Table of Contents
It is important to back up your databases in case problems occur so that you can recover your data and be up and running again. MySQL offers a variety of backup strategies from which you can choose to select whatever methods best suit the requirements for your installation.
Briefly summarized, backup concepts with which you should be familiar include the following:
Logical versus physical backups
Online versus offline backups
Local versus remote backups
Full versus incremental backups
Backup scheduling, compression, and encryption
More generally, the following discussion amplifies on the properties of different backup methods.
Logical versus physical (raw)
backups. Logical backups save information represented
as logical database structure (
statements) and content (
statements or delimited-text files). Physical backups consist of
raw copies of the directories and files that store database
Logical backup methods have these characteristics:
The backup is done by going through the MySQL server to obtain database structure and content information.
Backup is slower than physical methods because the server must access database information, convert it to logical format, and send it to the backup program.
Output is larger than for physical backup, particularly when saved in text format.
Backup and restore granularity is available at the server level (all databases), database level (all tables in a particular database), or table level. This is true regardless of storage engine.
The backup does not include log or configuration files, or other database-related files that are not part of databases.
Backups stored in logical format are machine independent and highly portable.
Logical backups are performed with the MySQL server running (the server is not taken offline).
Physical backup methods have these characteristics:
The backup consists of exact copies of database directories
and files. Typically this is a copy of all or part of the
MySQL data directory. Data from
tables cannot be backed up this way because their contents
are not stored on disk.
Physical backup methods are faster than logical because they involve only file copying without conversion.
Output is more compact than for logical backup.
Backup and restore granularity extends from the level of the
entire data directory down to the level of individual files.
This may or may not provide for table-level granularity,
depending on storage engine. (Each
table corresponds uniquely to a set of files, but an
InnoDB table shares file storage with
In addition to databases, the backup can include any related files such as log or configuration files.
Backups are portable only to other machines that have identical or similar hardware characteristics.
Backups can be performed while the MySQL server is not running. If the server is running, it is necessary to perform appropriate locking so that the server does not change database contents during the backup.
Physical backup tools include file system-level commands
(such as cp, scp,
START BACKUP for
For restore, files copied at the file system level or with
mysqlhotcopy can be copied back to their
original locations with file system commands;
tables, and ndb_restore restores
Online versus offline backups. Online backups take place while the MySQL server is running so that the database information can be obtained from the server. Offline backups take place while the server is stopped. (This distinction can also be described as “hot” versus “cold” backups; a “warm” backup is one where the server remains running but locked against modifying data while you access database files externally.)
Online backup methods have these characteristics:
Less intrusive to other clients, which can connect to the MySQL server during the backup and may be able to access data depending on what operations they need to perform.
Care must be taken to impose appropriate locking so that data modifications do not take place that compromise backup integrity.
Offline backup methods have these characteristics:
Affects clients adversely because the server is unavailable during backup.
Simpler backup procedure because there is no possibility of interference from client activity.
Local versus remote backups. A local backup is performed on the same host where the MySQL server runs, whereas a remote backup is initiated from a different host.
mysqldump can connect to local or remote
servers. For SQL output (
INSERT statements), local or
remote dumps can be done and generate output on the client.
For delimited-text output (with the
--tab option), data files
are created on the server host.
mysqlhotcopy performs only local backups: It connects to the server to lock it against data modifications and then copies local table files.
SELECT ... INTO
OUTFILE can be initiated from a remote client
host, but the output file is created on the server host.
Physical backup methods typically are initiated locally on the MySQL server host so that the server can be taken offline, although the destination for file copies might be remote.
Snapshot backups. Some file system implementations enable “snapshots” to be taken. These provide logical copies of the file system at a given point in time, without having to physically copy the entire file system. (For example, the implementation may use copy-on-write techniques so that only parts of the file system modified after the snapshot time need be copied.) MySQL itself does not provide the capability for taking file system snapshots. It is available through third-party solutions such as Veritas or LVM.
Full versus incremental backups. A full backup includes all data managed by a MySQL server at a given point in time. An incremental backup consists of the changes made to the data since the full backup. MySQL has different ways to perform full backups, such as those described in previous items. Incremental backups are made possible by enabling the server's binary log, which the server uses to record data changes.
