Consider a storage disk with 4 platters (numbered as 0, 1, 2 and 3), 200…
20182023
Consider a storage disk with 4 platters (numbered as 0, 1, 2 and 3), 200 cylinders (numbered as 0, 1, … , 199), and 256 sectors per track (numbered as 0, 1, … , 255). The following 6 disk requests of the form [sector number, cylinder number, platter number] are received by the disk controller at the same time:
[120, 72, 2] , [180, 134, 1] , [60, 20, 0] , [212, 86, 3] , [56, 116, 2] , [118, 16, 1]
Currently the head is positioned at sector number 100 of cylinder 80, and is moving towards higher cylinder numbers. The average power dissipation in moving the head over 100 cylinders is 20 milliwatts and for reversing the direction of the head movement once is 15 milliwatts. Power dissipation associated with rotational latency and switching of head between different platters is negligible.
The total power consumption in milliwatts to satisfy all of the above disk requests using the Shortest Seek Time First disk scheduling algorithm is _______.
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Correct answer: 85
Key idea: apply Shortest Seek Time First (SSTF) from the current head position (cylinder 80).
Request cylinders: 72, 134, 20, 86, 116, 16
SSTF service sequence (closest cylinder to current head each step):
80 → 86 (distance 6)
86 → 72 (distance 14)
72 → 116 (distance 44)
116 → 134 (distance 18)
134 → 20 (distance 114)
20 → 16 (distance 4)
Total head travel = 6 + 14 + 44 + 18 + 114 + 4 = 200 cylinders.
Power for movement: moving 100 cylinders uses 20 mW, so 200 cylinders uses (200/100)×20 = 40 mW.
Direction reversals: sequence of directions is + (80→86), − (86→72), + (72→116→134), − (134→20→16). That is 3 reversals total.
Power for reversals: 3 × 15 mW = 45 mW.
Total power = movement power + reversal power = 40 + 45 = 85 mW.
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