8.6.3 Optimizing REPAIR TABLE Statements
REPAIR TABLE, the same principle applies, but because the repair is done by the server, you set server system variables instead of myisamchk variables. Also, in addition to setting memory-allocation variables, increasing the
myisam_max_sort_file_sizesystem variable increases the likelihood that the repair will use the faster filesort method and avoid the slower repair by key cache method. Set the variable to the maximum file size for your system, after checking to be sure that there is enough free space to hold a copy of the table files. The free space must be available in the file system containing the original table files.
Suppose that a myisamchk table-repair operation is done using the following options to set its memory-allocation variables:
--key_buffer_size=128M --myisam_sort_buffer_size=256M --read_buffer_size=64M --write_buffer_size=64M
Some of those myisamchk variables correspond to server system variables:
|myisamchk Variable||System Variable|
Each of the server system variables can be set at runtime, and some of them (
read_buffer_size) have a session value in addition to a global value. Setting a session value limits the effect of the change to your current session and does not affect other users. Changing a global-only variable (
myisam_max_sort_file_size) affects other users as well. For
key_buffer_size, you must take into account that the buffer is shared with those users. For example, if you set the myisamchk
key_buffer_size variable to 128MB, you could set the corresponding
key_buffer_size system variable larger than that (if it is not already set larger), to permit key buffer use by activity in other sessions. However, changing the global key buffer size invalidates the buffer, causing increased disk I/O and slowdown for other sessions. An alternative that avoids this problem is to use a separate key cache, assign to it the indexes from the table to be repaired, and deallocate it when the repair is complete. See Section 18.104.22.168, “Multiple Key Caches”.
Based on the preceding remarks, a
REPAIR TABLE operation can be done as follows to use settings similar to the myisamchk command. Here a separate 128MB key buffer is allocated and the file system is assumed to permit a file size of at least 100GB.
SET SESSION myisam_sort_buffer_size = 256*1024*1024; SET SESSION read_buffer_size = 64*1024*1024; SET GLOBAL myisam_max_sort_file_size = 100*1024*1024*1024; SET GLOBAL repair_cache.key_buffer_size = 128*1024*1024; CACHE INDEX tbl_name IN repair_cache; LOAD INDEX INTO CACHE tbl_name; REPAIR TABLE tbl_name ; SET GLOBAL repair_cache.key_buffer_size = 0;
If you intend to change a global variable but want to do so only for the duration of a
REPAIR TABLE operation to minimally affect other users, save its value in a user variable and restore it afterward. For example:
SET @old_myisam_sort_buffer_size = @@GLOBAL.myisam_max_sort_file_size; SET GLOBAL myisam_max_sort_file_size = 100*1024*1024*1024; REPAIR TABLE tbl_name ; SET GLOBAL myisam_max_sort_file_size = @old_myisam_max_sort_file_size;
The system variables that affect
REPAIR TABLE can be set globally at server startup if you want the values to be in effect by default. For example, add these lines to the server
[mysqld] myisam_sort_buffer_size=256M key_buffer_size=1G myisam_max_sort_file_size=100G
These settings do not include
read_buffer_size globally to a large value does so for all sessions and can cause performance to suffer due to excessive memory allocation for a server with many simultaneous sessions.