This section describes how to check for and deal with data corruption in MySQL databases. If your tables become corrupted frequently, you should try to find the reason why. See What to Do If MySQL Keeps Crashing.
For an explanation of how
MyISAM tables can
become corrupted, see MyISAM Table Problems.
If you run mysqld with external locking disabled (which is the default), you cannot reliably use myisamchk to check a table when mysqld is using the same table. If you can be certain that no one can access the tables using mysqld while you run myisamchk, you only have to execute mysqladmin flush-tables before you start checking the tables. If you cannot guarantee this, you must stop mysqld while you check the tables. If you run myisamchk to check tables that mysqld is updating at the same time, you may get a warning that a table is corrupt even when it is not.
If the server is run with external locking enabled, you can use myisamchk to check tables at any time. In this case, if the server tries to update a table that myisamchk is using, the server waits for myisamchk to finish before it continues.
If you use myisamchk to repair or optimize tables, you must always ensure that the mysqld server is not using the table (this also applies if external locking is disabled). If you do not stop mysqld, you should 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.
When performing crash recovery, it is important to understand
tbl_name in a database corresponds to
the three files in the database directory shown in the following
Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.
myisamchk works by creating a copy of the
.MYD data file row by row. It ends the
repair stage by removing the old
and renaming the new file to the original file name. If you use
myisamchk does not create a temporary
.MYD file, but instead assumes that the
.MYD file is correct and generates only a
new index file without touching the
file. This is safe, because myisamchk
automatically detects whether the
is corrupt and aborts the repair if it is. You can also specify
--quick option twice to
myisamchk. In this case,
myisamchk does not abort on some errors (such
as duplicate-key errors) but instead tries to resolve them by
.MYD file. Normally the use
--quick options is
useful only if you have too little free disk space to perform a
normal repair. In this case, you should at least make a backup
of the table before running myisamchk.