Point-in-time recovery. One use for the binary log is to achieve point-in-time recovery. This is done by recovering first from the backup files to restore the server to its state when the backup was made, and then by re-executing changes in subsequently written binary log files to redo data modifications up to the desired point in time.
Backup scheduling, compression, and
encryption. Backup scheduling is valuable for
automating backup procedures. Compression of backup output
reduces space requirements, and encryption of the output
provides better security against unauthorized access of
backed-up data. MySQL itself does not provide these
capabilities. ibbackup can compress
InnoDB backups, and compression or encryption
of backup output can be achieved using file system utilities.
Other third-party solutions may be available.
Table maintenance. Data
integrity can be compromised if tables become corrupt. MySQL
provides programs for checking tables and repairing them should
problems be found. These programs apply primarily to
MyISAM tables. See
Section 6.4, “Table Maintenance and Crash Recovery”.
Resources related to backup or to maintaining data availability include the following:
A forum dedicated to backup issues is available at http://forums.mysql.com/list.php?93.
The syntax of the SQL statements described here is given in Chapter 12, SQL Statement Syntax.
For additional information about
backup procedures, see Section 13.2.6, “Backing Up and Recovering an
Replication enables you to maintain identical data on multiple servers. This has several benefits, such as allowing client load to be distributed over servers, availability of data even if a given server is taken offline or fails, and the ability to make backups using a slave server without affecting the master. See Chapter 16, Replication.
MySQL Cluster provides a high-availability, high-redundancy version of MySQL adapted for the distributed computing environment. See Chapter 17, MySQL Cluster. For information specifically about MySQL Cluster backup, see Section 17.7.3, “Online Backup of MySQL Cluster”.
This section summarizes some general methods for making backups.
Making Backups by Copying Files
MyISAM tables are stored as files, so it is
easy to do a backup by copying files. To get a consistent backup,
LOCK TABLES on the relevant
tables, followed by
TABLES for the tables. See
Section 12.4.5, “
LOCK TABLES and
TABLES Syntax”, and Section 22.214.171.124, “
FLUSH Syntax”. You
need only a read lock; this allows other clients to continue to
query the tables while you are making a copy of the files in the
database directory. The
statement is needed to ensure that the all active index pages are
written to disk before you start the backup.
Making Delimited-Text File Backups
To create a text file containing a table's data, you can use
SELECT * INTO OUTFILE
'. The file is created
on the MySQL server host, not the client host. For this statement,
the output file cannot already exist because allowing files to be
overwritten would constitute a security risk. See
Section 12.2.8, “
SELECT Syntax”. This method works for any kind of data
file, but saves only table data, not the table structure.
Another technique for backing up a database is to use the mysqldump program or the mysqlhotcopy script. mysqldump is more general because it can back up all kinds of tables. mysqlhotcopy works only with some storage engines. (See Section 4.5.4, “mysqldump — A Database Backup Program”, and Section 4.6.9, “mysqlhotcopy — A Database Backup Program”.)
Create a full backup of your database:
You can also create a binary backup simply by copying all table
*.MYI files), as long as the server isn't
updating anything. The mysqlhotcopy script uses
this method. (But note that these methods do not work if your
InnoDB does not necessarily store table
contents in database directories, and
mysqlhotcopy works only for
InnoDB tables, it is possible to perform an
online backup that takes no locks on tables; see
Section 4.5.4, “mysqldump — A Database Backup Program”.
Using the Binary Log to Enable Incremental Backups
MySQL supports incremental backups: You must start the server with
--log-bin option to enable
binary logging; see Section 5.2.3, “The Binary Log”. The binary log
files provide you with the information you need to replicate
changes to the database that are made subsequent to the point at
which you performed a backup. At the moment you want to make an
incremental backup (containing all changes that happened since the
last full or incremental backup), you should rotate the binary log
LOGS. This done, you need to copy to the backup location
all binary logs which range from the one of the moment of the last
full or incremental backup to the last but one. These binary logs
are the incremental backup; at restore time, you apply them as
explained in Section 6.3, “Point-in-Time Recovery”. The next
time you do a full backup, you should also rotate the binary log
mysqldump --flush-logs, or
mysqlhotcopy --flushlog. See
Section 4.5.4, “mysqldump — A Database Backup Program”, and Section 4.6.9, “mysqlhotcopy — A Database Backup Program”.
