A disk has cylinders 0 to 199. The head is at cylinder 70 and the request queue is 120, 35, 180, 10, 95, 150, 60. What is the total head movement under FCFS disk scheduling?
Correct answer D. 680 cylinders
Explanation
FCFS serves requests in arrival order: 70 to 120 is 50, then 85, 145, 170, 85, 55 and 90 cylinders, a total of 680. It is fair and simple but zig-zags across the disk, so seek distance is large. SSTF on the same queue needs only 230 cylinders, which shows why smarter disk schedulers reduce seek time so much.
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