Discussion
Suppose a disk has 201 cylinders, numbered 0 to 200. At some time the disk arm is at cylinder 100, and there is a queue of disk access requests for cylinder 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.
Answer: C
Cylinder 90 is serviced after 3 requests.
SSTF always serves the pending request nearest the current head position. Starting at 100:
- 100 (distance 0)
- 105 (distance 5, nearer than 90 at 10)
- 110 (distance 5, nearer than 90 at 15)
- 90 (distance 20, nearer than 135 at 25)
So the requests for 100, 105 and 110 are served before 90. Counting 1 or 2 forgets that the request at cylinder 100 itself counts, or that 110 is nearer than 90 once the head is at 105.
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