Disk Scheduling

• One of the responsibilities of the OS is to use the hardware efficiently. For the disk drives, meeting this responsibility entails having fast access time and large disk bandwidth.
• The access time has two major components.
  • The seek time is the time for the disk arm to move the heads to the cylinder containing the desired sector.
  • The rotational latency is the additional time for the disk to rotate the desired sector to the disk head.
• The disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer.
• We can improve both the access time and the bandwidth by scheduling the servicing of disk I/O requests in a good order.
• For a multiprogramming system with many processes, the disk queue may often have several pending requests.
  • Thus, when one request is completed, the OS chooses which pending request to service next.
  • How does the OS make this choice? Disk-scheduling algorithms.