CS 3733 Operating Systems Notes: Disk Head Scheduling Algorithms

This material is taken from Silberschatz, section 14.2, pp 492-498.

• Recall that a disk drive consists of a number of platters, each having one or two surfaces.
• Each surface is divided into tracks and each track is divided into sectors.
• The drive has one head per surface and the heads move together.
• For a given position of the heads, the collection of sectors that can be accessed is called a cylinder.
• There are three parts to accessing a sector on the disk:
  1. Move the head to the appropriate cylinder - seek time
  2. Wait for the desired sector to rotate under the head - rotational latency
  3. Transfer the data - transfer time
• Since I/O is slow, disk operations are usually queued.
• Example from book:
       queued requests to cylinders: 98, 183, 37, 122, 14, 124, 65, 67
• In what order should these accesses be made?
  1. FCFS: first-come, first-served
  2. SSTF: shortest seek time first
  3. SCAN and LOOK
  4. C-SCAN and C-LOOK

A simualtor As in the process scheduling simulator, perform experiments consisting of runs.

Look at the simulator running the above problem here.