Secondary Storage – Disk Scheduling Algorithms

Secondary storage

From magnetic tapes to disks

They are primarily used for storage and backup of infrequently used data due to their slow access times

As a one-dimensional linear array of logical blocks mapped onto sectors, with Sector 0 being the first sector

To use hardware efficiently, especially disk drives

Fast access time and increased disk bandwidth

Seek time and rotational latency

Time for the disk arm to move to the head of the cylinder containing the desired sector

Additional time waiting for the disk to rotate to the desired sector under the disk head

Total number of bytes transferred divided by the total time between the first request for service and the computation of the last transfer

To improve access time and bandwidth

System calls are issued to the OS; if the disk is available, the request is serviced, otherwise it is queued

To reduce seek time and rotational latency while increasing bandwidth

FCFS, SSTF, SCAN, C-SCAN, LOOK, C-LOOK

First Come First Serve (FCFS)

Simplest, fair, does not provide the fastest service, requests serviced in order of arrival

Services the request closest to the current head position to minimize seek time

Requests far from the head may wait a long time, leading to starvation

The disk arm moves from one end to the other, servicing requests along the way, then reverses direction

Requests just missed by the head must wait until the arm moves to the other end and back

C-SCAN moves in one direction, jumps to the opposite end without servicing during the jump, then continues servicing in the same direction

LOOK stops at the last request in its direction rather than going to the physical end of the disk

C-LOOK stops at the last request in its direction before jumping to the last request on the other side, rather than going to the physical end

Both service requests in one direction and then reverse

SCAN goes to the end of the disk before reversing, LOOK stops at the last request in the direction

Both move in one direction, jump back without servicing during the jump, and then service requests again while moving forward

C-SCAN goes to the physical end before jumping back, C-LOOK stops at the last request before jumping to the opposite end

Total Seek Time = Sum of absolute differences between consecutive head positions, i.e., Total Seek Time = Σ |Current Head Position - Next Request|