MYISAMCHK(1) MySQL Database System 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 that have .MYD and .MYI files for storing data and indexes).
The use of myisamchk with partitioned tables is not supported.
Caution
It is best to make a backup of a table before performing a table repair operation; under some
circumstances the operation might cause data loss. Possible causes include but are not limited to file
system errors.
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 does not 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:
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, which
can sometimes lead to the corruption of one or more MyISAM tables.
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
However, the easiest way to avoid this problem is to use CHECK TABLE instead of myisamchk to check tables.
See Section 12.4.2.3, “CHECK TABLE Syntax”.
myisamchk supports the following options, which can be specified on the command line or in the [myisamchk]
option file group. myisamchk also supports the options for processing option files described at
Section 4.2.3.3.1, “Command-Line Options that Affect Option-File Handling”.
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. Options are grouped by type of operation.
· --HELP, -H
Display a help message and exit. Options are presented in a single list.
· --debug=debug_options, -# debug_options
Write a debugging log. A typical debug_options string is ´d:t:o,file_name´. The default is
´d:t:o,/tmp/myisamchk.trace´.
· --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
├──────────────────┼───────────────────┤
│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 5.1.4, “Server System Variables”, and Section 7.4.7, “MyISAM Index Statistics
Collection”.
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.
[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.
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.
For a description of the output format, see the section called “MYISAMCHK TABLE INFORMATION”.
· --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
Do not 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 does not use locking, such as mysqld when run with external locking disabled.
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 9.5, “Character Set Configuration”.
· --correct-checksum
Correct the checksum information for the table.
· --data-file-length=len, -D len
The 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. Do not use this option unless you are desperate.
For a description of the output format, see the section called “MYISAMCHK TABLE INFORMATION”.
· --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.
· --no-symlinks, -l
Do not follow symbolic links. Normally myisamchk repairs the table that a symlink points to. This option
does not exist as of MySQL 4.0 because versions from 4.0 on do not remove symlinks during repair
operations.
· --max-record-length=len
Skip rows larger than the given length if myisamchk cannot allocate memory to hold them.
· --parallel-recover, -p
Use the same technique as -r and -n, but create 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 modifying only the index file, not 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
--recover. Normally, you should repair first using --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.
· --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
The 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 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
FROM tbl_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. Specifying the --verbose option once or twice produces
additional information. See the section called “MYISAMCHK TABLE INFORMATION”.
· --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.
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 TABLE INFORMATION
To obtain a description of a MyISAM table or statistics about it, use the commands shown here. The output from
these commands is explained later in this section.
· myisamchk -d tbl_name
Runs myisamchk in “describe mode” to produce a description of your table. If you start the MySQL server
with external locking disabled, myisamchk may report an error for a table that is updated while it runs.
However, because myisamchk does not change the table in describe mode, there is no risk of destroying
data.
· myisamchk -dv tbl_name
Adding -v runs myisamchk in verbose mode so that it produces more information about the table. Adding -v a
second time produces even more information.
· 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.
The tbl_name argument can be either the name of a MyISAM table or the name of its index file, as described in
myisamchk(1). Multiple tbl_name arguments can be given.
Suppose that a table named person has the following structure. (The MAX_ROWS table option is included so that
in the example output from myisamchk shown later, some values are smaller and fit the output format more
easily.)
CREATE TABLE person
(
id INT NOT NULL AUTO_INCREMENT,
last_name VARCHAR(20) NOT NULL,
first_name VARCHAR(20) NOT NULL,
birth DATE,
death DATE,
PRIMARY KEY (id),
INDEX (last_name, first_name),
INDEX (birth)
) MAX_ROWS = 1000000;
Suppose also that the table has these data and index file sizes:
-rw-rw---- 1 mysql mysql 9347072 Aug 19 11:47 person.MYD
-rw-rw---- 1 mysql mysql 6066176 Aug 19 11:47 person.MYI
Datafile parts: 306688 Deleted data: 0
Datafile pointer (bytes): 4 Keyfile pointer (bytes): 3
Datafile length: 9347072 Keyfile length: 6066176
Max datafile length: 4294967294 Max keyfile length: 17179868159
Recordlength: 54
table description:
Key Start Len Index Type Rec/key Root Blocksize
1 2 4 unique long 1 99328 1024
2 6 20 multip. varchar prefix 512 3563520 1024
27 20 varchar 512
3 48 3 multip. uint24 NULL 306688 6065152 1024
Field Start Length Nullpos Nullbit Type
1 1 1
2 2 4 no zeros
3 6 21 varchar
4 27 21 varchar
5 48 3 1 1 no zeros
6 51 3 1 2 no zeros
Explanations for the types of information myisamchk produces are given here. “Keyfile” refers to the index
file. “Record” and “row” are synonymous, as are “field” and “column.”
The initial part of the table description contains these values:
· MyISAM file
Name of the MyISAM (index) file.
· Record format
The format used to store table rows. The preceding examples use Fixed length. Other possible values are
Compressed and Packed. (Packed corresponds to what SHOW TABLE STATUS reports as Dynamic.)
· Chararacter set
The table default character set.
· File-version
Version of MyISAM format. Currently always 1.
· Creation time
When the data file was created.
· Recover time
When the index/data file was last reconstructed.
· Status
Table status flags. Possible values are crashed, open, changed, analyzed, optimized keys, and sorted index
pages.
