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MYISAMCHK(1) MariaDB 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).
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 MariaDB 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 MariaDB 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.
myisamchk supports the following options, which can be specified
on the command line or in the [myisamchk] option file group.
The options described in this section can be used for any type of
table maintenance operation performed by myisamchk. The sections
following this one describe options that pertain only to specific
operations, such as table checking or repairing.
• --help, -?
Display a help message and exit. Options are grouped by type
of operation.
• --HELP, -H
Display a help message and exit. Options are presented in a
single list.
• --debug=debug_options, -# debug_options
Write a debugging log. A typical debug_options string is
´d:t:o,file_name´. The default is
´d:t:o,/tmp/myisamchk.trace´.
• --silent, -s
Silent mode. Write output only when errors occur. You can use
-s twice (-ss) to make myisamchk very silent.
• --verbose, -v
Verbose mode. Print more information about what the program
does. This can be used with -d and -e. Use -v multiple times
(-vv, -vvv) for even more output.
• --version, -V
Display version information and exit.
• --wait, -w
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.
• --print-defaults
Print the program argument list and exit.
• --no-defaults
Don't read default options from any option file.
• --defaults-file=#
Only read default options from the given file.
• --defaults-extra-file=#
Read this file after the global files are read.
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 │
├──────────────────────┼───────────────────┤
│ key_cache_block_size │ 1024 │
├──────────────────────┼───────────────────┤
│ myisam_block_size │ 1024 │
├──────────────────────┼───────────────────┤
│ read_buffer_size │ 262136 │
├──────────────────────┼───────────────────┤
│ sort_buffer_size │ 2097144 │
├──────────────────────┼───────────────────┤
│ sort_key_blocks │ 16 │
├──────────────────────┼───────────────────┤
│ stats_method │ nulls_unequal │
├──────────────────────┼───────────────────┤
│ write_buffer_size │ 262136 │
└──────────────────────┴───────────────────┘
The possible myisamchk variables and their default values can be
examined with myisamchk --help:
sort_buffer_size is used when the keys are repaired by sorting
keys, which is the normal case when you use --recover.
key_buffer_size is used when you are checking the table with
--extend-check or when the keys are repaired by inserting keys
row by row into the table (like when doing normal inserts).
Repairing through the key buffer is used in the following cases:
• You use --safe-recover.
• The temporary files needed to sort the keys would be more
than twice as big as when creating the key file directly.
This is often the case when you have large key values for
CHAR, VARCHAR, or TEXT columns, because the sort operation
needs to store the complete key values as it proceeds. If you
have lots of temporary space and you can force myisamchk to
repair by sorting, you can use the --sort-recover option.
Repairing through the key buffer takes much less disk space than
using sorting, but is also much slower.
If you want a faster repair, set the key_buffer_size and
sort_buffer_size variables to about 25% of your available memory.
You can set both variables to large values, because only one of
them is used at a time.
myisam_block_size is the size used for index blocks.
stats_method influences how NULL values are treated for index
statistics collection when the --analyze option is given. It acts
like the myisam_stats_method system variable. For more
information, see the description of myisam_stats_method in
Section 5.1.4, “Server System Variables”, and Section 7.4.7,
“MyISAM Index Statistics Collection”.
ft_min_word_len and ft_max_word_len indicate the minimum and
maximum word length for FULLTEXT indexes. ft_stopword_file names
the stopword file. These need to be set under the following
circumstances.
If you use myisamchk to perform an operation that modifies table
indexes (such as repair or analyze), the FULLTEXT indexes are
rebuilt using the default full-text parameter values for minimum
and maximum word length and the stopword file unless you specify
otherwise. This can result in queries failing.
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 supports the following options for table checking
operations:
• --check, -c
Check the table for errors. This is the default operation if
you specify no option that selects an operation type
explicitly.
• --check-only-changed, -C
Check only tables that have changed since the last check.
• --extend-check, -e
Check the table very thoroughly. This is quite slow if the
table has many indexes. This option should only be used in
extreme cases. Normally, myisamchk or myisamchk
--medium-check should be able to determine whether there are
any errors in the table.
