There are 200 cylinders in a disk numbered from 200. The disk access request for cylinders is as follows:
240, 300, 250, 210, 201, 249, 275, 305, 307, 363, 337, 350, 397 and 320.
Elevator disk scheduling is used for disk scheduling. Calculate the total track movement made by r/w header (assume header is at 305).
(A) 292
(B) 293
(C) 722
(D) None of these.


Answer: (A)

Explanation: According to SCAN (also known as elevator) disk scheduling algorithm, scans down towards the nearest end (like an elevator does) and then when it hits the bottom it scans up servicing the requests that it didn\’t get going down.

So,

Hence, total path covered

= (399 - 305)+(399 - 201)
= 292

Option (A) is correct.


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