MySQL :: MySQL 5.7 Reference Manual :: 8.11.5 External Locking (original) (raw)

8.11.5 External Locking

External locking is the use of file system locking to manage contention for MyISAM database tables by multiple processes. External locking is used in situations where a single process such as the MySQL server cannot be assumed to be the only process that requires access to tables. Here are some examples:

• If you run multiple servers that use the same database directory (not recommended), each server must have external locking enabled.
• If you use myisamchk to perform table maintenance operations onMyISAM tables, you must either ensure that the server is not running, or that the server has external locking enabled so that it locks table files as necessary to coordinate with myisamchk for access to the tables. The same is true for use ofmyisampack to packMyISAM tables.
  If the server is run with external locking enabled, you can use myisamchk at any time for read operations such a checking tables. In this case, if the server tries to update a table thatmyisamchk is using, the server waits formyisamchk to finish before it continues.
  If you use myisamchk for write operations such as repairing or optimizing tables, or if you usemyisampack to pack tables, you_must_ always ensure that themysqld server is not using the table. If you do not stop mysqld, at least do amysqladmin flush-tables before you runmyisamchk. Your tables may become corrupted if the server andmyisamchk access the tables simultaneously.

With external locking in effect, each process that requires access to a table acquires a file system lock for the table files before proceeding to access the table. If all necessary locks cannot be acquired, the process is blocked from accessing the table until the locks can be obtained (after the process that currently holds the locks releases them).

External locking affects server performance because the server must sometimes wait for other processes before it can access tables.

External locking is unnecessary if you run a single server to access a given data directory (which is the usual case) and if no other programs such as myisamchk need to modify tables while the server is running. If you only_read_ tables with other programs, external locking is not required, although myisamchk might report warnings if the server changes tables whilemyisamchk is reading them.

With external locking disabled, to usemyisamchk, you must either stop the server while myisamchk executes or else lock and flush the tables before running myisamchk. (See Section 8.12.1, “System Factors”.) To avoid this requirement, use the CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables.

For mysqld, external locking is controlled by the value of theskip_external_locking system variable. When this variable is enabled, external locking is disabled, and vice versa. External locking is disabled by default.

Use of external locking can be controlled at server startup by using the --external-locking or--skip-external-locking option.

If you do use external locking option to enable updates toMyISAM tables from many MySQL processes, you must ensure that the following conditions are satisfied:

• Do not use the query cache for queries that use tables that are updated by another process.
• Do not start the server with thedelay_key_write system variable set to ALL or use theDELAY_KEY_WRITE=1 table option for any shared tables. Otherwise, index corruption can occur.

The easiest way to satisfy these conditions is to always use--external-locking together with--delay-key-write=OFF and--query-cache-size=0. (This is not done by default because in many setups it is useful to have a mixture of the preceding options.)