Suppose that a disk drive has 5,000 cylinders, numbered 0 to 4999, with the read

Question

Suppose that a disk drive has 5,000 cylinders, numbered 0 to 4999, with the read-write head having just finished a request at cylinder 1056.

The queue of pending requests, in order of arrival, is: 333, 1200, 922, 4545,3786, 3605.

Starting from the current head position, what is the order in which the requests will be serviced for each of the following disk-scheduling algorithms (assuming no other requests arrive and the head's initial movement is toward higher numbered cylinders for c-f)?

a. FCFS

b. SSTF

c. SCAN

d. LOOK

e. C-SCAN

f. C-LOOK

Explanation / Answer

a) FCFS - > request processed in order it arrives

1056 - > 333 - > 1200 - > 922 - > 4545 -> 3786 - > 3605

b) SSTF - > request processed in order of shortest seek time

1056 - > 922 - > 1200 - > 333 - > 3605 -> 3786 - > 4545

c) SCAN -> reads towards one side then return back

1056 - > 1200 - > 3605 - > 3786 - > 4545 -> 4999 - > 922 -> 333

d) LOOK -> it is same as scan but it will not read the last position i.e. 4999

1056 - > 1200 - > 3605 - > 3786 - > 4545 - > 922 -> 333

e) C-SCAN - > It is same as SCAN but after reaching it on one end it, come backs to other end and start in same direction

1056 - > 1200 - > 3605 - > 3786 - > 4545 -> 4999 - > 0 - > 333 -> 922

f) C- LOOK - > It is same as C-SCAN without touching the ends

1056 - > 1200 - > 3605 - > 3786 - > 4545 - > 333 -> 922

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