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).
You can also use the CHECK TABLE and REPAIR TABLE statements to check and
repair MyISAM tables. See Section 15.7.3.2, "CHECK TABLE Statement", and
Section 15.7.3.5, "REPAIR TABLE Statement".
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:
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 enables 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:
myisamchk *.MYI
If you are not in the database directory, you can check all the tables
there by specifying the path to the directory:
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:
myisamchk /path/to/datadir/*/*.MYI
The recommended way to quickly check all MyISAM tables is:
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:
myisamchk --silent --force --fast --update-state \
--key_buffer_size=64M --myisam_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 the section
called "MYISAMCHK MEMORY USAGE".
For additional information about using myisamchk, see Section 9.6,
"MyISAM Table Maintenance and Crash Recovery".
Important
You must ensure that no other program is using the tables while you
are running myisamchk. The most effective means of doing so is to
shut down the MySQL server while running myisamchk, or to lock all
tables that myisamchk is being used on.
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 15.7.3.2, "CHECK
TABLE Statement".
myisamchk supports the following options, which can be specified on the
command line or in the [myisamchk] group of an option file. For
information about option files used by MySQL programs, see
Section 6.2.2.2, "Using Option Files".
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.
o --help, -?
+--------------------+--------+
|Command-Line Format | --help |
+--------------------+--------+
Display a help message and exit. Options are grouped by type of
operation.
o --HELP, -H
+--------------------+--------+
|Command-Line Format | --HELP |
+--------------------+--------+
Display a help message and exit. Options are presented in a single
list.
o --debug=debug_options, -# debug_options
+--------------------+----------------------------+
|Command-Line Format | --debug[=debug_options] |
+--------------------+----------------------------+
|Type | String |
+--------------------+----------------------------+
|Default Value | d:t:o,/tmp/myisamchk.trace |
+--------------------+----------------------------+
Write a debugging log. A typical debug_options string is
d:t:o,file_name. The default is d:t:o,/tmp/myisamchk.trace.
This option is available only if MySQL was built using WITH_DEBUG.
MySQL release binaries provided by Oracle are not built using this
option.
o --defaults-extra-file=file_name
+--------------------+---------------------------------+
|Command-Line Format | --defaults-extra-file=file_name |
+--------------------+---------------------------------+
|Type | File name |
+--------------------+---------------------------------+
Read this option file after the global option file but (on Unix)
before the user option file. If the file does not exist or is
otherwise inaccessible, an error occurs. If file_name is not an
absolute path name, it is interpreted relative to the current
directory.
For additional information about this and other option-file options,
see Section 6.2.2.3, "Command-Line Options that Affect Option-File
Handling".
o --defaults-file=file_name
+--------------------+---------------------------+
|Command-Line Format | --defaults-file=file_name |
+--------------------+---------------------------+
|Type | File name |
+--------------------+---------------------------+
Use only the given option file. If the file does not exist or is
otherwise inaccessible, an error occurs. If file_name is not an
absolute path name, it is interpreted relative to the current
directory.
For additional information about this and other option-file options,
see Section 6.2.2.3, "Command-Line Options that Affect Option-File
Handling".
o --defaults-group-suffix=str
+--------------------+-----------------------------+
|Command-Line Format | --defaults-group-suffix=str |
+--------------------+-----------------------------+
|Type | String |
+--------------------+-----------------------------+
Read not only the usual option groups, but also groups with the usual
names and a suffix of str. For example, myisamchk normally reads the
[myisamchk] group. If this option is given as
--defaults-group-suffix=_other, myisamchk also reads the
[myisamchk_other] group.
For additional information about this and other option-file options,
see Section 6.2.2.3, "Command-Line Options that Affect Option-File
Handling".
o --no-defaults
+--------------------+---------------+
|Command-Line Format | --no-defaults |
+--------------------+---------------+
Do not read any option files. If program startup fails due to reading
unknown options from an option file, --no-defaults can be used to
prevent them from being read.
The exception is that the .mylogin.cnf file is read in all cases, if
it exists. This permits passwords to be specified in a safer way than
on the command line even when --no-defaults is used. To create
.mylogin.cnf, use the mysql_config_editor utility. See
mysql_config_editor(1).
