man pages section 1: User Commands

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Updated: July 2014

myisamchk (1)


myisamchk - maintenance utility


myisamchk [options] tbl_name ...


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).

     You can also use the CHECK TABLE and REPAIR TABLE statements
     to check and repair MyISAM tables. See Section,
     "CHECK TABLE Syntax", and Section, "REPAIR TABLE

     The use of myisamchk with partitioned tables is not


         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

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     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:

         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 --myisam_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".

     For additional information about using myisamchk, see
     Section 7.6, "MyISAM Table Maintenance and Crash Recovery".


         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

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         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, "CHECK TABLE Syntax".

     myisamchk supports the following options, which can be
     specified on the command line or in the [myisamchk] group of
     an option file.  myisamchk also supports the options for
     processing option files described at Section,
     "Command-Line Options that Affect Option-File Handling".

     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, -?

         Display a help message and exit. Options are grouped by
         type of operation.

     o   --HELP, -H

         Display a help message and exit. Options are presented
         in a single list.

     o   --debug=debug_options, -# debug_options

         Write a debugging log. A typical debug_options string is
         'd:t:o,file_name'. The default is

     o   --silent, -s

         Silent mode. Write output only when errors occur. You
         can use -s twice (-ss) to make myisamchk very silent.

     o   --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.

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     o   --version, -V

         Display version information and exit.

     o   --wait, -w

         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:

     sort_buffer_size is used when the keys are repaired by
     sorting keys, which is the normal case when you use
     --recover. As of MySQL 5.5.29, myisam_sort_buffer_size is
     available as an alternative name to sort_buffer_size.
     myisam_sort_buffer_size is preferable to sort_buffer_size
     because its name corresponds to the myisam_sort_buffer_size
     server system variable that has a similar meaning.

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     sort_buffer_size should be considered deprecated.

     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 5.1.4, "Server System
     Variables", and Section 8.3.7, "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.  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

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     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
     statements are performed by the server, which knows the
     proper full-text parameter values to use.

     myisamchk supports the following options for table checking

     o   --check, -c

         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

         Check only tables that have changed since the last

     o   --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.

         See also the description of this option under table
         repair options.

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         For a description of the output format, see the section

     o   --fast, -F

         Check only tables that haven't been closed properly.

     o   --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.

     o   --information, -i

         Print informational statistics about the table that is

     o   --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.

     o   --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.

     o   --update-state, -U

         Store information in the .MYI file to indicate when the
         table was checked and whether the table crashed. This
         should be used to get full benefit of the
         --check-only-changed option, but you shouldn't use this
         option if the mysqld server is using the table and you
         are running it with external locking disabled.

     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

         Make a backup of the .MYD file as file_name-time.BAK

     o   --character-sets-dir=path

         The directory where character sets are installed. See

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         Section 10.5, "Character Set Configuration".

     o   --correct-checksum

         Correct the checksum information for the table.

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

         The maximum length of the data file (when re-creating
         data file when it is "full").

     o   --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

         See also the description of this option under table
         checking options.

         For a description of the output format, see the section

     o   --force, -f

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

     o   --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.

     o   --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.

     o   --max-record-length=len

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

     o   --parallel-recover, -p

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         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!

     o   --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.

     o   --recover, -r

         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

         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-character-set=name

         Change the character set used by the table indexes. This
         option was replaced by --set-collation in MySQL 5.0.3.

     o   --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.

     o   --sort-recover, -n

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

     o   --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.

     o   --unpack, -u

         Unpack a table that was packed with myisampack.

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

     o   --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.

     o   --block-search=offset, -b offset

         Find the record that a block at the given offset belongs

     o   --description, -d

         Print some descriptive information about the table.
         Specifying the --verbose option once or twice produces
         additional information. See the section called

     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.

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     o   --sort-index, -S

         Sort the index tree blocks in high-low order. This
         optimizes seeks and makes table scans that use indexes

     o   --sort-records=N, -R N

         Sort records according to a particular index. This makes
         your data much more localized and may speed up
         range-based SELECT and ORDER BY operations that use this
         index. (The first time you use this option to sort a
         table, it may be very slow.) To determine a table's
         index numbers, use SHOW INDEX, which displays a table's
         indexes in the same order that myisamchk sees them.
         Indexes are numbered beginning with 1.

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

     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

     o   myisamchk -eiv tbl_name

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         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

         -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:       latin1_swedish_ci (8)
         File-version:        1
         Creation time:       2009-08-19 16:47:41
         Recover time:        2009-08-19 16:47:56
         Status:              checked,analyzed,optimized keys
         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        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

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         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

     The initial part of the table description contains these

     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. Currently 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

     o   Auto increment key, Last value

         The key number associated the table's AUTO_INCREMENT
         column, and the most recently generated value for this

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         column. These fields do not appear if there is no such

     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 7.6.4, "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 7.6.4, "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.

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     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.

     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

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         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

     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

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MySQL Database System                                MYISAMCHK(1)

             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

     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

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         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 7.6.4, "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

     o   Max levels

         How deep the B-tree for this key is. Large tables with
         long key values get high values.

     o   Records

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MySQL Database System                                MYISAMCHK(1)

         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 7.6.4, "MyISAM Table

     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.

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MySQL Database System                                MYISAMCHK(1)

     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

     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):

         shell> 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=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:

     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=path). The

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         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
         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.

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MySQL Database System                                MYISAMCHK(1)

     Computer Software License (December 2007). Oracle USA, Inc.,
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     This software is developed for general use in a variety of
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MySQL Database System                                MYISAMCHK(1)

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MySQL Database System                                MYISAMCHK(1)

     This software was built from source available at  The original
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