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Module 6: I/O & File Management

20227m

Suppose a disk has 100 cylinders, numbered from 0 to 99. At some time the disk arm is at cylinder 50, and there is a queue of disk access requests for cylinders 4, 34, 10, 7, 19, 73, 2, 15, 6 and 20. Assuming that shortest seek-time first (SSTF) is being used for scheduling the disk access. Calculate the total time taken to satisfy all requests if it takes 1 millisecond to move from one cylinder to adjacent one.

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SSTF calculation (arm at 50)

Requests: 4,34,10,7,19,73,2,15,6,20. Order by closest seek from current:
50→34(16),34→20(14)=30,20→19(1)=31,19→15(4)=35,15→10(5)=40,10→7(3)=43,7→6(1)=44,6→4(2)=46,4→2(2)=48,2→73(71)=119.
Total movement = 119 cylinders → 119ms (1ms per cylinder). FCFS would be larger (~ ~200). SSTF starves edge requests but minimizes seek; SCAN/C-SCAN would give different totals. Gantt/Sequence: 50-34-20-19-15-10-7-6-4-2-73.

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