MySQL, many writes waiting on disk i/o access
Switch to InnoDB.  Heed the advice here .
Change a few things in my.cnf because of the Engine change.
Don't do anything special with disk.


InnoDB works with 16KB chunks, not 4KB, like MyISAM for data; 16KB vs 1KB for indexes.

If you still have trouble,...  What indexes are on your table?  What order are the rows in?  Are the tables bigger than RAM?  How much RAM do you have?  Let's see SHOW CREATE TABLE.