Disk requests are received by a disk drive for cylinder 5, 25, 18, 3, 39, 8…
2007
Disk requests are received by a disk drive for cylinder 5, 25, 18, 3, 39, 8 and 35 in that order. A seek takes 5 msec per cylinder moved. How much seek time is needed to serve these requests for a Shortest Seek First (SSF) algorithm? Assume that the arm is at cylinder 20 when the last of these requests is made with none of the requests yet served
Answer: B. 295 msec — To solve this using Shortest Seek First (SSF), start at cylinder 20. Identify the nearest unserved request at each step. Sequence: 20 → 18 (dist 2) → 25 (dist…
- A.
125 msec
- B.
295 msec
- C.
575 msec
- D.
750 msec
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Correct answer: B
To solve this using Shortest Seek First (SSF), start at cylinder 20. Identify the nearest unserved request at each step. Sequence: 20 → 18 (dist 2) → 25 (dist 7) → 35 (dist 10) → 39 (dist 4) → 8 (dist 31) → 5 (dist 3) → 3 (dist 2). Total distance = 2 + 7 + 10 + 4 + 31 + 3 + 2 = 59 cylinders. Total seek time = 59 × 5 msec = 295 msec.
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