The first time a table is updated in MySQL, a counter in the header of the index files is incremented.

The counter is not changed during further updates.

When the last instance of a table is closed (because a FLUSH TABLES operation was performed or because there is no room in the table cache), the counter is decremented if the table has been updated at any point.

When you repair the table or check the table and it is found to be okay, the counter is reset to zero.

To avoid problems with interaction with other processes that might check the table, the counter is not decremented on close if it was zero.

A MyISAM table is copied without first issuing LOCK TABLES and FLUSH TABLES.

MySQL has crashed between an update and the final close. (Note that the table may still be okay, because MySQL always issues writes for everything between each statement.)

A table was modified by myisamchk --recover or myisamchk --update-state at the same time that it was in use by mysqld.

Multiple mysqld servers are using the table and one server performed a REPAIR TABLE or CHECK TABLE on the table while it was in use by another server. In this setup, it is safe to use CHECK TABLE, although you might get the warning from other servers. However, REPAIR TABLE should be avoided because when one server replaces the data file with a new one, this is not known to the other servers.

In general, it is a bad idea to share a data directory among multiple servers. See Section 5.7, “Running Multiple MySQL Servers on the Same Machine”, for additional discussion.