Backing Up Replication Slaves
If your MySQL server is a slave replication server, then
regardless of the backup method you choose, you should also back
relay-log.info files when you back up your
slave's data. These files are always needed to resume replication
after you restore the slave's data. If your slave is subject to
INFILE commands, you should also back up any
SQL_LOAD-* files that may exist in the
directory specified by the
--slave-load-tmpdir option. (This
location defaults to the value of the
system variable if not specified.) The slave needs these files to
resume replication of any interrupted
MySQL Enterprise The MySQL Enterprise Monitor provides numerous advisors that issue immediate warnings should replication issues arise. For more information, see http://www.mysql.com/products/enterprise/advisors.html.
If you have performance problems with your master server while making backups, one strategy that can help is to set up replication and perform backups on the slave rather than on the master. See Chapter 16, Replication.
Recovering Corrupt Tables
If you have to restore
MyISAM tables that have
become corrupt, try to recover them using
REPAIR TABLE or myisamchk
-r first. That should work in 99.9% of all cases. If
myisamchk fails, try the following procedure.
It is assumed that you have enabled binary logging by starting
MySQL with the
Restore the original mysqldump backup, or binary backup.
Execute the following command to re-run the updates in the binary logs:
mysqlbinlog binlog.[0-9]* | mysql
In some cases, you may want to re-run only certain binary logs, from certain positions (usually you want to re-run all binary logs from the date of the restored backup, excepting possibly some incorrect statements). See Section 6.3, “Point-in-Time Recovery”.
Making Backups Using a File System Snapshot
If you are using a Veritas file system, you can make a backup like this:
This section discusses a procedure for performing backups that allows you to recover data after several types of crashes:
Operating system crash
File system crash
Hardware problem (hard drive, motherboard, and so forth)
The example commands do not include options such as
--password for the
mysqldump and mysql
programs. You should include such options as necessary so that the
MySQL server allows you to connect to it.
We assume that data is stored in the
storage engine, which has support for transactions and automatic
crash recovery. We also assume that the MySQL server is under load
at the time of the crash. If it were not, no recovery would ever
For cases of operating system crashes or power failures, we can
assume that MySQL's disk data is available after a restart. The
InnoDB data files might not contain consistent
data due to the crash, but
InnoDB reads its
logs and finds in them the list of pending committed and
noncommitted transactions that have not been flushed to the data
InnoDB automatically rolls back those
transactions that were not committed, and flushes to its data
files those that were committed. Information about this recovery
process is conveyed to the user through the MySQL error log. The
following is an example log excerpt:
InnoDB: Database was not shut down normally. InnoDB: Starting recovery from log files... InnoDB: Starting log scan based on checkpoint at InnoDB: log sequence number 0 13674004 InnoDB: Doing recovery: scanned up to log sequence number 0 13739520 InnoDB: Doing recovery: scanned up to log sequence number 0 13805056 InnoDB: Doing recovery: scanned up to log sequence number 0 13870592 InnoDB: Doing recovery: scanned up to log sequence number 0 13936128 ... InnoDB: Doing recovery: scanned up to log sequence number 0 20555264 InnoDB: Doing recovery: scanned up to log sequence number 0 20620800 InnoDB: Doing recovery: scanned up to log sequence number 0 20664692 InnoDB: 1 uncommitted transaction(s) which must be rolled back InnoDB: Starting rollback of uncommitted transactions InnoDB: Rolling back trx no 16745 InnoDB: Rolling back of trx no 16745 completed InnoDB: Rollback of uncommitted transactions completed InnoDB: Starting an apply batch of log records to the database... InnoDB: Apply batch completed InnoDB: Started mysqld: ready for connections
For the cases of file system crashes or hardware problems, we can assume that the MySQL disk data is not available after a restart. This means that MySQL fails to start successfully because some blocks of disk data are no longer readable. In this case, it is necessary to reformat the disk, install a new one, or otherwise correct the underlying problem. Then it is necessary to recover our MySQL data from backups, which means that we must already have made backups. To make sure that is the case, we should design a backup policy.
We all know that backups must be scheduled periodically. A full
backup (a snapshot of the data at a point in time) can be done
in MySQL with several tools. For example,
Backup provides online nonblocking physical backup of
InnoDB data files, and
mysqldump provides online logical backup.
