Disaster Prevention and Recovery with the MySQL Database

If you need to administer MySQL, this article gets you off to a good start. In this section, we discuss the issues of disaster prevention and recovery. The fifth of a multi-part series, it is excerpted from chapter four of the book MySQL Administrator’s Guide, written by Paul Dubois (Sams; ISBN: 0672326345).

4.6 Disaster Prevention and Recovery

This section discusses how to make database backups and how to perform table maintenance. The syntax of the SQL statements described here is given in the MySQL Language Reference. Much of the information here pertains primarily to MyISAM tables. InnoDB backup procedures are given in Section 9.9, “Backing Up and Recovering an InnoDB Database.”

4.6.1 Database Backups

Because MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a LOCK TABLES on the relevant tables, followed by FLUSH TABLES for the tables. 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 FLUSH TABLES statement is needed to ensure that the all active index pages are written to disk before you start the backup.

If you want to make an SQL-level backup of a table, you can use SELECT INTO ... OUTFILE or BACKUP TABLE. For SELECT INTO ... OUTFILE, the output file cannot already exist. For BACKUP TABLE, the same is true as of MySQL 3.23.56 and 4.0.12, because this would be a security risk.

Another way to back up a database is to use the mysqldump program or the mysqlhotcopy script. See Section 7.8, “The mysqldump Database Backup Program,” and Section 7.9, “The mysqlhotcopy Database Backup Program.”

Do a full backup of your database:

shell> mysqldump --tab=/path/to/some/dir

–opt db_name

  1. Or:

shell> mysqlhotcopy db_name /path/to/some/dir
  1. You can also simply copy all table files (*.frm, *.MYD, and *.MYI files) as long as the server isn’t updating anything. The mysqlhotcopy script uses this method. (But note that these methods will not work if your database contains InnoDB tables. InnoDB does not store table contents in database directories, and mysqlhotcopy works only for MyISAM and ISAM tables.)

  2. Stop mysqld if it’s running, then start it with the --log-bin[=file_name] option. See Section 4.8.4, “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.

If your MySQL server is a slave replication server, then regardless of the backup method you choose, you should also back up the master.info and 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 --slave-load-tmpdir 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 will work only if you have enabled binary logging by starting MySQL with the --log-bin option; see Section 4.8.4, “The Binary Log.”

  1. Restore the original mysqldump backup, or binary backup.

  2. Execute the following command to re-run the updates in the binary logs:

shell> mysqlbinlog hostname-bin.[0-9]* |
mysql
  1. In your case, 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 queries). See Section 7.5, “The mysqlbinlog Binary Log Utility,” for more information on the mysqlbinlog utility and how to use it.

    If you are using the update logs instead, you can process their contents like this:

shell> ls -1 -t -r hostname.[0-9]* | xargs
cat | mysql
  1. ls is used to sort the update log filenames into the right order.

You can also do selective backups of individual files:

  • To dump the table, use SELECT * INTO OUTFILE 'file_name' FROM 'tbl_name.

  • To reload the table, use and restore with LOAD DATA INFILE 'file_name' REPLACE ... To avoid duplicate records, the table must have a PRIMARY KEY or a UNIQUE index. The REPLACE keyword causes old records to be replaced with new ones when a new record duplicates an old record 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 5.1, “Introduction to Replication.”

If you are using a Veritas filesystem, you can make a backup like this:

  1. From a client program, execute FLUSH TABLES WITH READ LOCK.

  2. From another shell, execute mount vxfs snapshot.

  3. From the first client, execute UNLOCK TABLES.

  4. Copy files from the snapshot.

  5. Unmount the snapshot.

4.6.2 Table Maintenance and Crash Recovery

The following text discusses how to use myisamchk to check or repair MyISAM tables (tables with .MYI and .MYD files). The same concepts apply to using isamchk to check or repair ISAM tables (tables with .ISM and .ISD files). See Chapter 8, “MySQL Storage Engines and Table Types.”

You can use the myisamchk utility to get information about your database tables or to check, repair, or optimize them. The following sections describe how to invoke myisamchk (including a description of its options), how to set up a table maintenance schedule, and how to use myisamchk to perform its various functions.

