Consider a disk system with 100 cylinders. The requests to access the cylinders occur in the following sequence:
4,34,10,7,19,73,2,15,6,20
Assuming that the head is current at cylinder 50, what is the time taken to satisfy all requests if it takes 1ms to move from the cylinder to adjacent one and the shortest seek time first policy is used?
(A) 375ms
(B) 238ms
(C) 276ms
(D) 119ms
Answer: (D)
Explanation: 4, 34, 10, 7, 19, 73, 2, 15, 6, 20
Since shortest seek time first policy is used, head will first move to 34.
This move will cause 16*1 ms.
After 34, head will move to 20 which will cause 14*1 ms. And so on.
So cylinders are accessed in following order 34, 20, 19, 15, 10, 7, 6, 4, 2, 73 and
total time will be (16 + 14 + 1 + 4 + 5 + 3 + 1 + 2 + 2 + 71)*1 = 119 ms.
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