For additional information about this and other option-file options,
see Section 6.2.2.3, "Command-Line Options that Affect Option-File
Handling".
o --print-defaults
+--------------------+------------------+
|Command-Line Format | --print-defaults |
+--------------------+------------------+
Print the program name and all options that it gets from option
files.
For additional information about this and other option-file options,
see Section 6.2.2.3, "Command-Line Options that Affect Option-File
Handling".
o --silent, -s
+--------------------+----------+
|Command-Line Format | --silent |
+--------------------+----------+
Silent mode. Write output only when errors occur. You can use -s
twice (-ss) to make myisamchk very silent.
o --verbose, -v
+--------------------+-----------+
|Command-Line Format | --verbose |
+--------------------+-----------+
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.
o --version, -V
+--------------------+-----------+
|Command-Line Format | --version |
+--------------------+-----------+
Display version information and exit.
o --wait, -w
+--------------------+---------+
|Command-Line Format | --wait |
+--------------------+---------+
|Type | Boolean |
+--------------------+---------+
|Default Value | false |
+--------------------+---------+
Instead of terminating with an error if the table is locked, wait
until the table is unlocked before continuing. 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 |
+-----------------------+-------------------+
|myisam_sort_key_blocks | 16 |
+-----------------------+-------------------+
|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:
myisam_sort_buffer_size is used when the keys are repaired by sorting
keys, which is the normal case when you use --recover. sort_buffer_size
is a deprecated synonym for myisam_sort_buffer_size.
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:
o You use --safe-recover.
o 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
myisam_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 7.1.8, "Server System
Variables", and Section 10.3.8, "InnoDB and MyISAM Index Statistics
Collection".
ft_min_word_len and ft_max_word_len indicate the minimum and maximum word
length for FULLTEXT indexes on MyISAM tables. 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:
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.
MYISAMCHK CHECK OPTIONS
myisamchk supports the following options for table checking operations:
o --check, -c
+--------------------+---------+
|Command-Line Format | --check |
+--------------------+---------+
Check the table for errors. This is the default operation if you
specify no option that selects an operation type explicitly.
o --check-only-changed, -C
+--------------------+----------------------+
|Command-Line Format | --check-only-changed |
+--------------------+----------------------+
Check only tables that have changed since the last check.
o --extend-check, -e
+--------------------+----------------+
|Command-Line Format | --extend-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.
See also the description of this option under table repair options.
For a description of the output format, see the section called
"OBTAINING TABLE INFORMATION WITH MYISAMCHK".
o --fast, -F
+--------------------+--------+
|Command-Line Format | --fast |
+--------------------+--------+
Check only tables that haven't been closed properly.
o --force, -f
+--------------------+---------+
|Command-Line Format | --force |
+--------------------+---------+
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.
o --information, -i
+--------------------+---------------+
|Command-Line Format | --information |
+--------------------+---------------+
Print informational statistics about the table that is checked.
o --medium-check, -m
+--------------------+----------------+
|Command-Line Format | --medium-check |
+--------------------+----------------+
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.
o --read-only, -T
+--------------------+-------------+
|Command-Line Format | --read-only |
+--------------------+-------------+
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.
o --update-state, -U
+--------------------+----------------+
|Command-Line Format | --update-state |
+--------------------+----------------+
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.
MYISAMCHK REPAIR OPTIONS
myisamchk supports the following options for table repair operations
(operations performed when an option such as --recover or --safe-recover
is given):
o --backup, -B
+--------------------+----------+
|Command-Line Format | --backup |
+--------------------+----------+
Make a backup of the .MYD file as file_name-time.BAK
o --character-sets-dir=dir_name
+--------------------+---------------------------+
|Command-Line Format | --character-sets-dir=path |
+--------------------+---------------------------+
|Type | String |
+--------------------+---------------------------+
|Default Value | [none] |
+--------------------+---------------------------+
The directory where character sets are installed. See Section 12.15,
"Character Set Configuration".
o --correct-checksum
+--------------------+--------------------+
|Command-Line Format | --correct-checksum |
+--------------------+--------------------+
Correct the checksum information for the table.
o --data-file-length=len, -D len
+--------------------+------------------------+
|Command-Line Format | --data-file-length=len |
+--------------------+------------------------+
|Type | Numeric |
+--------------------+------------------------+
The maximum length of the data file (when re-creating data file when
it is "full").
o --extend-check, -e
+--------------------+----------------+
|Command-Line Format | --extend-check |
+--------------------+----------------+
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.
