Chapter 7 Backup and Recovery

To be useful, backups must be scheduled regularly. A full backup (a snapshot of the data at a point in time) can be done in MySQL with several tools. For example, MySQL Enterprise Backup can perform a physical backup of an entire instance, with optimizations to minimize overhead and avoid disruption when backing up InnoDB data files; mysqldump provides online logical backup. This discussion uses mysqldump.

Assume that we make a full backup of all our InnoDB tables in all databases using the following command on Sunday at 1 p.m., when load is low:

shell> mysqldump --all-databases --master-data --single-transaction > backup_sunday_1_PM.sql

The resulting .sql file produced by mysqldump contains a set of SQL INSERT statements that can be used to reload the dumped tables at a later time.

This backup operation acquires a global read lock on all tables at the beginning of the dump (using FLUSH TABLES WITH READ LOCK). As soon as this lock has been acquired, the binary log coordinates are read and the lock is released. If long updating statements are running when the FLUSH statement is issued, the backup operation may stall until those statements finish. After that, the dump becomes lock-free and does not disturb reads and writes on the tables.

It was assumed earlier that the tables to back up are InnoDB tables, so --single-transaction uses a consistent read and guarantees that data seen by mysqldump does not change. (Changes made by other clients to InnoDB tables are not seen by the mysqldump process.) If the backup operation includes nontransactional tables, consistency requires that they do not change during the backup. For example, for the MyISAM tables in the mysql database, there must be no administrative changes to MySQL accounts during the backup.

Full backups are necessary, but it is not always convenient to create them. They produce large backup files and take time to generate. They are not optimal in the sense that each successive full backup includes all data, even that part that has not changed since the previous full backup. It is more efficient to make an initial full backup, and then to make incremental backups. The incremental backups are smaller and take less time to produce. The tradeoff is that, at recovery time, you cannot restore your data just by reloading the full backup. You must also process the incremental backups to recover the incremental changes.

To make incremental backups, we need to save the incremental changes. In MySQL, these changes are represented in the binary log, so the MySQL server should always be started with the --log-bin option to enable that log. With binary logging enabled, the server writes each data change into a file while it updates data. Looking at the data directory of a MySQL server that has been running for some days, we find these MySQL binary log files:

-rw-rw---- 1 guilhem  guilhem   1277324 Nov 10 23:59 gbichot2-bin.000001
-rw-rw---- 1 guilhem  guilhem         4 Nov 10 23:59 gbichot2-bin.000002
-rw-rw---- 1 guilhem  guilhem        79 Nov 11 11:06 gbichot2-bin.000003
-rw-rw---- 1 guilhem  guilhem       508 Nov 11 11:08 gbichot2-bin.000004
-rw-rw---- 1 guilhem  guilhem 220047446 Nov 12 16:47 gbichot2-bin.000005
-rw-rw---- 1 guilhem  guilhem    998412 Nov 14 10:08 gbichot2-bin.000006
-rw-rw---- 1 guilhem  guilhem       361 Nov 14 10:07 gbichot2-bin.index

Each time it restarts, the MySQL server creates a new binary log file using the next number in the sequence. While the server is running, you can also tell it to close the current binary log file and begin a new one manually by issuing a FLUSH LOGS SQL statement or with a mysqladmin flush-logs command. mysqldump also has an option to flush the logs. The .index file in the data directory contains the list of all MySQL binary logs in the directory.

The MySQL binary logs are important for recovery because they form the set of incremental backups. If you make sure to flush the logs when you make your full backup, the binary log files created afterward contain all the data changes made since the backup. Let's modify the previous mysqldump command a bit so that it flushes the MySQL binary logs at the moment of the full backup, and so that the dump file contains the name of the new current binary log:

shell> mysqldump --single-transaction --flush-logs --master-data=2 \
         --all-databases > backup_sunday_1_PM.sql

After executing this command, the data directory contains a new binary log file, gbichot2-bin.000007, because the --flush-logs option causes the server to flush its logs. The --master-data option causes mysqldump to write binary log information to its output, so the resulting .sql dump file includes these lines:

-- Position to start replication or point-in-time recovery from
-- CHANGE MASTER TO MASTER_LOG_FILE='gbichot2-bin.000007',MASTER_LOG_POS=4;

Because the mysqldump command made a full backup, those lines mean two things:

On Monday at 1 p.m., we can create an incremental backup by flushing the logs to begin a new binary log file. For example, executing a mysqladmin flush-logs command creates gbichot2-bin.000008. All changes between the Sunday 1 p.m. full backup and Monday 1 p.m. are written in gbichot2-bin.000007. This incremental backup is important, so it is a good idea to copy it to a safe place. (For example, back it up on tape or DVD, or copy it to another machine.) On Tuesday at 1 p.m., execute another mysqladmin flush-logs command. All changes between Monday 1 p.m. and Tuesday 1 p.m. are written in gbichot2-bin.000008 (which also should be copied somewhere safe).

The MySQL binary logs take up disk space. To free up space, purge them from time to time. One way to do this is by deleting the binary logs that are no longer needed, such as when we make a full backup:

shell> mysqldump --single-transaction --flush-logs --master-data=2 \
         --all-databases --delete-master-logs > backup_sunday_1_PM.sql

Now, suppose that we have a catastrophic unexpected exit on Wednesday at 8 a.m. that requires recovery from backups. To recover, first we restore the last full backup we have (the one from Sunday 1 p.m.). The full backup file is just a set of SQL statements, so restoring it is very easy:

shell> mysql < backup_sunday_1_PM.sql

At this point, the data is restored to its state as of Sunday 1 p.m.. To restore the changes made since then, we must use the incremental backups; that is, the gbichot2-bin.000007 and gbichot2-bin.000008 binary log files. Fetch the files if necessary from where they were backed up, and then process their contents like this:

shell> mysqlbinlog gbichot2-bin.000007 gbichot2-bin.000008 | mysql

We now have recovered the data to its state as of Tuesday 1 p.m., but still are missing the changes from that date to the date of the crash. To not lose them, we would have needed to have the MySQL server store its MySQL binary logs into a safe location (RAID disks, SAN, ...) different from the place where it stores its data files, so that these logs were not on the destroyed disk. (That is, we can start the server with a --log-bin option that specifies a location on a different physical device from the one on which the data directory resides. That way, the logs are safe even if the device containing the directory is lost.) If we had done this, we would have the gbichot2-bin.000009 file (and any subsequent files) at hand, and we could apply them using mysqlbinlog and mysql to restore the most recent data changes with no loss up to the moment of the crash:

shell> mysqlbinlog gbichot2-bin.000009 ... | mysql

For more information about using mysqlbinlog to process binary log files, see Section 7.5, “Point-in-Time (Incremental) Recovery”.

In case of an operating system crash or power failure, InnoDB itself does all the job of recovering data. But to make sure that you can sleep well, observe the following guidelines:

This section describes how to use mysqldump to create SQL-format dump files. For information about reloading such dump files, see Section 7.4.2, “Reloading SQL-Format Backups”.

By default, mysqldump writes information as SQL statements to the standard output. You can save the output in a file:

shell> mysqldump [arguments] > file_name

To dump all databases, invoke mysqldump with the --all-databases option:

shell> mysqldump --all-databases > dump.sql

To dump only specific databases, name them on the command line and use the --databases option:

shell> mysqldump --databases db1 db2 db3 > dump.sql

The --databases option causes all names on the command line to be treated as database names. Without this option, mysqldump treats the first name as a database name and those following as table names.

With --all-databases or --databases, mysqldump writes CREATE DATABASE and USE statements prior to the dump output for each database. This ensures that when the dump file is reloaded, it creates each database if it does not exist and makes it the default database so database contents are loaded into the same database from which they came. If you want to cause the dump file to force a drop of each database before recreating it, use the --add-drop-database option as well. In this case, mysqldump writes a DROP DATABASE statement preceding each CREATE DATABASE statement.