Even though table repair with myisamchk is quite secure, it’s 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 cause FULLTEXT indexes to be rebuilt with full-text parameters that are incompatible with the values used by the MySQL server. To avoid this, read the instructions in Section 4.6.2.2, “General Options for myisamchk.”

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 REPAIR TABLE.

  • To optimize MyISAM tables, use OPTIMIZE TABLE.

  • To analyze MyISAM tables, use ANALYZE TABLE.

These statements were introduced in different versions, but all are available from MySQL 3.23.14 on. The statements can be used directly, or by means of the mysqlcheck client program, which provides a command-line interface to them.

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. Otherwise, there can be unwanted interaction betweeen myisamchk and the server.

4.6.2.1 myisamchk Invocation Syntax

Invoke myisamchk like this:

shell> myisamchk [options] tbl_name

The options specify what you want myisamchk to do. They are described in the following sections. You can also get a list of options by invoking myisamchk --help.

With no options, myisamchk simply checks your table as the default operation. To get more information or to tell myisamchk to take corrective action, specify options as described in the following discussion.

tbl_name is the database table you want to check or repair. If you run myisamchk somewhere other than in the database directory, you must specify the path to the database directory, because myisamchk has no idea where the database is located. In fact, myisamchk doesn’t actually care whether the files you are working on are located in a database directory. You can copy the files that correspond to a database table into some other location and perform recovery operations on them there.

You can name several tables on the myisamchk command line if you wish. You can also specify a table by naming its index file (the file with the .MYI suffix). This allows you to specify all tables in a directory by using the pattern *.MYI. For example, if you are in a database directory, you can check all the MyISAM tables in that directory like this:

shell> myisamchk *.MYI

If you are not in the database directory, you can check all the tables there by specifying the path to the directory:

shell> myisamchk /path/to/database_dir/*.MYI

You can even check all tables in all databases by specifying a wildcard with the path to the MySQL data directory:

shell> myisamchk /path/to/datadir/*/*.MYI

The recommended way to quickly check all MyISAM and ISAM tables is:

shell> myisamchk --silent --fast
/path/to/datadir/*/*.MYI shell> isamchk --silent
/path/to/datadir/*/*.ISM

If you want to check all MyISAM and ISAM tables and repair any that are corrupted, you can use the following commands:

shell> myisamchk --silent --force --fast
--update-state
-O key_buffer=64M -O sort_buffer=64M
-O read_buffer=1M -O write_buffer=1M
/path/to/datadir/*/*.MYI shell> isamchk --silent --force -O
key_buffer=64M
-O sort_buffer=64M -O read_buffer=1M
-O write_buffer=1M
/path/to/datadir/*/*.ISM

These commands assume that you have more than 64MB free. For more information about memory allocation with myisamchk, see Section 4.6.2.6, “myisamchk Memory Usage.”

You must ensure that no other program is using the tables while you are running myisamchk. Otherwise, when you run myisamchk, it may display the following error message:

warning: clients are using or haven't closed
the table properly

This means that you are trying to check a table that has been updated by another program (such as the mysqld server) that hasn’t yet closed the file or that has died without closing the file properly.

If mysqld is running, you must force it to flush any table modifications that are still buffered in memory by using FLUSH TABLES. You should then ensure that no one is using the tables while you are running myisamchk. The easiest way to avoid this problem is to use CHECK TABLE instead of myisamchk to check tables.

{mospagebreak title=4.6.2.2 General Options for myisamchk}

The options described in this section can be used for any type of table maintenance operation performed by myisamchk. The sections following this one describe options that pertain only to specific operations, such as table checking or repairing.

  • --help, -?

    Display a help message and exit.

  • --debug=debug_options, -# debug_options

    Write a debugging log. The debug_options string often is 'd:t:o,file_name'.

  • --silent, -s

    Silent mode. Write output only when errors occur. You can use -s twice (-ss) to make myisamchk very silent.

  • --verbose, -v

    Verbose mode. Print more information. This can be used with -d and -e. Use -v multiple times (-vv, -vvv) for even more output.

  • --version, -V

    Display version information and exit.

