It is a good idea to perform table checks on a regular basis rather than waiting for problems to occur. One way to check and repair MyISAM tables is with the CHECK TABLE and REPAIR TABLE statements. These are available starting with MySQL 3.23.16. See Section 12.4.2, “Table Maintenance Statements”.

Another way to check tables is to use myisamchk. For maintenance purposes, you can use myisamchk -s. The -s option (short for --silent) causes myisamchk to run in silent mode, printing messages only when errors occur.

It is also a good idea to check tables when the server starts. For example, whenever the machine has done a restart in the middle of an update, you usually need to check all the tables that could have been affected. (These are “expected” crashed tables.) To cause the server to check MyISAM tables automatically, start it with the --myisam-recover option, available as of MySQL 3.23.25. If your server is too old to support this option, you could add a test to mysqld_safe that runs myisamchk to check all tables that have been modified during the last 24 hours if there is an old .pid (process ID) file left after a restart. (The .pid file is created by mysqld when it starts and removed when it terminates normally. The presence of a .pid file at system startup time indicates that mysqld terminated abnormally.)

It is also a good idea to enable automatic MyISAM table checking. For example, whenever the machine has done a restart in the middle of an update, you usually need to check each table that could have been affected before it is used further. (These are “expected crashed tables.”) To check MyISAM tables automatically, start the server with the --myisam-recover option, available as of MySQL 3.23.25. See Section 5.1.2, “Server Command Options”. If your server is too old to support this option, you could add a test to mysqld_safe that runs myisamchk to check all tables that have been modified during the last 24 hours if there is an old .pid (process ID) file left after a restart. (The .pid file is created by mysqld when it starts and removed when it terminates normally. The presence of a .pid file at system startup time indicates that mysqld terminated abnormally.)

You should also check your tables regularly during normal system operation. For example, you can run a cron job to check important tables once a week, using a line like this in a crontab file:

35 0 * * 0 /path/to/myisamchk --fast --silent /path/to/datadir/*/*.MYI

This prints out information about crashed tables so that you can examine and repair them as necessary.

To start with, execute myisamchk -s each night on all tables that have been updated during the last 24 hours. As you see that problems occur infrequently, you can back off the checking frequency to once a week or so.

Normally, MySQL tables need little maintenance. If you are performing many updates to MyISAM tables with dynamic-sized rows (tables with VARCHAR, BLOB, or TEXT columns) or have tables with many deleted rows you may want to defragment/reclaim space from the tables from time to time. You can do this by using OPTIMIZE TABLE on the tables in question. Alternatively, if you can stop the mysqld server for a while, change location into the data directory and use this command while the server is stopped:

shell> myisamchk -r -s --sort-index --sort_buffer_size=16M */*.MYI

For ISAM tables, the command is similar:

shell> isamchk -r -s --sort-index -O sort_buffer_size=16M */*.ISM