To dump a single database, name it on the command line:

shell> mysqldump --databases test > dump.sql

In the single-database case, it is permissible to omit the --databases option:

shell> mysqldump test > dump.sql

The difference between the two preceding commands is that without --databases, the dump output contains no CREATE DATABASE or USE statements. This has several implications:

To dump only specific tables from a database, name them on the command line following the database name:

shell> mysqldump test t1 t3 t7 > dump.sql

By default, if GTIDs are in use on the server where you create the dump file (gtid_mode=ON), mysqldump includes a SET @@GLOBAL.gtid_purged statement in the output to add the GTIDs from the gtid_executed set on the source server to the gtid_purged set on the target server. If you are dumping only specific databases or tables, it is important to note that the value that is included by mysqldump includes the GTIDs of all transactions in the gtid_executed set on the source server, even those that changed suppressed parts of the database, or other databases on the server that were not included in the partial dump. If you only replay one partial dump file on the target server, the extra GTIDs do not cause any problems with the future operation of that server. However, if you replay a second dump file on the target server that contains the same GTIDs (for example, another partial dump from the same source server), any SET @@GLOBAL.gtid_purged statement in the second dump file fails. To avoid this issue, either set the mysqldump option --set-gtid-purged to OFF or COMMENTED to output the second dump file without an active SET @@GLOBAL.gtid_purged statement, or remove the statement manually before replaying the dump file.

To reload a dump file written by mysqldump that consists of SQL statements, use it as input to the mysql client. If the dump file was created by mysqldump with the --all-databases or --databases option, it contains CREATE DATABASE and USE statements and it is not necessary to specify a default database into which to load the data:

shell> mysql < dump.sql

Alternatively, from within mysql, use a source command:

mysql> source dump.sql

If the file is a single-database dump not containing CREATE DATABASE and USE statements, create the database first (if necessary):

shell> mysqladmin create db1

Then specify the database name when you load the dump file:

shell> mysql db1 < dump.sql

Alternatively, from within mysql, create the database, select it as the default database, and load the dump file:

mysql> CREATE DATABASE IF NOT EXISTS db1;
mysql> USE db1;
mysql> source dump.sql

This section describes how to use mysqldump to create delimited-text dump files. For information about reloading such dump files, see Section 7.4.4, “Reloading Delimited-Text Format Backups”.

If you invoke mysqldump with the --tab=dir_name option, it uses dir_name as the output directory and dumps tables individually in that directory using two files for each table. The table name is the base name for these files. For a table named t1, the files are named t1.sql and t1.txt. The .sql file contains a CREATE TABLE statement for the table. The .txt file contains the table data, one line per table row.

The following command dumps the contents of the db1 database to files in the /tmp database:

shell> mysqldump --tab=/tmp db1

The .txt files containing table data are written by the server, so they are owned by the system account used for running the server. The server uses SELECT ... INTO OUTFILE to write the files, so you must have the FILE privilege to perform this operation, and an error occurs if a given .txt file already exists.

The server sends the CREATE definitions for dumped tables to mysqldump, which writes them to .sql files. These files therefore are owned by the user who executes mysqldump.

It is best that --tab be used only for dumping a local server. If you use it with a remote server, the --tab directory must exist on both the local and remote hosts, and the .txt files are written by the server in the remote directory (on the server host), whereas the .sql files are written by mysqldump in the local directory (on the client host).

For mysqldump --tab, the server by default writes table data to .txt files one line per row with tabs between column values, no quotation marks around column values, and newline as the line terminator. (These are the same defaults as for SELECT ... INTO OUTFILE.)

