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 InnoDB data files, and mysqldump provides online logical backup. This discussion uses mysqldump.

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:

shell> 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 the backup.

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

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 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:

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

After executing this command, the data directory contains a new binary log file, gbichot2-bin.000007. The resulting .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:

On Monday at 1 p.m., we can create an incremental backup by flushing the logs to begin a new binary log file. For example, executing a mysqladmin flush-logs command creates gbichot2-bin.000008. All changes between the Sunday 1 p.m. full backup and Monday 1 p.m. 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 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:

shell> 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 13.6.1.1, “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:

shell> mysql < backup_sunday_1_PM.sql

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

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

We now have recovered the data to its state as of Tuesday 1 p.m., but still are missing the changes from that date to the date of the crash. To not lose them, we would have needed to have the MySQL server store its MySQL binary logs into a safe location (RAID disks, SAN, ...) different from the place where it stores its data files, so that these logs were not on the destroyed disk. (That is, we can start the server with a --log-bin option that specifies a location on a different physical device from the one on which the data directory resides. That way, the logs are safe even if the device containing the directory is lost.) If we had done this, we would have the gbichot2-bin.000009 file 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:

To indicate the start and end times for recovery, specify the --start-date and --stop-date 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:

shell> 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:

shell> 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:

shell> 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 --start-position and --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 like so:

shell> 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:

shell> mysqlbinlog --stop-position="368312" /var/log/mysql/bin.123456 \
         | mysql -u root -p

shell> 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 mysqlbinlog includes SET 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 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 that each MyISAM table tbl_name in a database corresponds to three files in the database directory:

Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.

myisamchk works by creating a copy of the .MYD data file row by row. It ends the repair stage by removing the old .MYD file and renaming the new file to the original file name. If you use --quick, myisamchk does not create a temporary .MYD file, but instead assumes that the .MYD file is correct and generates only a new index file without touching the .MYD file. This is safe, because myisamchk automatically detects whether the .MYD file is corrupt and aborts the repair if it is. You can also specify the --quick option twice to myisamchk. In this case, myisamchk does not abort on some errors (such as duplicate-key errors) but instead tries to resolve them by modifying the .MYD file. Normally the use of two --quick options is useful only if you have too little free disk space to perform a normal repair. In this case, you should at least make a backup of the table before running myisamchk.

To check a MyISAM table, use the following commands:

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

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

You can also (and should, if possible) use the CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables. See Section 13.5.2.3, “CHECK TABLE Syntax”, and Section 13.5.2.6, “REPAIR TABLE Syntax”.

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

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

shell> perror 126 127 132 134 135 136 141 144 145
126 = 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 MAX_ROWS and AVG_ROW_LENGTH table option values:

ALTER TABLE tbl_name MAX_ROWS=xxx AVG_ROW_LENGTH=yyy;

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

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

The repair process involves up to 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 5.10.4.2, “How to Check MyISAM Tables for Errors”), or you want to use the extended features that myisamchk provides.

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 information.

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

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

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

Stage 2: Easy safe repair

First, try myisamchk -r -q tbl_name (-r -q means “quick recovery mode”). This attempts to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work, and the table is fixed. Start repairing the next table. Otherwise, use the following procedure:

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

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

Stage 3: Difficult repair

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

Go back to Stage 2. myisamchk -r -q should work. (This should not be an endless loop.)

You can also use the REPAIR TABLE tbl_name USE_FRM SQL statement, which performs the whole procedure automatically. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use REPAIR TABLE. See Section 13.5.2.6, “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:

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

shell> myisamchk -r tbl_name

You can optimize a table in the same way by using the OPTIMIZE TABLE SQL statement. OPTIMIZE TABLE does a table repair and a key analysis, and also sorts the index tree so that key lookups are faster. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use OPTIMIZE TABLE. See Section 13.5.2.5, “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.

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.

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 and repair MyISAM tables is with the CHECK TABLE and REPAIR TABLE statements. See Section 13.5.2.3, “CHECK TABLE Syntax”, and Section 13.5.2.6, “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 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 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 crontab file:

35 0 * * 0 /path/to/myisamchk --fast --silent /path/to/datadir/*/*.MYI

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 MyISAM tables with dynamic-sized rows (tables with VARCHAR, BLOB, or TEXT columns) or have tables with many deleted rows you may want to defragment/reclaim space from the tables from time to time. You can do this by using OPTIMIZE TABLE on the tables in question. Alternatively, if you can stop the mysqld server for a while, change location into the data directory and use this command while the server is stopped:

shell> myisamchk -r -s --sort-index --sort_buffer_size=16M */*.MYI