MySQL Reference Manual :: 14.1.4.1 Corrupted MyISAM Tables

Even though the MyISAM table format is very reliable (all changes to a table made by an SQL statement are written before the statement returns), you can still get corrupted tables if some of the following things happen:

Typical symptoms for a corrupt table are:

You can check whether a MyISAM table is okay with the CHECK TABLE statement. You can repair a corrupted MyISAM table with REPAIR TABLE. When mysqld is not running, you can also check or repair a table with the myisamchk command. See Section 13.5.2.3, “CHECK TABLE Syntax”, Section 13.5.2.6, “REPAIR TABLE Syntax”, and Section 5.8.3.1, “myisamchk Invocation Syntax”.

If your tables become corrupted frequently, you should try to determine why this is happening. The most important thing to know is whether the table became corrupted as a result of a server crash. You can verify this easily by looking for a recent restarted mysqld message in the error log. If there is such a message, it is likely that table corruption is a result of the server dying. Otherwise, corruption may have occurred during normal operation, which is a bug. You should try to create a reproducible test case that demonstrates the problem. See Section A.4.2, “What to Do If MySQL Keeps Crashing” and Section E.1.6, “Making a Test Case If You Experience Table Corruption”.