How to repair damaged MySQL tables

Once in a while something will happen to a server and the mysql database will get corrupted.

A specific instance comes to mind on one of our Cacti monitoring servers.

The /var partition filled up due to too many messages being sent to the root user in /var/spool/. This caused MySQL to crash as well since the cacti poller couldnt write to the poller_output table in MySQL.

The result was all graphs being blank within cacti.

In any case, a thorough analysis of the mysql database was in order and I decided to post this quick tutorial for performing quick / lengthy table checks for offline and online MySQL databases.

Stage 1: Checking your tables


myisamchk *.MYI 


myisamchk -e *.MYI 

if you have more time. Use the -s (silent) option to suppress unnecessary information.

If the mysqld server is stopped, you should use the –update-state option to tell myisamchk to mark the table as “checked.”

You have to repair only those tables for which myisamchk announces an error. For such tables, proceed to Stage 2.

If you get unexpected errors when checking (such as out of memory errors), or if myisamchk crashes, go to Stage 3.

Stage 2: Easy safe repair

First, try :

myisamchk -r -q tbl_name 

(-r -q means “quick recovery mode”).

This attempts to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work, and the table is fixed. Start repairing the next table. Otherwise, use the following procedure:

1. Make a backup of the data file before continuing.

2. Use

myisamchk -r tbl_name 

(-r means “recovery mode”)

This removes incorrect rows and deleted rows from the data file and reconstructs the index file.

3. If the preceding step fails, use

myisamchk --safe-recover tbl_name

Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode does not (but is slower).

Note: If you want a repair operation to go much faster, you should set the values of the sort_buffer_size and key_buffer_size variables each to about 25% of your available memory when running myisamchk.

If you get unexpected errors when repairing (such as out of memory errors), or if myisamchk crashes, go to Stage 3.

Stage 3: Difficult repair

You should reach this stage only if the first 16KB block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it is necessary to create a new index file. Do so as follows:

1. Move the data file to a safe place.

2. Use the table description file to create new (empty) data and index files:

   shell> mysql db_name
     mysql> SET AUTOCOMMIT=1;
     mysql> TRUNCATE TABLE tbl_name;
     mysql> quit

3. Copy the old data file back onto the newly created data file. (Do not just move the old file back onto the new file. You want to retain a copy in case something goes wrong.)

Go back to Stage 2. :

myisamchk -r -q should work.

(This should not be an endless loop.)

You can also use the REPAIR TABLE tbl_name USE_FRM SQL statement, which performs the whole procedure automatically. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use REPAIR TABLE. See Section, “REPAIR TABLE Syntax”.

Stage 4: Very difficult repair

You should reach this stage only if the .frm description file has also crashed. That should never happen, because the description file is not changed after the table is created:

1. Restore the description file from a backup and go back to Stage 3. You can also restore the index file and go back to Stage 2. In the latter case, you should start with

myisamchk -r

2. If you do not have a backup but know exactly how the table was created, create a copy of the table in another database. Remove the new data file, and then move the .frm description and .MYI index files from the other database to your crashed database. This gives you new description and index files, but leaves the .MYD data file alone. Go back to Stage 2 and attempt to reconstruct the index file.

Thats it!