14.1.4.2. Problems from Tables Not Being Closed Properly
Each MyISAM index file
(.MYI file) has a counter in the header
that can be used to check whether a table has been closed
properly. If you get the following warning from CHECK
TABLE or myisamchk, it means that
this counter has gone out of sync:
clients are using or haven't closed the table properly
This warning doesn't necessarily mean that the table is
corrupted, but you should at least check the table.
The counter works as follows:
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.
In other words, the counter can become incorrect only under
these conditions:
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 5.12, “Running Multiple MySQL Servers on the Same Machine”, for additional
discussion.