  • --wait, -w

    Instead of terminating with an error if the table is locked, wait until the table is unlocked before continuing. Note that if you are running mysqld with the --skip-external-locking option, the table can be locked only by another myisamchk command.

You can also set the following variables by using --var_name=value options:

Variable

Default Value

decode_bits

9

ft_max_word_len

version-dependent

ft_min_word_len

4

key_buffer_size

523264

myisam_block_size

1024

read_buffer_size

262136

sort_buffer_size

2097144

sort_key_blocks

16

write_buffer_size

262136


It is also possible to set variables by using --set-variable=var_name=value or -O var_name=value syntax. However, this syntax is deprecated as of MySQL 4.0.

The possible myisamchk variables and their default values can be examined with myisamchk --help:

sort_buffer_size is used when the keys are repaired by sorting keys, which is the normal case when you use --recover.

key_buffer_size is used when you are checking the table with --extend-check or when the keys are repaired by inserting keys row by row into the table (like when doing normal inserts). Repairing through the key buffer is used in the following cases:

  • You use --safe-recover.

  • The temporary files needed to sort the keys would be more than twice as big as when creating the key file directly. This is often the case when you have large key values for CHAR, VARCHAR, or TEXT columns, because the sort operation needs to store the complete key values as it proceeds. If you have lots of temporary space and you can force myisamchk to repair by sorting, you can use the --sort-recover option.

Repairing through the key buffer takes much less disk space than using sorting, but is also much slower.

If you want a faster repair, set the key_buffer_size and sort_buffer_size variables to about 25% of your available memory. You can set both variables to large values, because only one of them is used at a time.

myisam_block_size is the size used for index blocks. It is available as of MySQL 4.0.0.

The ft_min_word_len and ft_max_word_len variables are available as of MySQL 4.0.0. ft_stopword_file is available as of MySQL 4.0.19.

ft_min_word_len and ft_max_word_len indicate the minimum and maximum word length for FULLTEXT indexes. ft_stopword_file names the stopword file. These need to be set under the following circumstances.

If you use myisamchk to perform an operation that modifies table indexes (such as repair or analyze), the FULLTEXT indexes are rebuilt using the default full-text parameter values for minimum and maximum word length and the stopword file unless you specify otherwise. This can result in queries failing.

The problem occurs because these parameters are known only by the server. They are not stored in MyISAM index files. To avoid the problem if you have modified the minimum or maximum word length or the stopword file in the server, specify the same ft_min_word_len, ft_max_word_len, and ft_stopword_file values to myisamchk that you use for mysqld. For example, if you have set the minimum word length to 3, you can repair a table with myisamchk like this:

shell> myisamchk --recover
--ft_min_word_len=3
tbl_name.MYI

To ensure that myisamchk and the server use the same values for full-text parameters, you can place each one in both the [mysqld] and [myisamchk] sections of an option file:

[mysqld]
ft_min_word_len=3

[myisamchk]
ft_min_word_len=3

An alternative to using myisamchk is to use the REPAIR TABLE, ANALYZE TABLE, OPTIMIZE TABLE, or ALTER TABLE. These statements are performed by the server, which knows the proper full-text parameter values to use.

4.6.2.3 Check Options for myisamchk

myisamchk supports the following options for table checking operations:

  • --check, -c

    Check the table for errors. This is the default operation if you specify no option that selects an operation type explicitly.

  • --check-only-changed, -C

    Check only tables that have changed since the last check.

  • --extend-check, -e

    Check the table very thoroughly. This is quite slow if the table has many indexes. This option should only be used in extreme cases. Normally, myisamchk or myisamchk --medium-check should be able to determine whether there are any errors in the table.

    If you are using --extend-check and have plenty of memory, setting the key_buffer_size variable to a large value will help the repair operation run faster.

  • --fast, -F

    Check only tables that haven’t been closed properly.

  • --force, -f

    Do a repair operation automatically if myisamchk finds any errors in the table. The repair type is the same as that specified with the --repair or -r option.

  • --information, -i

    Print informational statistics about the table that is checked.

  • --medium-check, -m

    Do a check that is faster than an --extend-check operation. This finds only 99.99% of all errors, which should be good enough in most cases.