If you are using --extend-check and have plenty of memory,
setting the key_buffer_size variable to a large value helps
the repair operation run faster.
For a description of the output format, see the section
called “MYISAMCHK TABLE INFORMATION”.
• --fast, -F
Check only tables that haven´t been closed properly.
• --force, -f
Do a repair operation automatically if myisamchk finds any
errors in the table. The repair type is the same as that
specified with the --recover or -r option. States will be
updated as with --update-state.
• --information, -i
Print informational statistics about the table that is
checked.
• --medium-check, -m
Do a check that is faster than an --extend-check operation.
This finds only 99.99% of all errors, which should be good
enough in most cases.
• --read-only, -T
Do not mark the table as checked. This is useful if you use
myisamchk to check a table that is in use by some other
application that does not use locking, such as mysqld when
run with external locking disabled.
• --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):
• --backup, -B
Make a backup of the .MYD file as file_name-time.BAK
• --character-sets-dir=path
The directory where character sets are installed.
• --correct-checksum
Correct the checksum information for the table.
• --create-missing-keys
Create missing keys. This assumes that the data file is
correct and that the number of rows stored in the index file
is correct. Enables --quick.
• --data-file-length=len, -D len
The maximum length of the data file (when re-creating data
file when it is “full”).
• --extend-check, -e
Do a repair that tries to recover every possible row from the
data file. Normally, this also finds a lot of garbage rows.
Do not use this option unless you are desperate.
For a description of the output format, see the section
called “MYISAMCHK TABLE INFORMATION”.
• --force, -f
Overwrite old intermediate files (files with names like
tbl_name.TMD) instead of aborting. Add another --force to
avoid 'myisam_sort_buffer_size is too small' errors. In this
case we will attempt to do the repair with the given
myisam_sort_buffer_size and dynamically allocate as many
management buffers as needed.
• --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.
• --max-record-length=len
Skip rows larger than the given length if myisamchk cannot
allocate memory to hold them.
• --parallel-recover, -p
Use the same technique as -r and -n, but create all the keys
in parallel, using different threads. This is beta-quality
code. Use at your own risk!
• --quick, -q
Achieve a faster repair by modifying only the index file, not
the data file. You can specify this option twice to force
myisamchk to modify the original data file in case of
duplicate keys. NOTE: Tables where the data file is corrupted
can't be fixed with this option.
• --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.
• --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.
• --set-collation=name
Specify the collation to use for sorting table indexes. The
character set name is implied by the first part of the
collation name.
• --sort-recover, -n
Force myisamchk to use sorting to resolve the keys even if
the temporary files would be very large.
• --tmpdir=path, -t path
The path of the directory to be used for storing temporary
files. If this is not set, myisamchk uses the value of the
TMPDIR environment variable. tmpdir can be set to a list of
directory paths that are used successively in round-robin
fashion for creating temporary files. The separator character
between directory names is the colon (“:”) on Unix and the
semicolon (“;”) on Windows, NetWare, and OS/2.
• --unpack, -u
Unpack a table that was packed with myisampack.
myisamchk supports the following options for actions other than
table checks and repairs:
• --analyze, -a
Analyze the distribution of key values. This improves join
performance by enabling the join optimizer to better choose
the order in which to join the tables and which indexes it
should use. To obtain information about the key distribution,
use a myisamchk --description --verbose tbl_name command or
the SHOW INDEX FROM tbl_name statement.
• --block-search=offset, -b offset
Find the record that a block at the given offset belongs to.
• --description, -d
Print some descriptive information about the table.
Specifying the --verbose option once or twice produces
additional information. See the section called “MYISAMCHK
TABLE INFORMATION”.
• --set-auto-increment[=value], -A[value]
Force AUTO_INCREMENT numbering for new records to start at
the given value (or higher, if there are existing records
with AUTO_INCREMENT values this large). If value is not
specified, AUTO_INCREMENT numbers for new records begin with
the largest value currently in the table, plus one.
