This section discusses how to make database backups (full and
incremental) and how to perform table maintenance. The syntax of
the SQL statements described here is given in
Chapter 13, SQL Statement Syntax. Much of the information here
pertains primarily to
MyISAM tables. Additional
InnoDB backup procedures is
given in Section 14.2.8, “Backing Up and Recovering an
Because MySQL tables are stored as files, it is easy to do a
backup. To get a consistent backup, do a
TABLES on the relevant tables, followed by
FLUSH TABLES for the tables. See
Section 13.4.5, “
LOCK TABLES and
UNLOCK TABLES Syntax”, and Section 188.8.131.52, “
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.
To make an SQL-level backup of a table, you can use
SELECT INTO ... OUTFILE. For this statement,
the output file cannot already exist because allowing files to
be overwritten would constitute a security risk. See
Section 13.2.7, “
Another technique for backing up a database is to use the mysqldump program or the mysqlhotcopy script. See Section 8.12, “mysqldump — A Database Backup Program”, and Section 8.13, “mysqlhotcopy — A Database Backup Program”.
Create a full backup of your database:
You can also create a binary backup simply by copying all
table files (
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 database
InnoDB does not store table contents in
database directories, and mysqlhotcopy
works only for
Stop mysqld if it is running, then start
it with the
option. See Section 5.12.3, “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 executed mysqldump.
InnoDB tables, it is possible to perform
an online backup that takes no locks on tables; see
Section 8.12, “mysqldump — A Database Backup Program”.
MySQL supports incremental backups: You need to start the server
--log-bin option to enable binary
logging; see Section 5.12.3, “The Binary Log”. 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 by using
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 further below. The next time you do a full backup, you
should also rotate the binary log using
mysqldump --flush-logs, or
mysqlhotcopy --flushlog. See
Section 8.12, “mysqldump — A Database Backup Program”, and Section 8.13, “mysqlhotcopy — A Database Backup Program”.
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 replicating
LOAD DATA INFILE
commands, you should also back up any
SQL_LOAD-* files that may exist in the
directory specified by the
option. (This location defaults to the value of the
tmpdir variable if not specified.) The slave
needs these files to resume replication of any interrupted
LOAD DATA INFILE operations.
If you have to restore
MyISAM tables, 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. Note that it works only if 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 8.10, “mysqlbinlog — Utility for Processing Binary Log Files”, for more information on the mysqlbinlog utility and how to use it.
You can also make selective backups of individual files:
To dump the table, use
SELECT * INTO OUTFILE
To reload the table, use
LOAD DATA INFILE
'. To avoid duplicate rows, the table must have
PRIMARY KEY or a
UNIQUE index. The
REPLACE keyword causes old rows to be
replaced with new ones when a new row duplicates an old row
on a unique key value.
If you have performance problems with your 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 Section 6.1, “Introduction to Replication”.
If you are using a Veritas filesystem, you can make a backup like this:
From a client program, execute
FLUSH TABLES WITH
From another shell, execute
From the first client, execute
Copy files from the snapshot.
Unmount the snapshot.
This section discusses a procedure for performing backups that allows you to recover data after several types of crashes:
Operating 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 be needed.
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
reads its logs and finds in them the list of pending committed
and non-committed transactions that have not been flushed to the
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 filesystem 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 backups (a snapshot of the data at a point in time) can
be done in MySQL with several tools. For example,
InnoDB Hot Backup provides online
non-blocking physical backup of the
data files, and mysqldump provides online
logical backup. This discussion uses
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 is an online, non-blocking backup that does not disturb
the reads and writes on the tables. We assumed earlier that
our tables 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 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
mysql database, we must assume that no
administrative changes are being made to MySQL accounts during
.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.
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
--log-bin option and that
has been running for some days, we find these MySQL binary log
-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
.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
-- 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
All data changes logged after the backup are not present
.sql, but are present in the
gbichot2-bin.000007 binary log file
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 in the
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
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
Note: 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
PURGE MASTER LOGS
statement explains what should be verified before deleting the
MySQL binary logs. See Section 184.108.40.206, “
PURGE MASTER 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 following guidelines:
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
Make periodic full backups, using the mysqldump command shown earlier in Section 220.127.116.11, “Backup Policy”, that makes an online, non-blocking backup.
