Question

Some request lists might cause the disk scheduler to act the same when the three different algorithms are run. Create a request list of five track numbers that will cause all three algorithms to visit the same tracks in the same order. Enter those five track numbers here. If shortest seek time first starts with the disk head positioned at either 0 or 99, instead of at 50 (in the middle), which algorithm would it resemble: FCFS or SCAN? Why?

Answer 1

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1.

List of five track numbers that will cause all three algorithms to visit the same tracks in the same order are -

35

25

50

75

95

Below is the diagram how these 5 tracks will be traversed by various algorithms.

All the algorithms traverse in the order 35 -> 25 -> 50 -> 75 -> 95

2.

If shortest seek time first starts with the disk head positioned at either 0 or 99, instead of at 50 (in the middle) then it would resemble SCAN as for example it starts with 0 as the starting point then it will only move in the right direction servicing requests in between till it reaches the end point which is analogous to SCAN.It will only move in one direction because it is the only way to minimise the seek operations

As in SCAN disk scheduling algorithm, head starts from one end of the disk and moves towards the other end, servicing requests in between one by one and reach the other end. Then the direction of the head is reversed and the process continues as head continuously scan back and forth to access the disk.