This discussion uses mysqldump.
MySQL Enterprise For expert advice on backups and replication, subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.
Assume that we make a backup on Sunday at 1 p.m., when load is
low. The following command makes a full backup of all our
InnoDB tables in all databases:
mysqldump --single-transaction --all-databases > backup_sunday_1_PM.sql
This backup acquires a global read lock on all tables (using
FLUSH TABLES WITH READ
LOCK) at the beginning of the dump. 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
FLUSH statement is
issued, the MySQL server may get stalled until those statements
finish. After that, the dump becomes lock-free and does not
disturb reads and writes on the tables.
We assumed earlier that our tables are
--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 we do also have
other types of tables, we must assume that they are not changed
during the backup. For example, for the
MyISAM tables in the
database, we must assume that no administrative changes are
being made to MySQL accounts during the backup.
Full backups are necessary, but they are not always convenient. 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. After we have made the initial full backup, it is more efficient to make incremental backups. They 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. The MySQL server should always be started with the
--log-bin option so that it
stores these changes in a file while it updates data. This
option enables binary logging, so that the server writes each
SQL statement that updates data into a file called a MySQL
binary log. Looking at the data directory of a MySQL server that
was started with the
option and 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. This file is used for replication.
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, then any 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:
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,
.sql 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:
.sql file contains all changes made
before any changes written to the
gbichot2-bin.000007 binary log file or
All data changes logged after the backup are not present in
.sql, but are present in the
gbichot2-bin.000007 binary log file or
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
gbichot2-bin.000008. All changes
between the Sunday 1 p.m. full backup and Monday 1 p.m. will be
gbichot2-bin.000007 file. 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. will be in the
gbichot2-bin.000008 file (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:
mysqldump --single-transaction --flush-logs --master-data=2 \
--all-databases --delete-master-logs > backup_sunday_1_PM.sql
Deleting the MySQL binary logs with mysqldump
--delete-master-logs can be dangerous if your server
is a replication master server, because slave servers might
not yet fully have processed the contents of the binary log.
The description for the
LOGS statement explains what should be verified
before deleting the MySQL binary logs. See
Section 126.96.36.199, “
PURGE BINARY LOGS Syntax”.
Now, suppose that we have a catastrophic crash 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:
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.000008 binary log files. Fetch
the files if necessary from where they were backed up, and then
process their contents like this:
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
file at hand, and we could apply it using
mysqlbinlog and mysql to
restore the most recent data changes with no loss up to the
moment of the crash.
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
Always run the MySQL server with the
--log-bin option, or even
where the log file name is located on some safe media
different from the drive on which the data directory is
located. If you have such safe media, this technique can
also be good for disk load balancing (which results in a
If a MySQL server was started with the
--log-bin option to enable binary
logging, you can use the mysqlbinlog utility to
recover data from the binary log files, starting from a specified
point in time (for example, since your last backup) until the
present or another specified point in time. For information on
enabling the binary log and using mysqlbinlog,
see Section 5.2.3, “The Binary Log”, and
Section 4.6.7, “mysqlbinlog — Utility for Processing Binary Log Files”.
MySQL Enterprise For maximum data recovery, the MySQL Enterprise Monitor advises subscribers to synchronize to disk at each write. For more information, see http://www.mysql.com/products/enterprise/advisors.html.
To restore data from a binary log, you must know the location and
name of the current binary log file. By default, the server
creates binary log files in the data directory, but a path name
can be specified with the
option to place the files in a different location. Typically the
option is given in an option file (that is,
depending on your system). It can also be given on the command
line when the server is started. To determine the name of the
current binary log file, issue the following statement:
SHOW MASTER STATUS
If you prefer, you can execute the following command from the command line instead:
mysql -u root -p -E -e "SHOW MASTER STATUS"
root password for your server when
mysql prompts you for it.
To view the contents of a binary log, use
mysqlbinlog. See Section 4.6.7, “mysqlbinlog — Utility for Processing Binary Log Files”.
