MySQL之myisamchk

Description, check and repair of MyISAM tables.
Used without options all tables on the command will be checked for errors
Usage: myisamchk [OPTIONS] tables[.MYI]

 

Global options:
-H, --HELP Display this help and exit.
-?, --help Display this help and exit.
-O, --set-variable var=option.
Change the value of a variable. Please note that
this option is deprecated; you can set variables
directly with '--variable-name=value'.
-t, --tmpdir=path Path for temporary files. Multiple paths can be
specified, separated by colon (:), they will be used
in a round-robin fashion.
-s, --silent Only print errors. One can use two -s to make
myisamchk very silent.
-v, --verbose Print more information. This can be used with
--description and --check. Use many -v for more verbosity.
-V, --version Print version and exit.
-w, --wait Wait if table is locked.

Check options (check is the default action for myisamchk):
-c, --check Check table for errors.
-e, --extend-check Check the table VERY throughly. Only use this in
extreme cases as myisamchk should normally be able to
find out if the table is ok even without this switch.
-F, --fast Check only tables that haven't been closed properly.
-C, --check-only-changed
Check only tables that have changed since last check.
-f, --force Restart with '-r' if there are any errors in the table.
States will be updated as with '--update-state'.
-i, --information Print statistics information about table that is checked.
-m, --medium-check Faster than extend-check, but only finds 99.99% of
all errors. Should be good enough for most cases.
-U --update-state Mark tables as crashed if you find any errors.
-T, --read-only Don't mark table as checked.

Repair options (When using '-r' or '-o'):
-B, --backup Make a backup of the .MYD file as 'filename-time.BAK'.
--correct-checksum Correct checksum information for table.
-D, --data-file-length=# Max length of data file (when recreating data
file when it's full).
-e, --extend-check Try to recover every possible row from the data file
Normally this will also find a lot of garbage rows;
Don't use this option if you are not totally desperate.
-f, --force Overwrite old temporary files.
-k, --keys-used=# Tell MyISAM to update only some specific keys. # is a
bit mask of which keys to use. This can be used to
get faster inserts.
--max-record-length=#
Skip rows bigger than this if myisamchk can't allocate
memory to hold it.
-r, --recover Can fix almost anything except unique keys that aren't
unique.
-n, --sort-recover Forces recovering with sorting even if the temporary
file would be very big.
-p, --parallel-recover
Uses the same technique as '-r' and '-n', but creates
all the keys in parallel, in different threads.
-o, --safe-recover Uses old recovery method; Slower than '-r' but can
handle a couple of cases where '-r' reports that it
can't fix the data file.
--character-sets-dir=...
Directory where character sets are.
--set-collation=name
Change the collation used by the index.
-q, --quick Faster repair by not modifying the data file.
One can give a second '-q' to force myisamchk to
modify the original datafile in case of duplicate keys.
NOTE: Tables where the data file is currupted can't be
fixed with this option.
-u, --unpack Unpack file packed with myisampack.

Other actions:
-a, --analyze Analyze distribution of keys. Will make some joins in
MySQL faster. You can check the calculated distribution
by using '--description --verbose table_name'.
--stats_method=name Specifies how index statistics collection code should
treat NULLs. Possible values of name are "nulls_unequal"
(default for 4.1/5.0), "nulls_equal" (emulate 4.0), and
"nulls_ignored".
-d, --description Prints some information about table.
-A, --set-auto-increment[=value]
Force auto_increment to start at this or higher value
If no value is given, then sets the next auto_increment
value to the highest used value for the auto key + 1.
-S, --sort-index Sort index blocks. This speeds up 'read-next' in
applications.
-R, --sort-records=#
Sort records according to an index. This makes your
data much more localized and may speed up things
(It may be VERY slow to do a sort the first time!).
-b, --block-search=#
Find a record, a block at given offset belongs to.

 

由于临时断电，使用kill -9中止MySQL服务进程，所有的这些都可能会毁坏MySQL的数据文件。如果在被干扰时，服务正在改变文件，文件可能会留下错误的或不一致的状态。因为这样的毁坏有时是不容易被发现的，当你发现这个错误时可能是很久以后的事了。于是，当你发现这个问题时，也许所有的备份都有同样的错误。 

 

myisamchk通过一行一行地创建一个“.MYD”(数据 )文件的副本来工作，它通过由删除老的“.MYD 文件并且重命名新文件到原来的文件名结束修复阶段。如果你使用--quick，myisamchk不创建一个临时“.MYD”文件，只是假定“.MYD”文件是正确的并且仅创建一个新的索引文件，不接触“.MYD”文件，这是安全的，因为myisamchk自动检测“.MYD”文件是否损坏并且在这种情况下，放弃修复。你也可以给myisamchk两个--quick选项。在这种情况下，myisamchk不会在一些错误上(象重复键)放弃，相反试图通过修改“.MYD”文件解决它们。通常，只有在你在太少的空闲磁盘空间上实施一个正常修复，使用两个--quick选项才有用。在这种情况下，你应该至少在运行myisamchk前做一个备份。

 

1、myisamchk
使用myisamchk必须暂时停止MySQL服务器。例如，我们要检修test数据库。执行以下操作：

# service mysqld stop ;
# myisamchk -r /var/lib/mysql/test/*MYI
# service mysqld start;

myisamchk会自动检查并修复数据表中的索引错误。

 

2、mysqlcheck
使用mysqlcheck无需停止MySQL，可以进行热修复。

#mysqlcheck –r DBname Tablename –uuser –ppassword

 

注意，无论是myisamchk还是mysqlcheck，一般情况下不要使用-f强制修复，-f参数会在遇到一般修复无法成功的时候删除