• --sort-index, -S
Sort the index tree blocks in high-low order. This optimizes
seeks and makes table scans that use indexes faster.
• --sort-records=N, -R N
Sort records according to a particular index. This makes your
data much more localized and may speed up range-based SELECT
and ORDER BY operations that use this index. (The first time
you use this option to sort a table, it may be very slow.) To
determine a 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.)
• --stats-method=name
Specifies how index statistics collection code should treat
NULLs. Possible values of name are "nulls_unequal" (default),
"nulls_equal" (emulate MySQL 4 behavior), and
"nulls_ignored".
To obtain a description of a MyISAM table or statistics about it,
use the commands shown here. The output from these commands is
explained later in this section.
• myisamchk -d tbl_name
Runs myisamchk in “describe mode” to produce a description of
your table. If you start the MariaDB server with external
locking disabled, myisamchk may report an error for a table
that is updated while it runs. However, because myisamchk
does not change the table in describe mode, there is no risk
of destroying data.
• myisamchk -dv tbl_name
Adding -v runs myisamchk in verbose mode so that it produces
more information about the table. Adding -v a second time
produces even more information.
• myisamchk -eis tbl_name
Shows only the most important information from a table. This
operation is slow because it must read the entire table.
• myisamchk -eiv tbl_name
This is like -eis, but tells you what is being done.
The tbl_name argument can be either the name of a MyISAM table or
the name of its index file, as described in myisamchk(1).
Multiple tbl_name arguments can be given.
Suppose that a table named person has the following structure.
(The MAX_ROWS table option is included so that in the example
output from myisamchk shown later, some values are smaller and
fit the output format more easily.)
CREATE TABLE person
(
id INT NOT NULL AUTO_INCREMENT,
last_name VARCHAR(20) NOT NULL,
first_name VARCHAR(20) NOT NULL,
birth DATE,
death DATE,
PRIMARY KEY (id),
INDEX (last_name, first_name),
INDEX (birth)
) MAX_ROWS = 1000000;
Suppose also that the table has these data and index file sizes:
-rw-rw---- 1 mysql mysql 9347072 Aug 19 11:47 person.MYD
-rw-rw---- 1 mysql mysql 6066176 Aug 19 11:47 person.MYI
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:
• MyISAM file
Name of the MyISAM (index) file.
• Record format
The format used to store table rows. The preceding examples
use Fixed length. Other possible values are Compressed and
Packed. (Packed corresponds to what SHOW TABLE STATUS reports
as Dynamic.)
• Chararacter set
The table default character set.
• File-version
Version of MyISAM format. Currently always 1.
• Creation time
When the data file was created.
• Recover time
When the index/data file was last reconstructed.
• Status
Table status flags. Possible values are crashed, open,
changed, analyzed, optimized keys, and sorted index pages.
• 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.
• Data records
The number of rows in the table.
• 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”.
• Datafile parts
For dynamic-row format, this indicates how many data blocks
there are. For an optimized table without fragmented rows,
this is the same as Data records.
• Deleted data
How many bytes of unreclaimed deleted data there are. You can
optimize your table to minimize this space. See
Section 6.6.4, “MyISAM Table Optimization”.
• Datafile pointer
The size of the data file pointer, in bytes. It is usually 2,
3, 4, or 5 bytes. Most tables manage with 2 bytes, but this
cannot be controlled from MariaDB yet. For fixed tables, this
is a row address. For dynamic tables, this is a byte address.
• Keyfile pointer
The size of the index file pointer, in bytes. It is usually
1, 2, or 3 bytes. Most tables manage with 2 bytes, but this
is calculated automatically by MariaDB. It is always a block
address.
• Max datafile length
How long the table data file can become, in bytes.
• Max keyfile length
How long the table index file can become, in bytes.
• Recordlength
How much space each row takes, in bytes.
The table description part of the output includes a list of all
keys in the table. For each key, myisamchk displays some
low-level information:
• Key
This key´s number. This value is shown only for the first
column of the key. If this value is missing, the line
corresponds to the second or later column of a
multiple-column key. For the table shown in the example,
there are two table description lines for the second index.