  • --read-only, -T

    Don’t mark the table as checked. This is useful if you use myisamchk to check a table that is in use by some other application that doesn’t use locking, such as mysqld when run with the --skip-external-locking option.

  • --update-state, -U

    Store information in the .MYI file to indicate when the table was checked and whether the table crashed. This should be used to get full benefit of the --check-only-changed option, but you shouldn’t use this option if the mysqld server is using the table and you are running it with the --skip-external-locking option.

{mospagebreak title=4.6.2.4 Repair Options for myisamchk}

myisamchk supports the following options for table repair operations:

  • --backup, -B

    Make a backup of the .MYD file as file_name-time.BAK

  • --character-sets-dir=path

    The directory where character sets are installed. See Section 4.7.1, “The Character Set Used for Data and Sorting.”

  • --correct-checksum

    Correct the checksum information for the table.

  • --data-file-length=#, -D #

    Maximum length of the data file (when re-creating data file when it’s “full”).

  • --extend-check, -e

    Do a repair that tries to recover every possible row from the data file. Normally this will also find a lot of garbage rows. Don’t use this option unless you are totally desperate.

  • --force, -f

    Overwrite old temporary files (files with names like tbl_name.TMD) instead of aborting.

  • --keys-used=#, -k #

    For myisamchk, the option value indicates which indexes to update. Each binary bit of the option value corresponds to a table index, where the first index is bit 0. For isamchk, the option value indicates that only the first # of the table indexes should be updated. In either case, an option value of 0 disables updates to all indexes, which can be used to get faster inserts. Deactivated indexes can be reactivated by using myisamchk -r or (isamchk -r).

  • --no-symlinks, -l

    Do not follow symbolic links. Normally myisamchk repairs the table that a symlink points to. This option doesn’t exist as of MySQL 4.0, because versions from 4.0 on will not remove symlinks during repair operations.

  • --parallel-recover, -p

    Uses the same technique as -r and -n, but creates all the keys in parallel, using different threads. This option was added in MySQL 4.0.2. This is alpha code. Use at your own risk!

  • --quick, -q

    Achieve a faster repair by not modifying the data file. You can specify this option twice to force myisamchk to modify the original data file in case of duplicate keys.

  • --recover, -r

    Do a repair that can fix almost any problem except unique keys that aren’t unique (which is an extremely unlikely error with ISAM/MyISAM tables). If you want to recover a table, this is the option to try first. You should try -o only if myisamchk reports that the table can’t be recovered by -r. (In the unlikely case that -r fails, the data file is still intact.)

    If you have lots of memory, you should increase the value of sort_buffer_size.

  • --safe-recover, -o

    Do a repair using an old recovery method that reads through all rows in order and updates all index trees based on the rows found. This is an order of magnitude slower than -r, but can handle a couple of very unlikely cases that -r cannot. This recovery method also uses much less disk space than -r. Normally, you should repair first with -r, and then with -o only if -r fails.

    If you have lots of memory, you should increase the value of key_buffer_size.

  • --set-character-set=name

    Change the character set used by the table indexes.

  • --sort-recover, -n

    Force myisamchk to use sorting to resolve the keys even if the temporary files should not be very big.

  • --tmpdir=path, -t path

    Path of the directory to be used for storing temporary files. If this is not set, myisamchk uses the value of the TMPDIR environment variable. Starting from MySQL 4.1, tmpdir can be set to a list of directory paths that will be used successively in round-robin fashion for creating temporary files. The separator character between directory names should be colon (‘:‘) on Unix and semicolon (‘;‘) on Windows, NetWare, and OS/2.

  • --unpack, -u

    Unpack a table that was packed with myisampack.

4.6.2.5 Other Options for myisamchk

myisamchk supports the following options for actions other than table checks and repairs:

  • --analyze, -a

    Analyze the distribution of keys. This improves join performance by enabling the join optimizer to better choose the order in which to join the tables and which keys it should use. To obtain information about the distribution, use a myisamchk --description --verbose tbl_name command or the SHOW KEYS FROM tbl_name statement.

