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+12 votes

Consider a disk system with 100 cylinders. The requests to access the cylinders occur in following sequence:

4, 34, 10, 7, 19, 73, 2, 15, 6, 20

Assuming that the head is currently at cylinder 50, what is the time taken to satisfy all requests if it takes 1ms to move from one cylinder to adjacent one and shortest seek time first policy is used?

  1. 95 ms
  2. 119 ms
  3. 233 ms
  4. 276 ms


asked in Operating System by Veteran (69k points)
edited by | 1.5k views

2 Answers

+12 votes
Best answer

Answer is B.

 = (50-34)+(34-20)+(20-19)+(19-15)+(15-10)+(10-7)+(7-6)+(6-4)+(4-2)+(73-2)\\ = 16+14+1+4+5+3+1+2+2+71\\ = 119 \text{ ms.}

answered by Loyal (4.2k points)
selected by
+14 votes
Here in case of Shortest seek time first we don't need to do lot of  substraction & addition in this type of questions. It seems like it is necessary to actually calculate  small differences ! It is better to just draw actual movement & Then do computation.

Here sequence is like


So we can calculate like (50-2) + (73-2) = 71+48 = 119.
answered by Veteran (49.5k points)

So we can calculate like (50-2) + (73-2) = 71+48 = 119. what is reference for this trick and this is not working with other questions please someone confirm authenticity of this trick .

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