To enable data files to be written using a different format, mysqldump supports these options:

Depending on the value you specify for any of these options, it might be necessary on the command line to quote or escape the value appropriately for your command interpreter. Alternatively, specify the value using hex notation. Suppose that you want mysqldump to quote column values within double quotation marks. To do so, specify double quote as the value for the --fields-enclosed-by option. But this character is often special to command interpreters and must be treated specially. For example, on Unix, you can quote the double quote like this:

--fields-enclosed-by='"'

On any platform, you can specify the value in hex:

--fields-enclosed-by=0x22

It is common to use several of the data-formatting options together. For example, to dump tables in comma-separated values format with lines terminated by carriage-return/newline pairs (\r\n), use this command (enter it on a single line):

shell> mysqldump --tab=/tmp --fields-terminated-by=,
         --fields-enclosed-by='"' --lines-terminated-by=0x0d0a db1

Should you use any of the data-formatting options to dump table data, you need to specify the same format when you reload data files later, to ensure proper interpretation of the file contents.

For backups produced with mysqldump --tab, each table is represented in the output directory by an .sql file containing the CREATE TABLE statement for the table, and a .txt file containing the table data. To reload a table, first change location into the output directory. Then process the .sql file with mysql to create an empty table and process the .txt file to load the data into the table:

shell> mysql db1 < t1.sql
shell> mysqlimport db1 t1.txt

An alternative to using mysqlimport to load the data file is to use the LOAD DATA statement from within the mysql client:

mysql> USE db1;
mysql> LOAD DATA INFILE 't1.txt' INTO TABLE t1;

If you used any data-formatting options with mysqldump when you initially dumped the table, you must use the same options with mysqlimport or LOAD DATA to ensure proper interpretation of the data file contents:

shell> mysqlimport --fields-terminated-by=,
         --fields-enclosed-by='"' --lines-terminated-by=0x0d0a db1 t1.txt

Or:

mysql> USE db1;
mysql> LOAD DATA INFILE 't1.txt' INTO TABLE t1
       FIELDS TERMINATED BY ',' FIELDS ENCLOSED BY '"'
       LINES TERMINATED BY '\r\n';

This section surveys techniques that enable you to use mysqldump to solve specific problems:

shell> mysqldump db1 > dump.sql
shell> mysqladmin create db2
shell> mysql db2 < dump.sql

shell> mysqldump --databases db1 > dump.sql

shell> mysql < dump.sql

shell> mysqldump db1 > dump.sql

shell> mysqladmin create db1
shell> mysql db1 < dump.sql

shell> mysqldump --no-data test > dump-defs.sql
shell> mysqldump --no-create-info test > dump-data.sql

shell> mysqldump --no-data --routines --events test > dump-defs.sql

shell> mysqldump --all-databases --no-data --routines --events > dump-defs.sql

shell> mysql < dump-defs.sql

shell> mysqldump --all-databases --no-create-info > dump-data.sql

shell> mysql < dump-data.sql

This section explains the general idea of using the binary log to perform a point-in-time-recovery. The next section, Section 7.5.2, “Point-in-Time Recovery Using Event Positions”, explains the operation in details with an example.

The source of information for point-in-time recovery is the set of binary log files generated subsequent to the full backup operation. Therefore, to allow a server to be restored to a point-in-time, binary logging must be enabled on it, which is the default setting for MySQL 8.0 (see Section 5.4.4, “The Binary Log”).

To restore data from the binary log, you must know the name and location of the current binary log files. By default, the server creates binary log files in the data directory, but a path name can be specified with the --log-bin option to place the files in a different location. To see a listing of all binary log files, use this statement:

mysql> SHOW BINARY LOGS;

To determine the name of the current binary log file, issue the following statement:

mysql> SHOW MASTER STATUS;

The mysqlbinlog utility converts the events in the binary log files from binary format to text so that they can be viewed or applied. mysqlbinlog has options for selecting sections of the binary log based on event times or position of events within the log. See Section 4.6.8, “mysqlbinlog — Utility for Processing Binary Log Files”.

Applying events from the binary log causes the data modifications they represent to be reexecuted. This enables recovery of data changes for a given span of time. To apply events from the binary log, process mysqlbinlog output using the mysql client:

shell> mysqlbinlog binlog_files | mysql -u root -p

If binary log files have been encrypted, which can be done from MySQL 8.0.14 onwards, mysqlbinlog cannot read them directly as in the above example, but can read them from the server using the --read-from-remote-server (-R) option. For example:

shell> mysqlbinlog --read-from-remote-server --host=host_name --port=3306  --user=root --password --ssl-mode=required  binlog_files | mysql -u root -p

Here, the option --ssl-mode=required has been used to ensure that the data from the binary log files is protected in transit, because it is sent to mysqlbinlog in an unencrypted format.

