On a disk with 1000 cylinders, numbers 0 to 999, compute the number of tracks the disk arm must move to satisfy all the requests in the disk queue. Assume the last request serviced was at tracks 345 and the head is moving towards track 0. The queue in FIFO order contains requests for the following tracks. 123, 874, 692, 475, 105, 376. Perform the computation using C-SCAN scheduling algorithm. What is the total distance?a)1219b)1009c)1967d)1507Correct answer is option 'C'. Can you explain this answer?

Question

Answer

C-SCAN Scheduling Algorithm

C-SCAN scheduling algorithm is a variation of the SCAN algorithm. In the SCAN algorithm, the disk arm moves in one direction servicing all the requests until it reaches the end of the disk and then reverses its direction and services the requests in the opposite direction.

In the C-SCAN algorithm, the disk arm always moves in one direction, servicing all the requests in that direction. When it reaches the end of the disk, it immediately returns to the beginning of the disk without servicing any requests in the opposite direction. This results in a more uniform service time for requests.

Calculation of Total Distance

Given:

Disk has 1000 cylinders numbered 0 to 999.
Last request serviced was at track 345 and the head is moving towards track 0.
Queue in FIFO order contains requests for the following tracks: 123, 874, 692, 475, 105, 376.

Steps:

1. Arrange the requests in ascending order.

The queue in ascending order is: 105, 123, 376, 475, 692, 874.

2. Calculate the distance between the last serviced track (345) and the first request track in the current direction (105).

The distance between 345 and 105 is 240.

3. Calculate the distance between the last request track in the current direction (874) and the end of the disk (999).

The distance between 874 and 999 is 125.

4. Calculate the distance between the beginning of the disk (0) and the first request track in the opposite direction (692).

The distance between 0 and 692 is 692.

5. Calculate the total distance traveled by the disk arm.

Total distance = (distance between 345 and 105) + (distance between 999 and 874) + (distance between 692 and 0) + (distance between 475 and 123) + (distance between 376 and 0)

Total distance = 240 + 125 + 692 + 352 + 376

Total distance = 1967

Therefore, the total distance traveled by the disk arm to satisfy all the requests in the disk queue using the C-SCAN scheduling algorithm is 1967. The correct option is C.

Answer