  • --description, -d

    Print some descriptive information about the table.

  • --set-auto-increment[=value], -A[value]

    Force AUTO_INCREMENT numbering for new records to start at the given value (or higher, if there are already records with AUTO_INCREMENT values this large). If value is not specified, the AUTO_INCREMENT number for new records begins with the largest value currently in the table, plus one.

  • --sort-index, -S

    Sort the index tree blocks in high-low order. This optimizes seeks and makes table scanning by key faster.

  • --sort-records=#, -R #

    Sort records according to a particular index. This makes your data much more localized and may speed up range-based SELECT and ORDER BY operations that use this index. (The first time you use this option to sort a table, it may be very slow.) To determine a table’s index numbers, use SHOW KEYS, which displays a table’s indexes in the same order that myisamchk sees them. Indexes are numbered beginning with 1.

4.6.2.6 myisamchk Memory Usage

Memory allocation is important when you run myisamchk. myisamchk uses no more memory than you specify with the -O options. If you are going to use myisamchk on very large tables, you should first decide how much memory you want it to use. The default is to use only about 3MB to perform repairs. By using larger values, you can get myisamchk to operate faster. For example, if you have more than 32MB RAM, you could use options such as these (in addition to any other options you might specify):

shell> myisamchk -O sort=16M -O key=16M -O
read=1M -O write=1M ...

Using -O sort=16M should probably be enough for most cases.

Be aware that myisamchk uses temporary files in TMPDIR. If TMPDIR points to a memory filesystem, you may easily get out of memory errors. If this happens, set TMPDIR to point at some directory located on a filesystem with more space and run myisamchk again.

When repairing, myisamchk will also need a lot of disk space:

  • Double the size of the data file (the original one and a copy). This space is not needed if you do a repair with --quick; in this case, only the index file is re-created. This space is needed on the same filesystem as the original data file! (The copy is created in the same directory as the original.)

  • Space for the new index file that replaces the old one. The old index file is truncated at the start of the repair operation, so you usually ignore this space. This space is needed on the same filesystem as the original index file!

  • When using --recover or --sort-recover (but not when using --safe-recover), you will need space for a sort buffer. The amount of space required is:

    (largest_key + row_pointer_length) *
    number_of_rows * 2
  • You can check the length of the keys and the row_pointer_length with myisamchk -dv tbl_name. This space is allocated in the temporary directory (specified by TMPDIR or --tmpdir=path).

If you have a problem with disk space during repair, you can try to use --safe-recover instead of --recover.

{mospagebreak title=4.6.2.7 Using myisamchk for Crash Recovery}

If you run mysqld with --skip-external-locking (which is the default on some systems, such as Linux), you can’t reliably use myisamchk to check a table when mysqld is using the same table. If you can be sure that no one is accessing the tables through mysqld while you run myisamchk, you only have to do mysqladmin flush-tables before you start checking the tables. If you can’t guarantee this, then you must stop mysqld while you check the tables. If you run myisamchk while mysqld is updating the tables, you may get a warning that a table is corrupt even when it isn’t.

If you are not using --skip-external-locking, you can use myisamchk to check tables at any time. While you do this, all clients that try to update the table will wait until myisamchk is ready before continuing.

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 you are using --skip-external-locking). If you don’t take down 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.

This section describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted frequently you should try to find the reason why. See Section A.4.2, “What to Do If MySQL Keeps Crashing.”

The MyISAM table section contains reasons for why a table could be corrupted. See Section 8.1.4, “MyISAM Table Problems.”

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:

File

Purpose

tbl_name.frm

Definition (format) file

tbl_name.MYD

Data file

tbl_name.MYI

Index 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 .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 only generates 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 before running myisamchk.

4.6.2.8 How to Check MyISAM Tables for Errors

To check a MyISAM table, use the following commands:

  • myisamchk tbl_name

    This finds 99.99% of all errors. What it can’t 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 either the -s or --silent option.

  • myisamchk -m tbl_name

    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 keys in the rows and verifies that the checksum matches the checksum for the keys in the index tree.

  • myisamchk -e tbl_name

    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 --verbose (-v) option. This causes myisamchk to keep going, up through a maximum of 20 errors.

