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Suppose a disk has 201 cylinders, numbered from 0 to 200. At some time the disk arm is at cylinder 100, and there is a queue of disk access requests for cylinders 30, 85, 90, 100, 105, 110, 135 and 145. If Shortest-Seek Time First (SSTF) is being used for scheduling the disk access, the request for cylinder 90 is serviced after servicing ________ number of requests.

1. 1
2. 2
3. 3
4. 4

Correct answer: 3

Solution

Starting at 100 with SSTF, the service order by nearest-seek is 100, then 105, then 110, then 90 (distance 20, closer than 135 at 25). Thus cylinder 90 is the 4th serviced, i.e. after servicing 3 requests, so the answer is 3.

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