Suppose that a disk drive has 6,000 cylinders, numbered 0 to 5, 999. The drive i

Question

Suppose that a disk drive has 6,000 cylinders, numbered 0 to 5, 999. The drive is currently serving a request at cylinder 3, 250, and the previous request was at cylinder 4, 805. The queue of pending requests, in FIFO order, is: 1515, 1212, 1296, 3800, 544, 618, 356, 2523, 5965, 4681 Starting from the current head position, what is the total distance (in cylinders) that the disk arm moves to satisfy all the pending requests for each of the following disk scheduling algorithms? FCFS SSTF SCAN LOOK

Explanation / Answer

a)FCFS(First come First Serve)

Solution:

Queue : 1515,1212,1296,3800,544,618,356,2523,5965,4681

Head: 3250

Seek order: 1515,1212,1296,3800,544,618,356,2523,5965,4681

Seek distance: (1212-3250)+(1212-3800)+(544-3800)+(544-618)+(356-618)+(356-5965)+(4681-5965)

= -2038-2588-3256-74-262-5906-1284

= -15111

b)SSTF(Shortest Seek Time First)

Solution:

Queue : 1515,1212,1296,3800,544,618,356,2523,5965,4681

Head: 3250

Seek order: 3800,4681,5965,2523,1515,1296,1212,618,544,356

Seek distance: (5965-3250)+(5965-356)

= 2715+5609

= 8324

c)SCAN

Solution:

Queue : 1515,1212,1296,3800,544,618,356,2523,5965,4681

Head: 3250

Seek order: 2523,1515,1296,1212,618,544,356,0,3800,4681,5965

Seek distance: (3250-0)+(5965-0)

= 3250 + 5965

= 9215

d) LOOK

Solution:

Queue : 1515,1212,1296,3800,544,618,356,2523,5965,4681

Head: 3250

Seek order: 2523,1515,1296,1212,618,544,356,3800,4681,5965

Seek distance: (3250-356)+(5965-356)

= 2894 + 5609

= 8503

= 8324

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