Percona Server for MySQL offers the
LOCK TABLES FOR BACKUP statement as a
lightweight alternative to
FLUSH TABLES WITH READ LOCK for both physical and
LOCK TABLES FOR BACKUP¶
LOCK TABLES FOR BACKUP uses a new MDL lock type to block updates to
non-transactional tables and DDL statements for all tables. More specifically,
if there’s an active
LOCK TABLES FOR BACKUP lock, all DDL statements and all
updates to MyISAM, CSV, MEMORY and ARCHIVE tables will be blocked in the
Waiting for backup lock status as visible in
SELECT queries for all tables and
INSERT/REPLACE/UPDATE/DELETE against InnoDB, Blackhole and Federated
tables are not affected by
LOCK TABLES FOR BACKUP. Blackhole tables
obviously have no relevance for backups, and Federated tables are ignored by
both logical and physical backup tools.
FLUSH TABLES WITH READ LOCK,
LOCK TABLES FOR BACKUP does not
flush tables, i.e. storage engines are not forced to close tables and tables are
not expelled from the table cache. As a result,
LOCK TABLES FOR BACKUP only
waits for conflicting statements to complete (i.e. DDL and updates to
non-transactional tables). It never waits for SELECTs, or UPDATEs to InnoDB
tables to complete, for example.
If an “unsafe” statement is executed in the same connection that is holding a
LOCK TABLES FOR BACKUP lock, it fails with the following error:
ERROR 1880 (HY000): Can't execute the query because you have a conflicting backup lock UNLOCK TABLES releases the lock acquired by LOCK TABLES FOR BACKUP.
The intended use case for Percona XtraBackup is:
LOCK TABLES FOR BACKUP ... copy .frm, MyISAM, CSV, etc. ... UNLOCK TABLES ... get binlog coordinates ... ... wait for redo log copying to finish ...
LOCK TABLES FOR BACKUP requires the
Interaction with other global locks¶
LOCK TABLES FOR BACKUP has no effect if the current connection already
FLUSH TABLES WITH READ LOCK lock, as it is a more restrictive
FLUSH TABLES WITH READ LOCK is executed in a connection that has
LOCK TABLES FOR BACKUP,
FLUSH TABLES WITH READ LOCK fails with
If the server is operating in the read-only mode (i.e.
1), statements that are unsafe for backups will be either blocked or fail
with an error, depending on whether they are executed in the same connection
LOCK TABLES FOR BACKUP lock, or other connections.
MyISAM index and data buffering¶
MyISAM key buffering is normally write-through, i.e. by the time each update to a MyISAM table is completed, all index updates are written to disk. The only exception is delayed key writing feature which is disabled by default.
When the global system variable
delay_key_write is set to
key buffers for all MyISAM tables are not flushed between updates, so a
physical backup of those tables may result in broken MyISAM indexes. To
LOCK TABLES FOR BACKUP will fail with an error if
delay_key_write is set to
ALL. An attempt to set
ALL when there’s an active backup lock will
also fail with an error.
Another option to involve delayed key writing is to create MyISAM tables with
the DELAY_KEY_WRITE option and set the
delay_key_write variable to
ON (which is the default). In this case,
LOCK TABLES FOR BACKUP will not
be able to prevent stale index files from appearing in the backup. Users are
encouraged to set
OFF in the configuration
my.cnf, or repair MyISAM indexes after restoring from a physical
backup created with backup locks.
MyISAM may also cache data for bulk inserts, e.g. when executing multi-row
LOAD DATA statements. Those caches, however, are flushed between
statements, so have no effect on physical backups as long as all statements
updating MyISAM tables are blocked.
The mysqldump Command¶
mysqldump has also been extended with a new option,
lock-for-backup (disabled by default). When used together with the
--single-transaction option, the option makes
LOCK TABLES FOR BACKUP before starting the dump operation to prevent unsafe
statements that would normally result in an inconsistent backup.
When used without the
lock-for-backup is automatically converted to
lock-for-backup is mutually exclusive with
lock-all-tables, i.e. specifying both on the command line will lead to
If the backup locks feature is not supported by the target server, but
lock-for-backup is specified on the command line,
with an error.
Version Specific Information¶
- Feature ported from Percona Server for MySQL 5.7.
Command Line: Yes Config File: No Scope: Global Dynamic: No Variable Type: Boolean Default Value: YES
This is a server variable implemented to help other utilities decide what
locking strategy can be implemented for a server. When available, the backup
locks feature is supported by the server and the variable value is always
Variable Type: Numeric Scope: Global/Session
This status variable indicates the number of times the corresponding statements have been executed.
Client Command Line Parameter¶
Command Line: Yes Scope: Global Dynamic: No Variable Type: String Default Value: Off
When used together with the
--single-transaction option, the option
LOCK TABLES FOR BACKUP before starting the dump
operation to prevent unsafe statements that would normally result in an