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:
shell> myisamchk --silent --force --fast --update-state \
--key_buffer_size=64M --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".
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 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.
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 'd:t:o,/tmp/myisamchk.trace'.
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.
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 |
+------------------+-------------------+
|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:
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
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.
The problem occurs because these parameters are known only by the
server. They are not stored in MyISAM index files. To avoid the problem
if you have modified the minimum or maximum word length or the stopword
file in the server, specify the same ft_min_word_len, ft_max_word_len,
and ft_stopword_file values to myisamchk that you use for mysqld. For
example, if you have set the minimum word length to 3, you can repair a
table with myisamchk like this:
shell> myisamchk --recover --ft_min_word_len=3 tbl_name.MYI
To ensure that myisamchk and the server use the same values for
full-text parameters, you can place each one in both the [mysqld] and
[myisamchk] sections of an option file:
[mysqld]
ft_min_word_len=3
[myisamchk]
ft_min_word_len=3
An alternative to using myisamchk is to use the REPAIR TABLE, ANALYZE
TABLE, OPTIMIZE TABLE, or ALTER TABLE. These statements are performed
by the server, which knows the proper full-text parameter values to
use.
MYISAMCHK CHECK OPTIONS
myisamchk supports the following options for table checking operations:
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 check.
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.
For a description of the output format, see the section called
"MYISAMCHK TABLE INFORMATION".
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 checked.
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 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
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 Section 9.5,
"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 desperate.
For a description of the output format, see the section called
"MYISAMCHK TABLE INFORMATION".
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
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
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
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, NetWare, and OS/2.
o --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:
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 to.
o --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".
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
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
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.)
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.
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;
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: 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
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:
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 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 6.6.4, "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 6.6.4,
"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 6.6.4, "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 6.6.4,
"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 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:
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 following formula yields the amount of space
required:
(largest_key + row_pointer_length) x number_of_rows x 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.
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 02110-1335 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
Sun Microsystems, Inc. (http://www.mysql.com/).
MySQL 5.1 04/06/2010 MYISAMCHK(1)
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