Section 6.6.4, “MyISAM Table Optimization”.
· Datafile parts
For dynamic-row format, this indicates how many data blocks there are. For an optimized table without
fragmented rows, 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 6.6.4, “MyISAM 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 row 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 row takes, in bytes.
The table description part of the output includes a list of all keys in the table. For each key, myisamchk
displays some low-level information:
· Key
This key´s number. This value is shown only for the first column of the key. If this value is missing, the
line corresponds to the second or later column of a multiple-column key. For the table shown in the
example, there are two table description lines for the second index. This indicates that it is a
multiple-part index with two parts.
· Start
Where in the row this portion of the index starts.
· Len
How long this portion of the index 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. The total length of a multiple-part key is the sum of the Len values for all
· 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 rows there are per value for this
index. A unique index 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.
The last part of the output provides information about each column:
· Field
The column number.
· Start
The byte position of the column within table rows.
· Length
The length of the column in bytes.
· Nullpos, Nullbit
For columns that can be NULL, MyISAM stores NULL values as a flag in a byte. Depending on how many
nullable columns there are, there can be one or more bytes used for this purpose. The Nullpos and Nullbit
values, if nonempty, indicate which byte and bit contains that flag indicating whether the column is NULL.
The position and number of bytes used to store NULL flags is shown in the line for field 1. This is why
there are six Field lines for the person table even though it has only five columns.
· Type
The data type. The value may contain any of the following descriptors:
· constant
All rows have the same value.
· no endspace
Do not store endspace.
· no endspace, not_always
Do not store endspace and do not do endspace compression for all values.
· no zeros
Do not store zeros.
· always zero
Zero values are stored using one bit.
· Huff tree
The number of the Huffman tree associated with the column.
· Bits
The number of bits used in the Huffman tree.
The Huff tree and Bits fields are displayed if the table has been compressed with myisampack. See
myisampack(1), for an example of this information.
Example of myisamchk -eiv output:
Checking MyISAM file: person
Data records: 306688 Deleted blocks: 0
- check file-size
- check record delete-chain
No recordlinks
- check key delete-chain
block_size 1024:
- check index reference
- check data record references index: 1
Key: 1: Keyblocks used: 98% Packed: 0% Max levels: 3
- check data record references index: 2
Key: 2: Keyblocks used: 99% Packed: 97% Max levels: 3
- check data record references index: 3
Key: 3: Keyblocks used: 98% Packed: -14% Max levels: 3
Total: Keyblocks used: 98% Packed: 89%
- check records and index references
*** LOTS OF ROW NUMBERS DELETED ***
Records: 306688 M.recordlength: 25 Packed: 83%
Recordspace used: 97% Empty space: 2% Blocks/Record: 1.00
Record blocks: 306688 Delete blocks: 0
Record data: 7934464 Deleted data: 0
Lost space: 256512 Linkdata: 1156096
User time 43.08, System time 1.68
Maximum resident set size 0, Integral resident set size 0
Non-physical pagefaults 0, Physical pagefaults 0, Swaps 0
Blocks in 0 out 7, Messages in 0 out 0, Signals 0
Voluntary context switches 0, Involuntary context switches 0
Maximum memory usage: 1046926 bytes (1023k)
myisamchk -eiv output includes the following information:
· Data records
What percentage of the keyblocks are used. When a table has just been reorganized with myisamchk, the
values are very high (very near theoretical maximum).
· Packed
MySQL tries to pack key values that have a common suffix. This can only be used for indexes on CHAR and
VARCHAR columns. For long indexed strings that have similar leftmost parts, this can significantly reduce
the space used. In the preceding example, the second key is 40 bytes long and a 97% 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 row length. This is the exact row length for tables with fixed-length rows, because all rows
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 row (that is, how many links a fragmented row 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 large, you
can reorganize the table. See Section 6.6.4, “MyISAM Table Optimization”.
· Recordblocks
How many blocks (links) are used. For fixed-format tables, this is the same as the number of rows.
· Deleteblocks
How many blocks (links) are deleted.
· Recorddata
How many bytes in the data file are used.
Linkdata is the sum of the amount of storage used by all such pointers.
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 file system, out of
memory errors can easily occur. If this happens, run myisamchk with the --tmpdir=path option to specify a
directory located on a file system that has more space.
When performing repair operations, myisamchk also needs a lot of disk space:
· Twice 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 must be available on the
same file system as the original data file, as 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 must be available on the same file
system as the original data file.
· When using --recover or --sort-recover (but not when using --safe-recover), you need space on disk for
sorting. This space is allocated in the temporary directory (specified by TMPDIR or --tmpdir=path). 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 (see the
section called “MYISAMCHK TABLE INFORMATION”). The row_pointer_length and number_of_rows values are the
Datafile pointer and Data records values in the table description. To determine the largest_key value,
check the Key lines in the table description. The Len column indicates the number of bytes for each key
part. For a multiple-column index, the key size is the sum of the Len values for all key parts.
If you have a problem with disk space during repair, you can try --safe-recover instead of --recover.
COPYRIGHT
Copyright 2007-2008 MySQL AB, 2008-2010 Sun Microsystems, Inc.
This documentation is free software; you can redistribute it and/or modify it only under the terms of the GNU
General Public License as published by the Free Software Foundation; version 2 of the License.
This documentation is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
License for more details.
MySQL 5.1 04/06/2010 MYISAMCHK(1)
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