MYISAMCHK(1)

NAME
myisamchk - MyISAM table-maintenance utility

SYNOPSIS

myisamchk [options] tbl_name ...

DESCRIPTION

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

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 \
          -O key_buffer=64M -O sort_buffer=64M \
          -O read_buffer=1M -O write_buffer=1M \
          /path/to/datadir/*/*.MYI

This command assumes that you have more than 64MB free. For more information about memory allocation with myisamchk, see the section called “\FBMYISAMCHK\FR 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.

GENERAL OPTIONS FOR \FBMYISAMCHK\FR

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: VariableDefault Valuedecode_bits9ft_max_word_lenversion-dependentft_min_word_len4ft_stopword_filebuilt-in listkey_buffer_size523264myisam_block_size1024read_buffer_size262136sort_buffer_size2097144sort_key_blocks16stats_methodnulls_unequalwrite_buffer_size262136.PP 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 the section called “SERVER SYSTEM VARIABLES” and Section 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.

CHECK OPTIONS FOR \FBMYISAMCHK\FR

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 --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.

REPAIR OPTIONS FOR \FBMYISAMCHK\FR

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 7.1, “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's “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 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.

•   --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 do 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 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 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 using -r. (In the unlikely case that -r 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 -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. This option was replaced by --set-collation in MySQL 5.0.3.

•   --set-collation=name

    Change the collation used to sort 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.

OTHER OPTIONS FOR \FBMYISAMCHK\FR

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 existing records with AUTO_INCREMENT values this large). If value is not specified, 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=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 KEYS, 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 recreate indexes and pack the key blocks again. (In this case, recreating indexes is faster than updating offsets for each index.)

\FBMYISAMCHK\FR 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 also needs 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 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.

USING \FBMYISAMCHK\FR 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 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 4.2, “What to Do If MySQL Keeps Crashing”.

For an explanation of how MyISAM tables can become corrupted, see Section 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: FilePurposetbl_name.frmDefinition (format) filetbl_name.MYDData filetbl_name.MYIIndex file.PP 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.

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.

HOW TO REPAIR TABLES

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 5.2.3, “CHECK TABLE Syntax” and Section 5.2.6, “REPAIR TABLE Syntax”.

Symptoms of corrupted tables 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: nnn)

•   Unexpected end of file

•   Record file is crashed

•   Got error nnn from table handler

To get more information about the error you can run perrornnn, 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 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 or DESCRIBE.

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 are described in several of the earlier subsections of myisamchk(1).

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

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 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.

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:

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

2.  Use myisamchk -r tbl_name (-r means “recovery mode”). This removes incorrect records and deleted records from the data file and reconstructs 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
    If your version of MySQL doesn't have TRUNCATE TABLE, use DELETE FROM tbl_name instead.

3.  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. (This shouldn't be an endless loop.)

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.

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. See Section 5.2.5, “OPTIMIZE TABLE Syntax”.

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 these options, see myisamchk(1).

SEE ALSO
msql2mysql(1), myisamlog(1), myisampack(1), mysql(1), mysql.server(1), mysql_config(1), mysql_fix_privilege_tables(1), mysql_zap(1), mysqlaccess(1), mysqladmin(1), mysqlbinlog(1), mysqlcheck(1), mysqld(1), mysqld_multi(1), mysqld_safe(1), mysqldump(1), mysqlhotcopy(1), mysqlimport(1), mysqlmanager(1), mysqlshow(1), perror(1), replace(1), safe_mysqld(1)

For more information, please refer to the MySQL Reference Manual, which may already be installed locally and which is also available online at http://dev.mysql.com/doc/.

AUTHOR
MySQL AB (http://www.mysql.com/). This software comes with no warranty.