Viewing log contents can be useful when you need to determine event times or positions to select partial log contents prior to executing events. To view events from the log, send mysqlbinlog output into a paging program:

shell> mysqlbinlog binlog_files | more

Alternatively, save the output in a file and view the file in a text editor:

shell> mysqlbinlog binlog_files > tmpfile
shell> ... edit tmpfile ...

Saving the output in a file is useful as a preliminary to executing the log contents with certain events removed, such as an accidental DROP TABLE. You can delete from the file any statements not to be executed before executing its contents. After editing the file, apply the contents as follows:

shell> mysql -u root -p < tmpfile

If you have more than one binary log to apply on the MySQL server, the safe method is to process them all using a single connection to the server. Here is an example that demonstrates what may be unsafe:

shell> mysqlbinlog binlog.000001 | mysql -u root -p # DANGER!!
shell> mysqlbinlog binlog.000002 | mysql -u root -p # DANGER!!

Processing binary logs this way using different connections to the server causes problems if the first log file contains a CREATE TEMPORARY TABLE statement and the second log contains a statement that uses the temporary table. When the first mysql process terminates, the server drops the temporary table. When the second mysql process attempts to use the table, the server reports “unknown table.”

To avoid problems like this, use a single connection to apply the contents of all binary log files that you want to process. Here is one way to do so:

shell> mysqlbinlog binlog.000001 binlog.000002 | mysql -u root -p

Another approach is to write the whole log to a single file and then process the file:

shell> mysqlbinlog binlog.000001 >  /tmp/statements.sql
shell> mysqlbinlog binlog.000002 >> /tmp/statements.sql
shell> mysql -u root -p -e "source /tmp/statements.sql"

When writing to a dump file while reading back from a binary log containing GTIDs (see Section 17.1.3, “Replication with Global Transaction Identifiers”), use the --skip-gtids option with mysqlbinlog, like this:

shell> mysqlbinlog --skip-gtids binlog.000001 >  /tmp/dump.sql
shell> mysqlbinlog --skip-gtids binlog.000002 >> /tmp/dump.sql
shell> mysql -u root -p -e "source /tmp/dump.sql"

The last section, Section 7.5.1, “Point-in-Time Recovery Using Binary Log”, explains the general idea of using the binary log to perform a point-in-time-recovery. The section explains the operation in details with an example.

As an example, suppose that around 20:06:00 on March 11, 2020, an SQL statement was executed that deleted a table. You can perform a point-in-time recovery to restore the server up to its state right before the table deletion. These are some sample steps to achieve that:

This section describes how to check for and deal with data corruption in MySQL databases. If your tables become corrupted frequently, you should try to find the reason why. See Section B.3.3.3, “What to Do If MySQL Keeps Crashing”.

For an explanation of how MyISAM tables can become corrupted, see Section 16.2.4, “MyISAM Table Problems”.

If you run mysqld with external locking disabled (which is the default), you cannot reliably use myisamchk to check a table when mysqld is using the same table. If you can be certain that no one can access the tables using mysqld while you run myisamchk, you only have to execute mysqladmin flush-tables before you start checking the tables. If you cannot guarantee this, you must stop mysqld while you check the tables. If you run myisamchk to check tables that mysqld is updating at the same time, you may get a warning that a table is corrupt even when it is not.

If the server is run with external locking enabled, you can use myisamchk to check tables at any time. In this case, if the server tries to update a table that myisamchk is using, the server waits for myisamchk to finish before it continues.

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 external locking is disabled). If you do not stop mysqld, you should at least do a mysqladmin flush-tables before you run myisamchk. Your tables may become corrupted if the server and myisamchk access the tables simultaneously.

