex can kill all or only SELECT queries which are preventing the global lock from being acquired
Waiting for queries to finish¶
Good moment to issue a global lock is the moment when there are no long queries
running. But waiting for a good moment to issue the global lock for extended
period of time isn’t always good approach, as it can extend the time needed for
backup to take place. To prevent innobackupex from waiting to issue FLUSH
TABLES WITH READ LOCK for too long, new option has been implemented:
innobackupex –ftwrl-wait-timeout option can be used to limit the
waiting time. If the good moment to issue the lock did not happen during this
time, innobackupex will give up and exit with an error message and backup will
not be taken. Zero value for this option turns off the feature (which is
default).
Another possibility is to specify the type of query to wait on. In this case
innobackupex --ftwrl-wait-query-type. Possible values are all and
update. When all is used innobackupex will wait for all long running
queries (execution time longer than allowed by innobackupex --ftwrl-wait-threshold) to finish before running the FLUSH TABLES WITH READ LOCK. When update is used innobackupex will wait on UPDATE/ALTER/REPLACE/INSERT queries to finish.
Although time needed for specific query to complete is hard to predict, we can
assume that queries that are running for a long time already will likely not be
completed soon, and queries which are running for a short time will likely be
completed shortly. innobackupex can use the value of innobackupex --ftwrl-wait-threshold option to specify which query is long running and will
likely block global lock for a while. In order to use this option xtrabackup user should have PROCESS and SUPER privileges.
Killing the blocking queries¶
Second option is to kill all the queries which prevent global lock from being
acquired. In this case all the queries which run longer than FLUSH TABLES WITH
READ LOCK are possible blockers. Although all queries can be killed,
additional time can be specified for the short running queries to complete. This
can be specified by innobackupex --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. Default value is zero, which
turns this feature off.
innobackupex --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¶
-
innobackupex --ftwrl-wait-timeout(seconds) - how long to wait for a good moment. Default is 0, not to wait. -
innobackupex --ftwrl-wait-query-type- which long queries should be finished beforeFLUSH TABLES WITH READ LOCKis run. Default is all. -
innobackupex --ftwrl-wait-threshold(seconds) - how long query should be running before we consider it long running and potential blocker of global lock. -
innobackupex --kill-long-queries-timeout(seconds) - how many time we give for queries to complete afterFLUSH TABLES WITH READ LOCKis issued before start to kill. Default if0, not to kill. -
innobackupex --kill-long-query-type- which queries should be killed oncekill-long-queries-timeouthas expired.
Example¶
Running the innobackupex with the following options will cause innobackupex to spend no longer than 3 minutes waiting for all queries older than 40 seconds to complete.
$ innobackupex --ftwrl-wait-threshold=40 --ftwrl-wait-query-type=all --ftwrl-wait-timeout=180 --kill-long-queries-timeout=20 --kill-long-query-type=all /data/backups/
After FLUSH TABLES WITH READ LOCK is issued, innobackupex will wait 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.