8.3. myisamchk — MyISAM Table-Maintenance Utility

MySQL 5.0

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8.3. myisamchk — MyISAM Table-Maintenance Utility

The myisamchk utility gets information about your database tables or checks, repairs, or optimizes them. myisamchk works with MyISAM tables (tables that have .MYD and .MYI files for storing data and indexes).

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 tables is:

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

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

shell> myisamchk --silent --force --fast --update-state \ --key_buffer_size=64M --sort_buffer_size=64M \ --read_buffer_size=1M --write_buffer_size=1M \ /path/to/datadir/*/*.MYI

This command assumes that you have more than 64MB free. For more information about memory allocation with myisamchk, see Section 8.3.5, “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.

8.3.1. myisamchk General Options

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 about what the program does. 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 external locking disabled, the table can be locked only by another myisamchk command.

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

Variable Default Value

decode_bits 9

ft_max_word_len version-dependent

ft_min_word_len 4

ft_stopword_file built-in list

key_buffer_size 523264

myisam_block_size 1024

read_buffer_size 262136

sort_buffer_size 2097144

sort_key_blocks 16

stats_method nulls_unequal

write_buffer_size 262136

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.

stats_method influences how NULL values are treated for index statistics collection when the --analyze option is given. It acts like the myisam_stats_method system variable. For more information, see the description of myisam_stats_method in Section 5.2.2, “Server System Variables”, and Section 7.4.7, “MyISAM Index Statistics Collection”. For MySQL 5.0, stats_method was added in MySQL 5.0.14. For older versions, the statistics collection method is equivalent to nulls_equal.

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.

8.3.2. myisamchk Check Options

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 helps 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 --recover 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 external locking disabled.

--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 external locking disabled.

8.3.3. myisamchk Repair Options

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 5.11.1, “The Character Set Used for Data and Sorting”.

--correct-checksum

Correct the checksum information for the table.

--data-file-length=len, -D len

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

--extend-check, -e

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

--force, -f

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

--keys-used=val, -k val

For myisamchk, the option value is a bit-value that indicates which indexes to update. Each binary bit of the option value corresponds to a table index, where the first index is bit 0. 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.

--max-record-length=len

Skip rows larger than the given length if myisamchk cannot allocate memory to hold them.

--parallel-recover, -p

Uses the same technique as -r and -n, but creates all the keys in parallel, using different threads. This is beta-quality 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 MyISAM tables). If you want to recover a table, this is the option to try first. You should try --safe-recover only if myisamchk reports that the table can't be recovered using --recover. (In the unlikely case that --recover fails, the data file remains 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 --recover, but can handle a couple of very unlikely cases that --recover cannot. This recovery method also uses much less disk space than --recover. Normally, you should repair first with --recover, and then with --safe-recover only if --recover 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. This option was replaced by --set-collation in MySQL 5.0.3.

--set-collation=name

Specify the collation to use for sorting table indexes. The character set name is implied by the first part of the collation name. This option was added in MySQL 5.0.3.

--sort-recover, -n

Force myisamchk to use sorting to resolve the keys even if the temporary files would be very large.

--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. tmpdir can be set to a list of directory paths that are used successively in round-robin fashion for creating temporary files. The separator character between directory names is the colon (‘:’) on Unix and the semicolon (‘;’) on Windows, NetWare, and OS/2.

--unpack, -u

Unpack a table that was packed with myisampack.

8.3.4. Other myisamchk Options

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

--analyze, -a

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

--block-search=offset, -b offset

Find the record that a block at the given offset belongs to.

--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 existing records with AUTO_INCREMENT values this large). If value is not specified, AUTO_INCREMENT numbers for new records begin 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 scans that use indexes faster.

--sort-records=N, -R N

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 INDEX, which displays a table's indexes in the same order that myisamchk sees them. Indexes are numbered beginning with 1.

If keys are not packed (PACK_KEYS=0)), they have the same length, so when myisamchk sorts and moves records, it just overwrites record offsets in the index. If keys are packed (PACK_KEYS=1), myisamchk must unpack key blocks first, then re-create indexes and pack the key blocks again. (In this case, re-creating indexes is faster than updating offsets for each index.)

8.3.5. myisamchk Memory Usage

Memory allocation is important when you run myisamchk. myisamchk uses no more memory than its memory-related variables are set to. 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 --sort_buffer_size=16M --key_buffer_size=16M \ --read_buffer_size=1M --write_buffer_size=1M ...

Using --sort_buffer_size=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, run myisamchk with the --tmpdir=path option to specify some directory located on a filesystem that has more space.

When repairing, myisamchk also needs a lot of disk space:

Double the size of the data file (the original file 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 need space for a sort buffer. The following formula yields the amount of space required:

(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 --safe-recover instead of --recover.

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8.3. myisamchk — MyISAM Table-Maintenance Utility

MySQL 5.0