See also the description of this option under table checking options.
For a description of the output format, see the section called
"OBTAINING TABLE INFORMATION WITH MYISAMCHK".
o --force, -f
+--------------------+---------+
|Command-Line Format | --force |
+--------------------+---------+
Overwrite old intermediate files (files with names like tbl_name.TMD)
instead of aborting.
o --keys-used=val, -k val
+--------------------+-----------------+
|Command-Line Format | --keys-used=val |
+--------------------+-----------------+
|Type | Numeric |
+--------------------+-----------------+
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.
o --max-record-length=len
+--------------------+-------------------------+
|Command-Line Format | --max-record-length=len |
+--------------------+-------------------------+
|Type | Numeric |
+--------------------+-------------------------+
Skip rows larger than the given length if myisamchk cannot allocate
memory to hold them.
o --quick, -q
+--------------------+---------+
|Command-Line Format | --quick |
+--------------------+---------+
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.
o --recover, -r
+--------------------+-----------+
|Command-Line Format | --recover |
+--------------------+-----------+
Do a repair that can fix almost any problem except unique keys that
are not 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 cannot 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
myisam_sort_buffer_size.
o --safe-recover, -o
+--------------------+----------------+
|Command-Line Format | --safe-recover |
+--------------------+----------------+
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 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.
o --set-collation=name
+--------------------+----------------------+
|Command-Line Format | --set-collation=name |
+--------------------+----------------------+
|Type | String |
+--------------------+----------------------+
Specify the collation to use for sorting table indexes. The character
set name is implied by the first part of the collation name.
o --sort-recover, -n
+--------------------+----------------+
|Command-Line Format | --sort-recover |
+--------------------+----------------+
Force myisamchk to use sorting to resolve the keys even if the
temporary files would be very large.
o --tmpdir=dir_name, -t dir_name
+--------------------+-------------------+
|Command-Line Format | --tmpdir=dir_name |
+--------------------+-------------------+
|Type | Directory name |
+--------------------+-------------------+
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.
o --unpack, -u
+--------------------+----------+
|Command-Line Format | --unpack |
+--------------------+----------+
Unpack a table that was packed with myisampack.
OTHER MYISAMCHK OPTIONS
myisamchk supports the following options for actions other than table
checks and repairs:
o --analyze, -a
+--------------------+-----------+
|Command-Line Format | --analyze |
+--------------------+-----------+
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.
o --block-search=offset, -b offset
+--------------------+-----------------------+
|Command-Line Format | --block-search=offset |
+--------------------+-----------------------+
|Type | Numeric |
+--------------------+-----------------------+
Find the record that a block at the given offset belongs to.
o --description, -d
+--------------------+---------------+
|Command-Line Format | --description |
+--------------------+---------------+
Print some descriptive information about the table. Specifying the
--verbose option once or twice produces additional information. See
the section called "OBTAINING TABLE INFORMATION WITH MYISAMCHK".
o --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.
o --sort-index, -S
+--------------------+--------------+
|Command-Line Format | --sort-index |
+--------------------+--------------+
Sort the index tree blocks in high-low order. This optimizes seeks
and makes table scans that use indexes faster.
o --sort-records=N, -R N
+--------------------+------------------+
|Command-Line Format | --sort-records=# |
+--------------------+------------------+
|Type | Numeric |
+--------------------+------------------+
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.)
OBTAINING TABLE INFORMATION WITH MYISAMCHK
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.
o 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.
o 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.
o myisamchk -eis tbl_name
Shows only the most important information from a table. This
operation is slow because it must read the entire table.
o 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 ENGINE=MYISAM;
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
Example of myisamchk -dvv output:
MyISAM file: person
Record format: Packed
Character set: utf8mb4_0900_ai_ci (255)
File-version: 1
Creation time: 2017-03-30 21:21:30
Status: checked,analyzed,optimized keys,sorted index pages
Auto increment key: 1 Last value: 306688
Data records: 306688 Deleted blocks: 0
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 1024
2 6 80 multip. varchar prefix 0 1024
87 80 varchar 0
3 168 3 multip. uint24 NULL 0 1024
Field Start Length Nullpos Nullbit Type
1 1 1
2 2 4 no zeros
3 6 81 varchar
4 87 81 varchar
5 168 3 1 1 no zeros
6 171 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:
o MyISAM file
Name of the MyISAM (index) file.
