MYISAMCHK(1) MySQL Database System MYISAMCHK(1)
myisamchk - MyISAM table-maintenance utility
myisamchk [options] tbl_name ...
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
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 doesnt 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
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 \
This command assumes that you have more than 64MB free. For more
information about memory allocation with myisamchk, see the section
called 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 havent 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 hasnt 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.
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.
Display a help message and exit.
--debug=debug_options, -# debug_options
Write a debugging log. The debug_options string often is
Silent mode. Write output only when errors occur. You can use -s
twice (-ss) to make myisamchk very silent.
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.
Display version information and exit.
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
|Variable | Default Value |
|sort_key_blocks | 16 |
|stats_method | nulls_unequal |
|write_buffer_size | 262136 |
|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 |
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
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 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:
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
MYISAMCHK CHECK OPTIONS
myisamchk supports the following options for table checking operations:
Check the table for errors. This is the default operation if you
specify no option that selects an operation type explicitly.
Check only tables that have changed since the last check.
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.
Check only tables that havent been closed properly.
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.
Print informational statistics about the table that is checked.
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
Dont 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
doesnt use locking, such as mysqld when run with external locking
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 shouldnt
use this option if the mysqld server is using the table and you are
running it with external locking disabled.
MYISAMCHK REPAIR OPTIONS
myisamchk supports the following options for table repair operations:
Make a backup of the .MYD file as file_name-time.BAK
The directory where character sets are installed. See Section 9.1,
The Character Set Used for Data and Sorting.
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
Do a repair that tries to recover every possible row from the data
file. Normally, this also finds a lot of garbage rows. Dont use
this option unless you are desperate.
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
Skip rows larger than the given length if myisamchk cannot allocate
memory to hold them.
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!
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.
Do a repair that can fix almost any problem except unique keys that
arent 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 cant 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
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
Change the character set used by the table indexes. This option was
replaced by --set-collation in MySQL 5.0.3.
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.
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 a table that was packed with myisampack.
OTHER MYISAMCHK OPTIONS
myisamchk supports the following options for actions other than table
checks and repairs:
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 FROM
--block-search=offset, -b offset
Find the record that a block at the given offset belongs to.
Print some descriptive information about the table.
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 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 tables index
numbers, use SHOW INDEX, which displays a tables indexes in the
same order that myisamchk sees them. Indexes are numbered beginning
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.)
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
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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MySQL 5.0 12/20/2006 MYISAMCHK(1)