Backup Locks

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 logical backups.

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. If there is an active LOCK TABLES FOR BACKUP lock then all DDL statements and all updates to MyISAM, CSV, MEMORY, ARCHIVE, TokuDB, and MyRocks tables will be blocked in the Waiting for backup lock status, visible in PERFORMANCE_SCHEMA or PROCESSLIST.

LOCK TABLES FOR BACKUP has no effect on SELECT queries for all mentioned storage engines. Against InnoDB, MyRocks, Blackhole and Federated tables, the LOCK TABLES FOR BACKUP is not applicable to the INSERT, REPLACE, UPDATE, DELETE statements: Blackhole tables obviously have no relevance to backups, and Federated tables are ignored by both logical and physical backup tools.

Unlike 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 or MyRocks 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 ...

Privileges

The LOCK TABLES FOR BACKUP requires the BACKUP_ADMIN privilege.

Interaction with other global locks

The LOCK TABLES FOR BACKUP has no effect if the current connection already owns a FLUSH TABLES WITH READ LOCK lock, as it is a more restrictive lock. If FLUSH TABLES WITH READ LOCK is executed in a connection that has acquired LOCK TABLES FOR BACKUP, FLUSH TABLES WITH READ LOCK fails with an error.

If the server is operating in the read-only mode (i.e. read_only set to 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 that owns 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 ALL, 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 prevent this, LOCK TABLES FOR BACKUP will fail with an error if delay_key_write is set to ALL. An attempt to set delay_key_write to 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 delay_key_writes to OFF in the configuration file, 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 INSERTs or 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 mysqldump issue 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 single-transaction option, lock-for-backup is automatically converted to lock-all-tables.

The option lock-for-backup is mutually exclusive with lock-all-tables, i.e. specifying both on the command line will lead to an error.

If the backup locks feature is not supported by the target server, but lock-for-backup is specified on the command line, mysqldump aborts with an error.

Version Specific Information

  • 8.0.12-1
    Feature ported from Percona Server for MySQL 5.7.

System Variables

variable have_backup_locks
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 YES.

Status Variables

variable Com_lock_tables_for_backup
Variable Type:Numeric
Scope:Global/Session

This status variable indicates the number of times the corresponding statements have been executed.

Client Command Line Parameter

option lock-for-backup
Command Line:Yes
Scope:Global
Dynamic:No
Variable Type:String
Default Value:Off

When used together with the --single-transaction option, the option makes mysqldump issue LOCK TABLES FOR BACKUP before starting the dump operation to prevent unsafe statements that would normally result in an inconsistent backup.