MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6
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 does not 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. (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 Instances on One Machine”, for additional discussion.