o 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.)
o Chararacter set
The table default character set.
o File-version
Version of MyISAM format. Always 1.
o Creation time
When the data file was created.
o Recover time
When the index/data file was last reconstructed.
o Status
Table status flags. Possible values are crashed, open, changed,
analyzed, optimized keys, and sorted index pages.
o Auto increment key, Last value
The key number associated the table's AUTO_INCREMENT column, and the
most recently generated value for this column. These fields do not
appear if there is no such column.
o Data records
The number of rows in the table.
o Deleted blocks
How many deleted blocks still have reserved space. You can optimize
your table to minimize this space. See Section 9.6.4, "MyISAM Table
Optimization".
o 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.
o Deleted data
How many bytes of unreclaimed deleted data there are. You can
optimize your table to minimize this space. See Section 9.6.4,
"MyISAM Table Optimization".
o 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.
o 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.
o Max datafile length
How long the table data file can become, in bytes.
o Max keyfile length
How long the table index file can become, in bytes.
o 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:
o 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.
o Start
Where in the row this portion of the index starts.
o 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 key parts.
o Index
Whether a key value can exist multiple times in the index. Possible
values are unique or multip. (multiple).
o Type
What data type this portion of the index has. This is a MyISAM data
type with the possible values packed, stripped, or empty.
o Root
Address of the root index block.
o 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.
o 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:
o Field
The column number.
o Start
The byte position of the column within table rows.
o Length
The length of the column in bytes.
o 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.
o Type
The data type. The value may contain any of the following
descriptors:
o constant
All rows have the same value.
o no endspace
Do not store endspace.
o no endspace, not_always
Do not store endspace and do not do endspace compression for all
values.
o no endspace, no empty
Do not store endspace. Do not store empty values.
o table-lookup
The column was converted to an ENUM.
o zerofill(N)
The most significant N bytes in the value are always 0 and are
not stored.
o no zeros
Do not store zeros.
o always zero
Zero values are stored using one bit.
o Huff tree
The number of the Huffman tree associated with the column.
o 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:
o Data records
The number of rows in the table.
o Deleted blocks
How many deleted blocks still have reserved space. You can optimize
your table to minimize this space. See Section 9.6.4, "MyISAM Table
Optimization".
o Key
The key number.
o Keyblocks used
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).
o 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.
o Max levels
How deep the B-tree for this key is. Large tables with long key
values get high values.
o Records
How many rows are in the table.
o 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.
o Packed
MySQL strips spaces from the end of strings. The Packed value
indicates the percentage of savings achieved by doing this.
o Recordspace used
What percentage of the data file is used.
o Empty space
What percentage of the data file is unused.
o 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 9.6.4,
"MyISAM Table Optimization".
o Recordblocks
How many blocks (links) are used. For fixed-format tables, this is
the same as the number of rows.
o Deleteblocks
How many blocks (links) are deleted.
o Recorddata
How many bytes in the data file are used.
o Deleted data
How many bytes in the data file are deleted (unused).
o Lost space
If a row is updated to a shorter length, some space is lost. This is
the sum of all such losses, in bytes.
o Linkdata
When the dynamic table format is used, row fragments are linked with
pointers (4 to 7 bytes each). 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 512MB RAM available, you could
use options such as these (in addition to any other options you might
specify):
myisamchk --myisam_sort_buffer_size=256M \
--key_buffer_size=512M \
--read_buffer_size=64M \
--write_buffer_size=64M ...
Using --myisam_sort_buffer_size=16M is probably 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=dir_name 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:
o 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.
o 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.
o 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=dir_name). 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 "OBTAINING TABLE
INFORMATION WITH MYISAMCHK"). 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 (C) 1997, 2026, Oracle and/or its affiliates.
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.
You should have received a copy of the GNU General Public License along
with the program; if not, write to the Free Software Foundation, Inc., 51
Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA or see
http://www.gnu.org/licenses/.
SEE ALSO
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
Oracle Corporation (http://dev.mysql.com/).
MySQL 8.4 09/15/2026 myisamchk(1)
mysql 8.4.13 - Generated Sun Oct 25 16:32:16 CDT 2026