Make periodic incremental backups by flushing the logs
FLUSH LOGS or mysqladmin
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 5.12.3, “The Binary Log”, and Section 8.10, “mysqlbinlog — Utility for Processing Binary Log Files”.
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 pathname
can be specified with the
--log-bin option to
place the files in a different location. Typically the option is
given in an option file (that is,
my.ini, 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
SHOW BINLOG EVENTS\G
If you prefer, you can execute the following command from the command line instead:
mysql -u root -p -E -e "SHOW BINLOG EVENTS"
root password for your server when
mysql prompts you for it.
To indicate the start and end times for recovery, specify the
options for mysqlbinlog, in
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-date="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
--stop-date 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-date="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-date="2005-04-20 9:55:00" \
--stop-date="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
This section discusses how to use myisamchk
to check or repair
MyISAM tables (tables that
files for storing data and indexes). For general
myisamchk background, see
Section 8.3, “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 8.3.1, “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:
To check or repair
MyISAM tables, use
CHECK TABLE or
MyISAM tables, use
MyISAM tables, use
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 18.104.22.168, “
ANALYZE TABLE Syntax”, Section 22.214.171.124, “
CHECK TABLE Syntax”,
Section 126.96.36.199, “
OPTIMIZE TABLE Syntax”, and
Section 188.8.131.52, “
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 A.4.2, “What to Do If MySQL Keeps Crashing”.
For an explanation of how
MyISAM tables can
become corrupted, see Section 14.1.4, “
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 don't 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 three files in the database directory:
|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
file and renaming the new file to the original file name. If
does not create a temporary
but instead assumes that the
.MYD file is
correct and generates only a new index file without touching
.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
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 the
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
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
REPAIR TABLE statements
to check and repair
MyISAM tables. See
Section 184.108.40.206, “
CHECK TABLE Syntax”, and
Section 220.127.116.11, “
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
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 145126 = Index file is crashed / Wrong file format 127 = Record-file is crashed 132 = Old database file 134 = Record was already deleted (or record file crashed) 135 = No more room in record file 136 = No more room in index file 141 = Duplicate unique key or constraint on write or update 144 = Table is crashed and last repair failed 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 18.104.22.168, “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 8.3, “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
Run myisamchk *.MYI or myisamchk
-e *.MYI if you have more time. Use the
-s (silent) option to suppress unnecessary
If the mysqld server is stopped, you should
--update-state option to tell
myisamchk to mark the table as
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
-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
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
recovery mode uses an old recovery method that handles a
few cases that regular recovery mode does not (but is
Note: If you want a repair operation to go much faster, you
should set the values of the
key_buffer_size variables each to about 25%
of your available memory when running
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.)
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 22.214.171.124, “
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 126.96.36.199, “
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 8.3, “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.
myisamchk -d -v
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
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 188.8.131.52, “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 184.108.40.206, “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
Fixed length. Other
possible values are
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
MyISAM data type with the
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 two parts.
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 the theoretical maximum).
MySQL tries to pack key values that have a common suffix.
This can only be used for indexes on
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
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 220.127.116.11, “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 8.5, “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
MyISAM tables is with the
CHECK TABLE and
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
-s option (short for
--silent) causes myisamchk
to run in silent mode, printing messages only when errors
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
MyISAM tables automatically, start the
server with the
--myisam-recover option. See
Section 5.2.1, “mysqld Command Options”.
You should also check your tables regularly during normal
system operation. At MySQL AB, we run a
cron job to check all our important tables
once a week, using a line like this in a
35 0 * * 0
This prints out information about crashed tables so that we can examine and repair them when needed.
Because we have not had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for several years, once a week is more than sufficient for us.
We recommend that to start with, you execute myisamchk -s each night on all tables that have been updated during the last 24 hours, until you come to trust MySQL as much as we do.
Normally, MySQL tables need little maintenance. If you are
performing many updates to
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 stopped:
myisamchk -r -s --sort-index --sort_buffer_size=16M */*.MYI