  • myisamchk -e -i tbl_name

    Like the previous command, but the -i option tells myisamchk to print some informational statistics, too.

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

{mospagebreak title=4.6.2.9 How to Repair Tables}

The discussion in this section describes how to use myisamchk on MyISAM tables (extensions .MYI and .MYD). If you are using ISAM tables (extensions .ISM and .ISD), you should use isamchk instead; the concepts are similar.

If you are using MySQL 3.23.16 and above, you can (and should) use the CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables.

The symptoms of a corrupted table include queries that abort unexpectedly and observable errors such as these:

  • tbl_name.frm is locked against change

  • Can’t find file tbl_name.MYI (Errcode: ###)

  • Unexpected end of file

  • Record file is crashed

  • Got error ### from table handler

To get more information about the error you can run perror ###, where ### 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 have to 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 don’t know the current table option values, use SHOW CREATE TABLE tbl_name.

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.

The options that you can use for table maintenance with myisamchk and isamchk are described in several of the earlier subsections of Section 4.6.2, “Table Maintenance and Crash Recovery.”

The following section is for the cases where the above command fails or if you want to use the extended features that myisamchk and isamchk provide.

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 will still be alive for a while after mysqladmin returns, until all queries are stopped and all keys 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 down, 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 weird errors when checking (such as out of memory errors), or if myisamchk crashes, go to Stage 3.

Stage 2: Easy safe repair

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 or isamchk.

First, try myisamchk -r -q tbl_name (-r -q means “quick recovery mode”). This will attempt 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:

  1. Make a backup of the data file before continuing.

  2. Use myisamchk -r tbl_name (-r means “recovery mode”). This will remove incorrect records and deleted records from the data file and reconstruct the index file.

  3. If the preceding step fails, use myisamchk --safe-recover tbl_name. Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode doesn’t (but is slower).

If you get weird 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’s necessary to create a new index file. Do so as follows:

  1. Move the data file to some safe place.

  2. Use the table description file to create new (empty) data and index files:

    shell> mysql db_name
    mysql> SET AUTOCOMMIT=1;
    mysql> TRUNCATE TABLE tbl_name;
    mysql> quit
  3. If your version of MySQL doesn’t have TRUNCATE TABLE, use DELETE FROM tbl_name instead.

  4. Copy the old data file back onto the newly created data file. (Don’t 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 now. (This shouldn’t be an endless loop.)

As of MySQL 4.0.2, you can also use REPAIR TABLE tbl_name USE_FRM, which performs the whole procedure automatically.

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 isn’t changed after the table is created:

  1. 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.

  2. If you don’t 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, 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.

4.6.2.10 Table Optimization

To coalesce fragmented records and eliminate wasted space resulting from deleting or updating records, run myisamchk in recovery mode:

shell> myisamchk -r tbl_name

You can optimize a table in the same way by using the SQL OPTIMIZE TABLE statement. OPTIMIZE TABLE does a repair of the table and a key analysis, and also sorts the index tree to give faster key lookups. 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.

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

  • -S, --sort-index

  • -R index_num, --sort-records=index_num

  • -a, --analyze

For a full description of the options, see Section 4.6.2.1, “myisamchk Invocation Syntax.”

{mospagebreak title=4.6.3 Setting Up a Table Maintenance Schedule}

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. These are available starting with MySQL 3.23.16.

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’s also a good idea to check tables when the server starts. For example, whenever the machine has done a restart in the middle of an update, you usually need to check all the tables that could have been affected. (These are “expected crashed tables.”) To check MyISAM tables automatically, start the server with the --myisam-recover option, available as of MySQL 3.23.25. If your server is too old to support this option, you could add a test to mysqld_safe that runs myisamchk to check all tables that have been modified during the last 24 hours if there is an old .pid (process ID) file left after a restart. (The .pid file is created by mysqld when it starts and removed when it terminates normally. The presence of a .pid file at system startup time indicates that mysqld terminated abnormally.)

An even better test would be to check any table whose last-modified time is more recent than that of the .pid file.

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 haven’t had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough 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 changing MyISAM tables with dynamic size 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 (once a month?).

