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4 June, 07:38

The outstanding disk requests are for tracks 6,10,4,20,36,8, and 40 in that order. Assume that the seek time speed is 5 msec/track, there are 50 cylinders total, and presently the read-write head is at track 15 and moving up towards 50. Calculate the seek time for each of the following scheduling methods.

a. FCFS

b. SSTF

c. C-LOOK

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  1. 4 June, 08:08
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    a. FCFS

    seek: 9+4+6+16+16+28+32 = 111 cylinders, 555 ms

    b. SSTF

    seek: 5+16+4+30+2+2+2 = 61 cylinders, 305 ms

    c. C-LOOK

    seek: 5+16+4+30+2+2+2 = 61 cylinders, 305 ms

    Explanation:

    a. FCFS: requests are served in the order they are received. It is used when load is low and has long waiting time for long requests queues.

    order: (15), 6, 10, 4, 20, 36, 8, 40

    seek: 9+4+6+16+16+28+32 = 111 cylinders, 555 ms

    b. SSTF: requests are served using shortest seek time first. It minimizes arm movement, and maximizes request rate

    order: (15), 20, 36, 40, 10, 8, 6, 4 15 10 8 6 4 40 36 20

    seek: 5+16+4+30+2+2+2 = 61 cylinders, 305 ms

    c. C-LOOK: requests are served in one direction until when done then reverses. It does not go to end of disk.

    Assume maximum track is 50 then reverses

    order: (15), 20, 36, 40, (50), 10, 8, 6, 4

    seek: 5+16+4+30+2+2+2 = 61 cylinders, 305 ms
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