To indicate the start and end times for recovery, specify the
--stop-datetime options for
DATETIME format. As an example,
suppose that exactly at 10:00 a.m. on April 20, 2005 an SQL
statement was executed that deleted a large table. To restore
the table and data, you could restore the previous night's
backup, and then execute the following command:
mysqlbinlog --stop-datetime="2005-04-20 9:59:59" \
/var/log/mysql/bin.123456 | mysql -u root -p
This command recovers all of the data up until the date and time
given by the
option. If you did not detect the erroneous SQL statement that
was entered until hours later, you will probably also want to
recover the activity that occurred afterward. Based on this, you
could run mysqlbinlog again with a start date
and time, like so:
mysqlbinlog --start-datetime="2005-04-20 10:01:00" \
/var/log/mysql/bin.123456 | mysql -u root -p
In this command, the SQL statements logged from 10:01 a.m. on will be re-executed. The combination of restoring of the previous night's dump file and the two mysqlbinlog commands restores everything up until one second before 10:00 a.m. and everything from 10:01 a.m. on. You should examine the log to be sure of the exact times to specify for the commands. To display the log file contents without executing them, use this command:
mysqlbinlog /var/log/mysql/bin.123456 > /tmp/mysql_restore.sql
Then open the file with a text editor to examine it.
Instead of specifying dates and times, the
--stop-position options for
mysqlbinlog can be used for specifying log
positions. They work the same as the start and stop date
options, except that you specify log position numbers rather
than dates. Using positions may enable you to be more precise
about which part of the log to recover, especially if many
transactions occurred around the same time as a damaging SQL
statement. To determine the position numbers, run
mysqlbinlog for a range of times near the
time when the unwanted transaction was executed, but redirect
the results to a text file for examination. This can be done
mysqlbinlog --start-datetime="2005-04-20 9:55:00" \
--stop-datetime="2005-04-20 10:05:00" \
/var/log/mysql/bin.123456 > /tmp/mysql_restore.sql
This command creates a small text file in the
/tmp directory that contains the SQL
statements around the time that the deleterious SQL statement
was executed. Open this file with a text editor and look for the
statement that you don't want to repeat. Determine the positions
in the binary log for stopping and resuming the recovery and
make note of them. Positions are labeled as
log_pos followed by a number. After restoring
the previous backup file, use the position numbers to process
the binary log file. For example, you would use commands
something like these:
mysqlbinlog --stop-position="368312" /var/log/mysql/bin.123456 \
| mysql -u root -pshell>
mysqlbinlog --start-position="368315" /var/log/mysql/bin.123456 \
| mysql -u root -p
The first command recovers all the transactions up until the
stop position given. The second command recovers all
transactions from the starting position given until the end of
the binary log. Because the output of
TIMESTAMP statements before each SQL statement
recorded, the recovered data and related MySQL logs will reflect
the original times at which the transactions were executed.
This section discusses how to use myisamchk to
check or repair
MyISAM tables (tables that have
.MYI files for
storing data and indexes). For general
myisamchk background, see
Section 4.6.3, “myisamchk — MyISAM Table-Maintenance Utility”.
You can use myisamchk to get information about your database tables or to check, repair, or optimize them. The following sections describe how to perform these operations and how to set up a table maintenance schedule.
Even though table repair with myisamchk is quite secure, it is always a good idea to make a backup before doing a repair or any maintenance operation that could make a lot of changes to a table.
myisamchk operations that affect indexes can
FULLTEXT indexes to be rebuilt with
full-text parameters that are incompatible with the values used by
the MySQL server. To avoid this problem, follow the guidelines in
Section 188.8.131.52, “myisamchk General Options”.
In many cases, you may find it simpler to do
MyISAM table maintenance using the SQL
statements that perform operations that
myisamchk can do:
These statements can be used directly or by means of the
mysqlcheck client program. One advantage of
these statements over myisamchk is that the
server does all the work. With myisamchk, you
must make sure that the server does not use the tables at the same
time so that there is no unwanted interaction between
myisamchk and the server. See
Section 184.108.40.206, “
ANALYZE TABLE Syntax”, Section 220.127.116.11, “
CHECK TABLE Syntax”,
Section 18.104.22.168, “
OPTIMIZE TABLE Syntax”, and
Section 22.214.171.124, “
REPAIR TABLE Syntax”.
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.1.4.2, “What to Do If MySQL Keeps Crashing”.
