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.
FLUST TABLES WITH READ LOCK¶
The FLUSH TABLES WITH READ LOCK statement performs two main actions:
-
Flushes any changes to the tables currently in memory but have not yet been written to disk. This operation ensures that the data on the disk is up-to-date with the most recent changes.
-
Acquires a read lock on all tables. This means that other sessions can continue to read from the tables but cannot execute any write operations (like INSERT, UPDATE, or DELETE) until the lock is released. This action is beneficial when creating a consistent data snapshot for backup purposes.
Please note that FLUSH TABLES WITH READ LOCK applies to all databases unless you specify tables individually. Also, this statement causes an implicit commit.
The following command connects to the server and executes the FLUSH TABLES WITH READ LOCK statement. After running this command, no write operations can be performed until the lock is released.
mysql --user="user" --password="password" --host="host" --execute="FLUSH TABLES WITH READ LOCK"
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, and MyRocks tables will be blocked
in the Waiting for backup lock status, visible in PERFORMANCE_SCHEMA or
PROCESSLIST.
LOCK TABLES FOR BACKUP does not affect SELECT queries for all mentioned
storage engines. Against InnoDB, MyRocks, Blackhole and Federated tables,
the LOCK TABLES FOR BACKUP does not apply to the INSERT, REPLACE,
UPDATE, DELETE statements: Blackhole tables 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, the statement fails with the following error:
Expected output
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 that lock is more restrictive. 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 (for example, 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 a 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 the 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 do not affect physical backups as long as all statements
updating MyISAM tables are blocked.
The mysqldump Command¶
The mysqldump tool 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 a
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.
System Variables¶
have_backup_locks¶
| Option | Description |
|---|---|
| Command Line: | Yes |
| Config file | No |
| Scope: | Global |
| Dynamic: | No |
| Data 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¶
Com_lock_tables_for_backup¶
| Option | Description |
|---|---|
| Scope: | Global/Session |
| Data type | Numeric |
This status variable indicates the number of times the corresponding statements have been executed.
Client command line parameter¶
lock-for-backup¶
| Option | Description |
|---|---|
| Command Line: | Yes |
| Scope: | Global |
| Dynamic: | No |
| Data 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.
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