myisamchk — MyISAM Table-Maintenance Utility How many bytes in the data file are used. How many bytes in the data file are deleted (unused). If a row is updated to a shorter length, some space is lost. This is the sum of all such losses, in bytes. 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 512MB RAM available, you could use options such as these (in addition to any other options you might specify): 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= dir_name  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: ( 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 Section 4.6.4.5, “Obtaining Table Information with 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 578 •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= dir_name ). The following formula yields the amount of space required: •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. •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. • Linkdata • Lost space • Deleted data • Recorddata 4.6.4.6 myisamchk Memory Usage myisamchk --myisam_sort_buffer_size=256M \ --key_buffer_size=512M \ --read_buffer_size=64M \ --write_buffer_size=64M ...