For an explanation of how
MyISAM tables can
become corrupted, see Section 13.1.4, “
MyISAM Table Problems”.
If you run mysqld with external locking disabled (which is the default as of MySQL 4.0), you cannot reliably use myisamchk to check a table when mysqld is using the same table. If you can be certain that no one will access the tables through 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 will wait 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
tbl_name in a database corresponds to
the three files in the database directory shown in the following
|Definition (format) 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
and renaming the new file to the original file name. If you use
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
file. This is safe, because myisamchk
automatically detects whether the
is corrupt and aborts the repair if it is. You can also specify
--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
.MYD file. Normally the use
--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
This finds 99.99% of all errors. What it cannot find is
corruption that involves only the data
file (which is very unusual). If you want to check a table,
you should normally run myisamchk without
options or with the
-s (silent) option.
This finds 99.999% of all errors. It first checks all index entries for errors and then reads through all rows. It calculates a checksum for all key values in the rows and verifies that the checksum matches the checksum for the keys in the index tree.
This does a complete and thorough check of all data
-e means “extended check”).
It does a check-read of every key for each row to verify
that they indeed point to the correct row. This may take a
long time for a large table that has many indexes. Normally,
myisamchk stops after the first error it
finds. If you want to obtain more information, you can add
-v (verbose) option. This causes
myisamchk to keep going, up through a
maximum of 20 errors.
myisamchk -e -i
This is like the previous command, but the
-i option tells
myisamchk to print additional statistical
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
You can also (and should, if possible) use the
CHECK TABLE and
REPAIR TABLE statements to check
MyISAM tables. See
Section 126.96.36.199, “
CHECK TABLE Syntax”, and
Section 188.8.131.52, “
REPAIR TABLE Syntax”.
Symptoms of corrupted tables include queries that abort unexpectedly and observable errors such as these:
is locked against change
Can't find file
Unexpected end of file
Record file is crashed
nnn from table handler
To get more information about the error, run
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:
perror 126 127 132 134 135 136 141 144 145MySQL 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
AVG_ROW_LENGTH table option values:
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 four 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 6.4.2, “How to Check
MyISAM Tables for Errors”), or you want to use
the extended features that myisamchk
The options that you can use for table maintenance with myisamchk are described in Section 4.6.3, “myisamchk — MyISAM Table-Maintenance Utility”.
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 alive 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
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
of memory errors), or if myisamchk
crashes, go to Stage 3.
Stage 2: Easy safe repair
First, try myisamchk -r -q
-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:
Make a backup of the data file before continuing.
Use myisamchk -r
-r means “recovery mode”).
This removes incorrect rows and deleted rows from the data
file and reconstructs the index file.
If the preceding step fails, use myisamchk
tbl_name. Safe recovery
mode uses an old recovery method that handles a few cases
that regular recovery mode does not (but is slower).
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:
Move the data file to a safe place.
Use the table description file to create new (empty) data and index files:
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.)
If you are using replication, you should stop it prior to performing the above procedure, since it involves file system operations, and these are not logged by MySQL.
Go back to Stage 2. myisamchk -r -q should work. (This should not be an endless loop.)
You can also use the
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 184.108.40.206, “
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:
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.
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.
To coalesce fragmented rows and eliminate wasted space that results from deleting or updating rows, run myisamchk in recovery mode:
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
TABLE. See Section 220.127.116.11, “
OPTIMIZE TABLE Syntax”.
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 4.6.3, “myisamchk — MyISAM Table-Maintenance Utility”.
To obtain a description of a table or statistics about it, use the commands shown here. We explain some of the information in more detail later.
Runs myisamchk in “describe mode” to produce a description of your table. If you start the MySQL server with external locking disabled, myisamchk may report an error for a table that is updated while it runs. However, because myisamchk does not change the table in describe mode, there is no risk of destroying data.
-v runs myisamchk
in verbose mode so that it produces more information about
what it is doing.
Shows only the most important information from a table. This operation is slow because it must read the entire table.
