The default value is 1k, this is possibly too small for best performance on modern systems; in particular, many filesystems used a bigger block size, so writing a single index block requires a read followed by a write. Random reads are really slow on hard discs (writes are mostly fast as they go into your battery-backed raid controller which has lots of RAM).
I am currently in the process of experimenting with myisam_block_size, and so far have determined the following:
- myisam_block_size is settable only at server start time, either in my.cnf or on the command-line
- myisam_block_size only affects newly created tables or tables rebuilt using ALTER TABLE; existing MyISAM tables keep their old index block size and work no differently.
Advantages of higher myisam_block_size
- Fewer IO requests required to satisfy some requests (e.g. range scans) as fewer blocks are needed
- Better index prefix compression, as the blocks are larger - particularly with PACK_KEYS enabled (index compression works on a block-by-block basis)
- No read-then-write required by the OS to write to the underlying disc, if the block size is >= the filesystem block size
- More wasted space in partially filled blocks
- Possibly less efficient use of key cache, as more of the "wrong" blocks are cached
- Larger IOs are required to fulfil the same request often - as it requires the same number of bigger blocks