GNU Info

(mysql.info)Crash recovery

---

Next: Check Prev: myisamchk memory Up: Table maintenance

---

Enter node , (file) or (file)node

Using `myisamchk' for Crash Recovery
....................................

If you run `mysqld' with `--skip-locking' (which is the default on some
systems, like Linux), you can't reliably use `myisamchk' to check a
table when `mysqld' is using the same table.  If you can be sure that
no one is accessing the tables through `mysqld' while you run
`myisamchk', you only have to do `mysqladmin flush-tables' before you
start checking the tables.  If you can't guarantee the above, then you
must take down `mysqld' while you check the tables.  If you run
`myisamchk' while `mysqld' is updating the tables, you may get a
warning that a table is corrupt even if it isn't.

If you are not using `--skip-locking', you can use `myisamchk' to check
tables at any time.  While you do this, all clients that try to update
the table will wait until `myisamchk' is ready before continuing.

If you use `myisamchk' to repair or optimize tables, you *MUST* always
ensure that the `mysqld' server is not using the table (this also
applies if you are using `--skip-locking').  If you don't take down
`mysqld' you should at least do a `mysqladmin flush-tables' before you
run `myisamchk'.

This chapter describes how to check for and deal with data corruption
in MySQL databases.  If your tables get corrupted a lot you should try
to find the reason for this! Note: Crashing.

The `MyISAM' table section contains reason for why a table could be
corrupted. Note: MyISAM table problems.

When performing crash recovery, it is important to understand that each
table `tbl_name' in a database corresponds to three files in the
database directory:

*File*         *Purpose*
`tbl_name.frm' Table definition (form) file
`tbl_name.MYD' Data file
`tbl_name.MYI' Index file

Each of these three file types is subject to corruption in various
ways, but problems occur most often in data files and index files.

`myisamchk' works by creating a copy of the `.MYD' (data) file row by
row. It ends the repair stage by removing the old `.MYD' file and
renaming the new file to the original file name.  If you use `--quick',
`myisamchk' does not create a temporary `.MYD' file, but instead
assumes that the `.MYD' file is correct and only generates a new index
file without touching the `.MYD' file. This is safe, because
`myisamchk' automatically detects if the `.MYD' file is corrupt and
aborts the repair in this case.  You can also give two `--quick'
options to `myisamchk'.  In this case, `myisamchk' does not abort on
some errors (like duplicate key) but instead tries to resolve them by
modifying the `.MYD' file. Normally the use of two `--quick' options is
useful only if you have too little free disk space to perform a normal
repair.  In this case you should at least make a backup before running
`myisamchk'.