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 any of the following events occur:
The mysqld process is killed in the middle of a write.
An unexpected computer shutdown occurs (for example, the computer is turned off).
Hardware failures.
You are using an external program (such as myisamchk) to modify a table that is being modified by the server at the same time.
A software bug in the MySQL or
MyISAMcode.
Typical symptoms of a corrupt table are:
You get the following error while selecting data from the table:
Incorrect key file for table: '...'. Try to repair it
Queries don't find rows in the table or return incomplete results.
You can check the health of a MyISAM table
using the CHECK TABLE statement,
and 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 12.4.2.2, “CHECK TABLE Syntax”,
Section 12.4.2.5, “REPAIR TABLE Syntax”, and Section 4.6.3, “myisamchk — MyISAM Table-Maintenance Utility”.
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. This is a bug. You should try
to create a reproducible test case that demonstrates the
problem. See Section C.5.4.2, “What to Do If MySQL Keeps Crashing”, and
MySQL
Internals: Porting.