This is like
-eis, but tells you what is
tbl_name argument can be either
the name of a
MyISAM table or the name of its
index file, as described in Section 4.6.3, “myisamchk — MyISAM Table-Maintenance Utility”.
tbl_name arguments can be
Sample output for some of these commands follows. They are based on a table with these data and index file sizes:
-rw-rw-r-- 1 monty tcx 317235748 Jan 12 17:30 company.MYD -rw-rw-r-- 1 davida tcx 96482304 Jan 12 18:35 company.MYI
Example of myisamchk -d output:
MyISAM file: company.MYI Record format: Fixed length Data records: 1403698 Deleted blocks: 0 Recordlength: 226 table description: Key Start Len Index Type 1 2 8 unique double 2 15 10 multip. text packed stripped 3 219 8 multip. double 4 63 10 multip. text packed stripped 5 167 2 multip. unsigned short 6 177 4 multip. unsigned long 7 155 4 multip. text 8 138 4 multip. unsigned long 9 177 4 multip. unsigned long 193 1 text
Example of myisamchk -d -v output:
MyISAM file: company Record format: Fixed length File-version: 1 Creation time: 1999-10-30 12:12:51 Recover time: 1999-10-31 19:13:01 Status: checked Data records: 1403698 Deleted blocks: 0 Datafile parts: 1403698 Deleted data: 0 Datafile pointer (bytes): 3 Keyfile pointer (bytes): 3 Max datafile length: 3791650815 Max keyfile length: 4294967294 Recordlength: 226 table description: Key Start Len Index Type Rec/key Root Blocksize 1 2 8 unique double 1 15845376 1024 2 15 10 multip. text packed stripped 2 25062400 1024 3 219 8 multip. double 73 40907776 1024 4 63 10 multip. text packed stripped 5 48097280 1024 5 167 2 multip. unsigned short 4840 55200768 1024 6 177 4 multip. unsigned long 1346 65145856 1024 7 155 4 multip. text 4995 75090944 1024 8 138 4 multip. unsigned long 87 85036032 1024 9 177 4 multip. unsigned long 178 96481280 1024 193 1 text
Example of myisamchk -eis output:
Checking MyISAM file: company Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4 Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4 Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4 Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3 Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3 Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4 Total: Keyblocks used: 98% Packed: 17% Records: 1403698 M.recordlength: 226 Packed: 0% Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00 Record blocks: 1403698 Delete blocks: 0 Recorddata: 317235748 Deleted data: 0 Lost space: 0 Linkdata: 0 User time 1626.51, System time 232.36 Maximum resident set size 0, Integral resident set size 0 Non physical pagefaults 0, Physical pagefaults 627, Swaps 0 Blocks in 0 out 0, Messages in 0 out 0, Signals 0 Voluntary context switches 639, Involuntary context switches 28966
Example of myisamchk -eiv output:
Checking MyISAM file: company Data records: 1403698 Deleted blocks: 0 - check file-size - check delete-chain block_size 1024: index 1: index 2: index 3: index 4: index 5: index 6: index 7: index 8: index 9: No recordlinks - check index reference - check data record references index: 1 Key: 1: Keyblocks used: 97% Packed: 0% Max levels: 4 - check data record references index: 2 Key: 2: Keyblocks used: 98% Packed: 50% Max levels: 4 - check data record references index: 3 Key: 3: Keyblocks used: 97% Packed: 0% Max levels: 4 - check data record references index: 4 Key: 4: Keyblocks used: 99% Packed: 60% Max levels: 3 - check data record references index: 5 Key: 5: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 6 Key: 6: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 7 Key: 7: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 8 Key: 8: Keyblocks used: 99% Packed: 0% Max levels: 3 - check data record references index: 9 Key: 9: Keyblocks used: 98% Packed: 0% Max levels: 4 Total: Keyblocks used: 9% Packed: 17% - check records and index references
*** LOTS OF ROW NUMBERS DELETED ***Records: 1403698 M.recordlength: 226 Packed: 0% Recordspace used: 100% Empty space: 0% Blocks/Record: 1.00 Record blocks: 1403698 Delete blocks: 0 Recorddata: 317235748 Deleted data: 0 Lost space: 0 Linkdata: 0 User time 1639.63, System time 251.61 Maximum resident set size 0, Integral resident set size 0 Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0 Blocks in 4 out 0, Messages in 0 out 0, Signals 0 Voluntary context switches 10604, Involuntary context switches 122798
Explanations for the types of information myisamchk produces are given here. “Keyfile” refers to the index file. “Record” and “row” are synonymous.
