IO Scheduler

Modern processor speed is much higher than disk access.  A disk seek can take 8 msec and is equivalent to 25M processor cycle.  Therefore, the function of IO scheduler is to optimize seek optimization.

There are 2 basic tasks of IO schedule - sorting and merging.  Sorting is to arrange pending IO by block order to minimize head movement.  Merging is to coalesce several IO requests into one, thus reducing the number of requests.  For example, one IO to read block 6 and one to read block 7 can be merged into one IO to read block 6 to 7.