It is a good idea to perform table checks on a regular basis
rather than waiting for problems to occur. One way to check and
repair MyISAM
tables is with the
CHECK TABLE
and
REPAIR TABLE
statements. These
are available starting with MySQL 3.23.16. See
Section 12.4.2, “Table Maintenance Statements”.
Another way to check tables is to use
myisamchk. For maintenance purposes, you can
use myisamchk -s. The -s
option (short for --silent
)
causes myisamchk to run in silent mode,
printing messages only when errors occur.
It is also a good idea to check tables when the server starts.
For example, whenever the machine has done a restart in the
middle of an update, you usually need to check all the tables
that could have been affected. (These are
“expected” crashed tables.) To cause the server to
check MyISAM
tables automatically, start it
with the --myisam-recover
option,
available as of MySQL 3.23.25. If your server is too old to
support this option, you could add a test to
mysqld_safe that runs
myisamchk to check all tables that have been
modified during the last 24 hours if there is an old
.pid
(process ID) file left after a
restart. (The .pid
file is created by
mysqld when it starts and removed when it
terminates normally. The presence of a .pid
file at system startup time indicates that
mysqld terminated abnormally.)
It is also a good idea to enable automatic
MyISAM
table checking. For example, whenever
the machine has done a restart in the middle of an update, you
usually need to check each table that could have been affected
before it is used further. (These are “expected crashed
tables.”) To check MyISAM
tables
automatically, start the server with the
--myisam-recover
option,
available as of MySQL 3.23.25. See
Section 5.1.2, “Server Command Options”. If your server is too old to
support this option, you could add a test to
mysqld_safe that runs
myisamchk to check all tables that have been
modified during the last 24 hours if there is an old
.pid
(process ID) file left after a
restart. (The .pid
file is created by
mysqld when it starts and removed when it
terminates normally. The presence of a .pid
file at system startup time indicates that
mysqld terminated abnormally.)
You should also check your tables regularly during normal system
operation. For example, you can run a cron
job to check important tables once a week, using a line like
this in a crontab
file:
35 0 * * 0/path/to/myisamchk
--fast --silent/path/to/datadir/
*/*.MYI
This prints out information about crashed tables so that you can examine and repair them as necessary.
To start with, execute myisamchk -s each night on all tables that have been updated during the last 24 hours. As you see that problems occur infrequently, you can back off the checking frequency to once a week or so.
Normally, MySQL tables need little maintenance. If you are
performing many updates to MyISAM
tables with
dynamic-sized rows (tables with
VARCHAR
,
BLOB
, or
TEXT
columns) or have tables with
many deleted rows you may want to defragment/reclaim space from
the tables from time to time. You can do this by using
OPTIMIZE TABLE
on the tables in
question. Alternatively, if you can stop the
mysqld server for a while, change location
into the data directory and use this command while the server is
stopped:
shell> myisamchk -r -s --sort-index --sort_buffer_size=16M */*.MYI
For ISAM
tables, the command is similar:
shell> isamchk -r -s --sort-index -O sort_buffer_size=16M */*.ISM
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