Big Alice plays the disk Crockett - the basic elevator algorithm

• When writing to a hard disk - size and location matters.
• I/O time depends on seek time plus data transfer time.
• The disk always rotates - so we need to schedule I/Os as close to the current read-head location as possible to reduce seek time.
• The problem - the disk does not expose its physical geometry.
• So the solution: ordering requests based on sector address
• and making sure not to make the disk head "jump":
  • First move the elevator up: perform all I/Os from low address to high address.
  • If a request arrives, whose address is lower than the current I/O - it will wait for the elevator to come back down.
  • When there are no I/Os in the elevator's current direction - switch direction.

Originally written by guy keren