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 on
MyISAM 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 of myisampack to pack
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 that myisamchk is using, the server will wait for myisamchk to finish before it continues.
If you use myisamchk for write operations such as repairing or optimizing tables, or if you use myisampack to pack tables, you must always ensure that the mysqld server is not using the table. If you don't stop mysqld, at least do a mysqladmin flush-tables before you run myisamchk. Your tables may become corrupted if the server and myisamchk 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 while myisamchk is reading them.
With external locking disabled, to use myisamchk, you must either stop the server while myisamchk executes or else lock and flush the tables before
running myisamchk. (See Section
8.11.1, "System Factors and Startup Parameter Tuning".) To avoid this requirement, use the
CHECK TABLE and
REPAIR TABLE statements to check and repair
For mysqld, external locking is controlled by the value of the
system variable. When this variable is enabled, external locking is disabled, and vice versa. From MySQL 4.0 on,
external locking is disabled by default.
If you do use external locking option to enable updates to
MyISAM 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 the
--delay-key-write=ALL option or use the
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
--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.)