When performing crash recovery, it is important to understand that each MyISAM table tbl_name in a database corresponds to the three files in the database directory shown in the following table.

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 generates only a new index file without touching the .MYD file. This is safe, because myisamchk automatically detects whether the .MYD file is corrupt and aborts the repair if it is. You can also specify the --quick option twice to myisamchk. In this case, myisamchk does not abort on some errors (such as duplicate-key errors) 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 of the table before running myisamchk.

To check a MyISAM table, use the following commands:

In most cases, a simple myisamchk command with no arguments other than the table name is sufficient to check a table.

The discussion in this section describes how to use myisamchk on MyISAM tables (extensions .MYI and .MYD).

You can also use the CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables. See Section 13.7.3.2, “CHECK TABLE Statement”, and Section 13.7.3.5, “REPAIR TABLE Statement”.

Symptoms of corrupted tables include queries that abort unexpectedly and observable errors such as these:

To get more information about the error, run perror nnn, where nnn is the error number. The following example shows how to use perror to find the meanings for the most common error numbers that indicate a problem with a table:

shell> perror 126 127 132 134 135 136 141 144 145
MySQL error code 126 = Index file is crashed
MySQL error code 127 = Record-file is crashed
MySQL error code 132 = Old database file
MySQL error code 134 = Record was already deleted (or record file crashed)
MySQL error code 135 = No more room in record file
MySQL error code 136 = No more room in index file
MySQL error code 141 = Duplicate unique key or constraint on write or update
MySQL error code 144 = Table is crashed and last repair failed
MySQL error code 145 = Table was marked as crashed and should be repaired

Note that error 135 (no more room in record file) and error 136 (no more room in index file) are not errors that can be fixed by a simple repair. In this case, you must use ALTER TABLE to increase the MAX_ROWS and AVG_ROW_LENGTH table option values:

ALTER TABLE tbl_name MAX_ROWS=xxx AVG_ROW_LENGTH=yyy;

If you do not know the current table option values, use SHOW CREATE TABLE.

For the other errors, you must repair your tables. myisamchk can usually detect and fix most problems that occur.

The repair process involves up to three stages, described here. Before you begin, you should change location to the database directory and check the permissions of the table files. On Unix, make sure that they are readable by the user that mysqld runs as (and to you, because you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you.

This section is for the cases where a table check fails (such as those described in Section 7.6.2, “How to Check MyISAM Tables for Errors”), or you want to use the extended features that myisamchk provides.

The myisamchk options used for table maintenance with are described in Section 4.6.4, “myisamchk — MyISAM Table-Maintenance Utility”. myisamchk also has variables that you can set to control memory allocation that may improve performance. See Section 4.6.4.6, “myisamchk Memory Usage”.

If you are going to repair a table from the command line, you must first stop the mysqld server. Note that when you do mysqladmin shutdown on a remote server, the mysqld server is still available for a while after mysqladmin returns, until all statement-processing has stopped and all index changes have been flushed to disk.

Stage 1: Checking your tables

Run myisamchk *.MYI or 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:

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:

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 13.7.3.5, “REPAIR TABLE Statement”.

To coalesce fragmented rows and eliminate wasted space that results from deleting or updating rows, run myisamchk in recovery mode:

shell> myisamchk -r tbl_name

You can optimize a table in the same way by using the OPTIMIZE TABLE SQL statement. OPTIMIZE TABLE does a table repair and a key analysis, and also sorts the index tree so that key lookups are faster. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use OPTIMIZE TABLE. See Section 13.7.3.4, “OPTIMIZE TABLE Statement”.

myisamchk has a number of other options that you can use to improve the performance of a table:

For a full description of all available options, see Section 4.6.4, “myisamchk — MyISAM Table-Maintenance Utility”.

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. See Section 13.7.3, “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 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 cause the server to check MyISAM tables automatically, start it with the myisam_recover_options system variable set. See Section 5.1.8, “Server System Variables”.

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 --myisam_sort_buffer_size=16M */*.MYI