You can do this by using OPTIMIZE TABLE on the tables in question. Or, 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 -O
sort_buffer_size=16M */*.MYI

For ISAM tables, the command is similar:

shell> isamchk -r -s --sort-index -O
sort_buffer_size=16M */*.ISM

{mospagebreak title=4.6.4 Getting Information About a Table}

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:

  • myisamchk -d tbl_name

    Runs myisamchk in “describe mode” to produce a description of your table. If you start the MySQL server using the --skip-external-locking option, myisamchk may report an error for a table that is updated while it runs. However, because myisamchk doesn’t change the table in describe mode, there is no risk of destroying data.

  • myisamchk -d -v tbl_name

    Adding -v runs myisamchk in verbose mode so that it produces more information about what it is doing.

  • myisamchk -eis tbl_name

    Shows only the most important information from a table. This operation is slow because it must read the entire table.

  • myisamchk -eiv tbl_name

    This is like -eis, but tells you what is being done.

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.MYM

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.

  • MyISAM file

    Name of the MyISAM (index) file.

  • File-version

    Version of MyISAM format. Currently always 2.

  • Creation time

    When the data file was created.

  • Recover time

    When the index/data file was last reconstructed.

  • Data records

    How many records are in the table.

  • Deleted blocks

    How many deleted blocks still have reserved space. You can optimize your table to minimize this space. See Section 4.6.2.10, “Table Optimization.”

  • Datafile parts

    For dynamic record format, this indicates how many data blocks there are. For an optimized table without fragmented records, this is the same as Data records.

  • Deleted data

    How many bytes of unreclaimed deleted data there are. You can optimize your table to minimize this space. See Section 4.6.2.10, “Table Optimization.”

  • Datafile pointer

    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 record address. For dynamic tables, this is a byte address.

  • Keyfile pointer

    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.

  • Recordlength

    How much space each record takes, in bytes.

  • Record format

    The format used to store table rows. The preceding examples use Fixed length. Other possible values are Compressed and Packed.

  • table description

    A list of all keys in the table. For each key, myisamchk displays some low-level information:

  • Key

    This key’s number.

  • Start

    Where in the record this index part starts.

  • Len

    How long this index part 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.

  • Index

    Whether a key value can exist multiple times in the index. Values are unique or multip. (multiple).

  • Type

    What data type this index part has. This is a MyISAM data type with the options packed, stripped, or empty.

  • Root

    Address of the root index block.

  • Blocksize

    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.

  • Rec/key

    This is a statistical value used by the optimizer. It tells how many records there are per value for this key. A unique key 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.

  • Keyblocks used

    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).

  • Packed

    MySQL tries to pack keys with a common suffix. This can only be used for indexes on CHAR, VARCHAR, or DECIMAL columns. For long indexed strings that have similar leftmost parts, this can significantly reduce the space used. In the third example above, the fourth key is 10 characters long and a 60% reduction in space is achieved.

  • Max levels

    How deep the B-tree for this key is. Large tables with long key values get high values.

  • Records

    How many rows are in the table.

  • M.recordlength

    The average record length. This is the exact record length for tables with fixed-length records, because all records have the same length.

  • Packed

    MySQL strips spaces from the end of strings. The Packed value indicates the percentage of savings achieved by doing this.

  • Recordspace used

    What percentage of the data file is used.

  • Empty space

    What percentage of the data file is unused.

  • Blocks/Record

    Average number of blocks per record (that is, how many links a fragmented record 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 big, you can reorganize the table.

  • Recordblocks

    How many blocks (links) are used. For fixed format, this is the same as the number of records.

  • Deleteblocks

    How many blocks (links) are deleted.

  • Recorddata

    How many bytes in the data file are used.

  • Deleted data

    How many bytes in the data file are deleted (unused).

  • Lost space

    If a record is updated to a shorter length, some space is lost. This is the sum of all such losses, in bytes.

  • Linkdata

    When the dynamic table format is used, record 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 7.2, “myisampack, the MySQL Compressed Read-Only Table Generator,” for an example of this information and a description of what it means.

Please check back next week for the continuation of this article.

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