Question

A disk request queue has requests for blocks on the following cylinders (ordered by time of arrival): 18, 50, 45, 69, 9, 49 The disk has 100 cylinders numbered trough 0 to 99. The disk head is currently at cylinder 46 and is moving towards cylinder 99. Calculate the total seek distance for each of the following disk seek algorithms: FCFS, SSTF and SCAN. (Note: For SCAN, you should assume that the disk head should go until the last cylinder in one direction and then reverse from the last cylinder.)

Answer 1

The total seek distance for FCFS is 149, for SSTF is 87, and for SCAN is 202.

Step-by-step explanation:

The correct answer is option C

In FCFS (First-Come, First-Served), the total seek distance is calculated by summing the absolute differences between consecutive cylinders in the request queue. Following this algorithm, the total seek distance for the given requests is 149.

In SSTF (Shortest Seek Time First), the disk head always moves to the closest cylinder with pending requests. In this case, the total seek distance would be:

1. From 46 to 45 (1 distance)
2. From 45 to 50 (5 distances)
3. From 50 to 49 (1 distance)
4. From 49 to 69 (20 distances)
5. From 69 to 18 (51 distances)
6. From 18 to 9 (9 distances)

The total seek distance for SSTF is 87.

In SCAN, the disk head moves in one direction until it reaches the last cylinder, and then reverses direction. The total seek distance would be:

1. From 46 to 99 (53 distances)
2. From 99 to 0 (99 distances)
3. From 0 to 9 (9 distances)
4. From 9 to 18 (9 distances)
5. From 18 to 45 (27 distances)
6. From 45 to 49 (4 distances)
7. From 49 to 50 (1 distance)

The total seek distance for SCAN is 202.