This indicates that it is a multiple-part index with two
parts.
• Start
Where in the row this portion of the index starts.
• Len
How long this portion of the index is. For packed numbers,
this should always be the full length of the column. For
strings, it may be shorter than the full length of the
indexed column, because you can index a prefix of a string
column. The total length of a multiple-part key is the sum of
the Len values for all key parts.
• Index
Whether a key value can exist multiple times in the index.
Possible values are unique or multip. (multiple).
• Type
What data type this portion of the index has. This is a
MyISAM data type with the possible values packed, stripped,
or empty.
• Root
Address of the root index block.
• Blocksize
The size of each index block. By default this is 1024, but
the value may be changed at compile time when MariaDB is
built from source.
• Rec/key
This is a statistical value used by the optimizer. It tells
how many rows there are per value for this index. A unique
index always has a value of 1. This may be updated after a
table is loaded (or greatly changed) with myisamchk -a. If
this is not updated at all, a default value of 30 is given.
The last part of the output provides information about each
column:
• Field
The column number.
• Start
The byte position of the column within table rows.
• Length
The length of the column in bytes.
• Nullpos, Nullbit
For columns that can be NULL, MyISAM stores NULL values as a
flag in a byte. Depending on how many nullable columns there
are, there can be one or more bytes used for this purpose.
The Nullpos and Nullbit values, if nonempty, indicate which
byte and bit contains that flag indicating whether the column
is NULL.
The position and number of bytes used to store NULL flags is
shown in the line for field 1. This is why there are six
Field lines for the person table even though it has only five
columns.
• Type
The data type. The value may contain any of the following
descriptors:
• constant
All rows have the same value.
• no endspace
Do not store endspace.
• no endspace, not_always
Do not store endspace and do not do endspace compression
for all values.
• no endspace, no empty
Do not store endspace. Do not store empty values.
• table-lookup
The column was converted to an ENUM.
• zerofill(N)
The most significant N bytes in the value are always 0
and are not stored.
• no zeros
Do not store zeros.
• always zero
Zero values are stored using one bit.
• Huff tree
The number of the Huffman tree associated with the column.
• Bits
The number of bits used in the Huffman tree.
The Huff tree and Bits fields are displayed if the table has been
compressed with myisampack. See myisampack(1), for an example of
this information.
Example of myisamchk -eiv output:
Checking MyISAM file: person
Data records: 306688 Deleted blocks: 0
- check file-size
- check record delete-chain
No recordlinks
- check key delete-chain
block_size 1024:
- check index reference
- check data record references index: 1
Key: 1: Keyblocks used: 98% Packed: 0% Max levels: 3
- check data record references index: 2
Key: 2: Keyblocks used: 99% Packed: 97% Max levels: 3
- check data record references index: 3
Key: 3: Keyblocks used: 98% Packed: -14% Max levels: 3
Total: Keyblocks used: 98% Packed: 89%
- check records and index references
*** LOTS OF ROW NUMBERS DELETED ***
Records: 306688 M.recordlength: 25 Packed: 83%
Recordspace used: 97% Empty space: 2% Blocks/Record: 1.00
Record blocks: 306688 Delete blocks: 0
Record data: 7934464 Deleted data: 0
Lost space: 256512 Linkdata: 1156096
User time 43.08, System time 1.68
Maximum resident set size 0, Integral resident set size 0
Non-physical pagefaults 0, Physical pagefaults 0, Swaps 0
Blocks in 0 out 7, Messages in 0 out 0, Signals 0
Voluntary context switches 0, Involuntary context switches 0
Maximum memory usage: 1046926 bytes (1023k)
myisamchk -eiv output includes the following information:
• Data records
The number of rows in the table.
• 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”.
• Key
The key number.
• 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).
• Packed
MariaDB tries to pack key values that have a common suffix.
This can only be used for indexes on CHAR and VARCHAR
columns. For long indexed strings that have similar leftmost
parts, this can significantly reduce the space used. In the
preceding example, the second key is 40 bytes long and a 97%
reduction in space is achieved.
