How to Repair MySQL databases and tables

Last modified: July 2, 2022

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How to Repair MySQL databases and tables

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STEP 1: BACKING UP THE DATABASES

Before you attempt to repair any database, you should back it up first. To back up all of the files from all of your databases, follow these steps:

1. Log in to your server using SSH.

2. Stop the MySQL server using the appropriate command for your Linux distribution:

- For CentOS and Fedora, type:
```
service mysqld stop
```
- For Debian and Ubuntu, type:
```
service mysql stop
```

3. Type the following command:

cp -rfv /var/lib/mysql /var/lib/mysql$(date +%s)

This command copies all of the files from all of your databases to a directory name based on the current time (more precisely, the number of seconds elapsed since January 1, 1970). This ensures that each database backup is stored in a directory that has a unique name. For added protection, you can (and should) back up the database files to a remote location, not on the server.

4. Restart the MySQL server using the appropriate command for your Linux distribution:

- For CentOS and Fedora, type:
```
service mysqld start
```
- For Debian and Ubuntu, type:
```
service mysql start
```

STEP 2: CHECKING AND REPAIRING A TABLE WITH MYSQLCHECK

You are prepared to begin debugging after backing up your databases. While MySQL is operating, you can check and fix databases using the mysqlcheckprogram. When you need to operate on a database without shutting down the entire MySQL service, this capability is helpful.

Additionally, mysqlcheck works on tables that use the MyISAM or InnoDB database engines.

1. As the root user, type the following command:

cd /var/lib/mysql

2. Type the following command, replacing DATABASE with the name of the database that you want to check:

mysqlcheck DATABASE

The previous command checks all of the tables in the specified database. Alternatively, to check a specific table in a database, type the following command. Replace DATABASE with the name of the database, and replace TABLE with the name of the table that you want to check:

mysqlcheck DATABASE TABLE

3. Mysqlcheck checks the specified database and tables. If a table passes the check, mysqlcheck displays OK for the table. However, if mysqlcheck reports an error for a table, type the following command to try to repair it. Replace DATABASE with the database name, and TABLE with the table name:

mysqlcheck -r DATABASE TABLE

4. If mysqlcheck cannot successfully repair the table or tables, go to the following procedure.

STEP 3: RUNNING ENGINE-SPECIFIC DIAGNOSTICS

If running mysqlcheck does not fix the problem, the next step is to run diagnostics specific to the engine used by the database table or tables. Follow the appropriate procedure below for your table’s database storage engine.

Repairing MyISAM tables with myisamchk

un the myisamchkprogram to repair it. To do this, follow these steps:

Note: The myisamchk program only works for tables that use the MyISAM storage engine. It does not work for the InnoDB engine.

Firstly, stop the MySQL server using the appropriate command for your Linux distribution:

For CentOS and Fedora, type:
```
service mysqld stop
```
For Debian and Ubuntu, type:
```
service mysql stop
```

2. Secondly, ype the following command: cd /var/lib/mysql

3. Change to the directory where the database is located. For example, if the database is named customers, type cd customers.

4. Next, ype the following command, replacing the TABLE with the name of the table that you want to check:

myisamchk TABLE

To check all of the tables in a database, type the following command:

myisamchk *.MYI

If the previous command does not work, you can try deleting temporary files that may be preventing myisamchk from running correctly. To do this, change back to the /var/lib/mysql directory, and then type the following command:

ls */*.TMD

If there are any .TMD files listed, type the following command to delete them:

rm */*.TMD

Then try to run myisamchk again.

5. To try to repair a table, type the following command, replacing the TABLE with the name of the table that you want to repair:

myisamchk --recover TABLE

6. After that, estart the MySQL server using the appropriate command for your Linux distribution:

- For CentOS and Fedora, type:
```
service mysqld start
```
- For Debian and Ubuntu, type:
```
service mysql start
```

7. Lastly, test the repaired table or tables.

Running the InnoDB recovery process

If you are using the InnoDB storage engine for a database table, you can run the InnoDB recovery process. To do this, follow these steps:

1. Firstly, use your preferred text editor to open the my.cnf file on your server. The location of the my.cnf file depends on your Linux distribution:

On CentOS and Fedora, the my.cnf file is located in the /etc directory.
On Debian and Ubuntu, the my.cnf file is located in the /etc/mysql directory.

2. Secondly, in the my.cnf file, locate the [mysqld] section.

3. Add the following line to the [mysqld] section:

innodb_force_recovery=4

4. Next, save the changes to the my.cnf file, and then restart the MySQL server using the appropriate command for your Linux distribution:

For CentOS and Fedora, type:
```
service mysqld restart
```
For Debian and Ubuntu, type:
```
service mysql restart
```

5. After that, type the following command to export all of the databases to the databases.sql file:

mysqldump --all-databases --add-drop-database --add-drop-table > databases.sql

6. Start the mysql program, and then try to drop the affected database or databases using the DROP DATABASE command.

If MySQL is unable to drop a database, you can delete it manually in step 8 below after you stop the MySQL server.

7. Stop the MySQL server using the appropriate command for your Linux distribution:

For CentOS and Fedora, type:
```
service mysqld stop
```
For Debian and Ubuntu, type:
```
service mysql stop
```

8. If you were unable to drop a database in step 6, type the following commands to delete it manually. Replace DBNAME with the name of the database that you want to delete:

cd /var/lib/mysql
rm -rf DBNAME

Make sure you do not delete the mysql or performance_schemadirectories!

9. Next, use your preferred text editor to open the my.cnf file on your server, and then comment out the following line in the [mysqld] section as shown:

#innodb_force_recovery=4

This disables InnoDB recovery mode.

10. Save the changes to the my.cnf file, and then start the MySQL server using the appropriate command for your Linux distribution:

For CentOS and Fedora, type:
```
service mysqld start
```
For Debian and Ubuntu, type:
```
service mysql start
```

11. Then, type the following command to restore the databases from the backup file you created in step 5:

mysql < databases.sql

12. Lastly, test the restored database.

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