Name of the
MyISAM (index) file.
MyISAM format. Currently
When the data file was created.
When the index/data file was last reconstructed.
How many rows are in the table.
How many deleted blocks still have reserved space. You can optimize your table to minimize this space. See Section 6.4.4, “Table Optimization”.
For dynamic-row format, this indicates how many data blocks
there are. For an optimized table without fragmented rows,
this is the same as
How many bytes of unreclaimed deleted data there are. You can optimize your table to minimize this space. See Section 6.4.4, “Table Optimization”.
The size of the data file pointer, in bytes. It is usually 2, 3, 4, or 5 bytes. Most tables manage with 2 bytes, but this cannot be controlled from MySQL yet. For fixed tables, this is a row address. For dynamic tables, this is a byte address.
The size of the index file pointer, in bytes. It is usually 1, 2, or 3 bytes. Most tables manage with 2 bytes, but this is calculated automatically by MySQL. It is always a block address.
Max datafile length
How long the table data file can become, in bytes.
Max keyfile length
How long the table index file can become, in bytes.
How much space each row takes, in bytes.
The format used to store table rows. The preceding examples
Fixed length. Other possible values
A list of all keys in the table. For each key, myisamchk displays some low-level information:
This key's number.
Where in the row this portion of the index starts.
How long this portion of the index is. For packed numbers, this should always be the full length of the column. For strings, it may be shorter than the full length of the indexed column, because you can index a prefix of a string column.
Whether a key value can exist multiple times in the
index. Possible values are
What data type this portion of the index has. This is a
MyISAM data type with the possible
Address of the root index block.
The size of each index block. By default this is 1024, but the value may be changed at compile time when MySQL is built from source.
This is a statistical value used by the optimizer. It tells how many rows there are per value for this index. A unique index always has a value of 1. This may be updated after a table is loaded (or greatly changed) with myisamchk -a. If this is not updated at all, a default value of 30 is given.
For the table shown in the examples, there are two
table description lines for the ninth
index. This indicates that it is a multiple-part index with
What percentage of the keyblocks are used. When a table has just been reorganized with myisamchk, as for the table in the examples, the values are very high (very near theoretical maximum).
MySQL tries to pack key values that have a common suffix.
This can only be used for indexes on
VARCHAR columns. For long
indexed strings that have similar leftmost parts, this can
significantly reduce the space used. In the third of the
preceding examples, the fourth key is 10 characters long and
a 60% reduction in space is achieved.
How deep the B-tree for this key is. Large tables with long key values get high values.
How many rows are in the table.
The average row length. This is the exact row length for tables with fixed-length rows, because all rows have the same length.
MySQL strips spaces from the end of strings. The
Packed value indicates the percentage of
savings achieved by doing this.
What percentage of the data file is used.
What percentage of the data file is unused.
Average number of blocks per row (that is, how many links a fragmented row is composed of). This is always 1.0 for fixed-format tables. This value should stay as close to 1.0 as possible. If it gets too large, you can reorganize the table. See Section 6.4.4, “Table Optimization”.
How many blocks (links) are used. For fixed-format tables, this is the same as the number of rows.
How many blocks (links) are deleted.
How many bytes in the data file are used.
How many bytes in the data file are deleted (unused).
If a row is updated to a shorter length, some space is lost. This is the sum of all such losses, in bytes.
When the dynamic table format is used, row fragments are
linked with pointers (4 to 7 bytes each).
Linkdata is the sum of the amount of
storage used by all such pointers.
If a table has been compressed with myisampack, myisamchk -d prints additional information about each table column. See Section 4.6.5, “myisampack — Generate Compressed, Read-Only MyISAM Tables”, for an example of this information and a description of what it means.
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
MyISAM tables is with the
CHECK TABLE and
REPAIR TABLE statements. See
Section 18.104.22.168, “
CHECK TABLE Syntax”, and
Section 22.214.171.124, “
REPAIR TABLE Syntax”.
Another way to check tables is to use
myisamchk. For maintenance purposes, you can
use myisamchk -s. The
option (short for
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 check
automatically, start the server with the
--myisam-recover option. See
Section 5.1.2, “Server Command Options”.
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
35 0 * * 0
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
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
myisamchk -r -s --sort-index --sort_buffer_size=16M */*.MYI