• Max levels
How deep the B-tree for this key is. Large tables with long
key values get high values.
• Records
How many rows are in the table.
• M.recordlength
The average row length. This is the exact row length for
tables with fixed-length rows, because all rows have the same
length.
• Packed
MariaDB strips spaces from the end of strings. The Packed
value indicates the percentage of savings achieved by doing
this.
• Recordspace used
What percentage of the data file is used.
• Empty space
What percentage of the data file is unused.
• Blocks/Record
Average number of blocks per row (that is, how many links a
fragmented row is composed of). This is always 1.0 for
fixed-format tables. This value should stay as close to 1.0
as possible. If it gets too large, you can reorganize the
table. See Section 6.6.4, “MyISAM Table Optimization”.
• Recordblocks
How many blocks (links) are used. For fixed-format tables,
this is the same as the number of rows.
• Deleteblocks
How many blocks (links) are deleted.
• Recorddata
How many bytes in the data file are used.
• Deleted data
How many bytes in the data file are deleted (unused).
• 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.
• 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.
Memory allocation is important when you run myisamchk. myisamchk
uses no more memory than its memory-related variables are set to.
If you are going to use myisamchk on very large tables, you
should first decide how much memory you want it to use. The
default is to use only about 3MB to perform repairs. By using
larger values, you can get myisamchk to operate faster. For
example, if you have more than 32MB RAM, you could use options
such as these (in addition to any other options you might
specify):
shell> myisamchk --sort_buffer_size=16M \
--key_buffer_size=16M \
--read_buffer_size=1M \
--write_buffer_size=1M ...
Using --sort_buffer_size=16M should probably be enough for most
cases.
Be aware that myisamchk uses temporary files in TMPDIR. If TMPDIR
points to a memory file system, out of memory errors can easily
occur. If this happens, run myisamchk with the --tmpdir=path
option to specify a directory located on a file system that has
more space.
When performing repair operations, myisamchk also needs a lot of
disk space:
• Twice the size of the data file (the original file and a
copy). This space is not needed if you do a repair with
--quick; in this case, only the index file is re-created.
This space must be available on the same file system as the
original data file, as the copy is created in the same
directory as the original.
• Space for the new index file that replaces the old one. The
old index file is truncated at the start of the repair
operation, so you usually ignore this space. This space must
be available on the same file system as the original data
file.
• When using --recover or --sort-recover (but not when using
--safe-recover), you need space on disk for sorting. This
space is allocated in the temporary directory (specified by
TMPDIR or --tmpdir=path). The following formula yields the
amount of space required:
(largest_key + row_pointer_length) × number_of_rows × 2
You can check the length of the keys and the
row_pointer_length with myisamchk -dv tbl_name (see the
section called “MYISAMCHK TABLE INFORMATION”). The
row_pointer_length and number_of_rows values are the Datafile
pointer and Data records values in the table description. To
determine the largest_key value, check the Key lines in the
table description. The Len column indicates the number of
bytes for each key part. For a multiple-column index, the key
size is the sum of the Len values for all key parts.
If you have a problem with disk space during repair, you can try
--safe-recover instead of --recover.
Copyright 2007-2008 MySQL AB, 2008-2010 Sun Microsystems, Inc.,
2010-2020 MariaDB Foundation
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/.
For more information, please refer to the MariaDB Knowledge Base,
available online at https://mariadb.com/kb/
MariaDB Foundation (http://www.mariadb.org/).
This page is part of the MariaDB (MariaDB database server)
project. Information about the project can be found at
⟨http://mariadb.org/⟩. If you have a bug report for this manual
page, see ⟨https://mariadb.com/kb/en/mariadb/reporting-bugs/⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/MariaDB/server⟩ on 2023-12-22. (At that
time, the date of the most recent commit that was found in the
repository was 2023-05-11.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
MariaDB 10.8 15 May 2020 MYISAMCHK(1)
Pages that refer to this page: aria_chk(1), myisamchk(1), myisampack(1)
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