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Consider the following five disk five disk access requests of the form (request id, cylinder number) that are present in the disk scheduler queue at a given time.

$(P, 155), (Q,85), (R,110),(S, 30), (T,115)$

Assume the head is positioned at cylinder $100$. The scheduler follows Shortest Seek Time First scheduling to service the requests.

Which one of the following statements is FALSE?

  1. $T$ is serviced before $P$.
  2. $Q$ is serviced after $S$,but before $T$.
  3. The head reverses its direction of movement between servicing of $Q$ and $P$.
  4. $R$ is serviced before $P$.
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1 Answer

13 votes
13 votes
Best answer

Shortest Seek Time First (SSTF), selects the request with minimum to seek time first from the current head position. 
In the given question disk requests are given in the form of $\langle request \ id, \ cylinder \ number \rangle$

Cylinder Queue: $(P,155), (Q,85), (R,110), (S,30), (T,115)$

Head starts at: $100$

  • It is clear that $R$ and $T$ are serviced before $P$.
  • $Q$ is serviced when head is moving towards lower cylinders and $P$ is serviced when head is moving towards higher cylinders thus reverses it direction at $S$.

Option B) is the correct answer

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2 Comments

1 flag:
✌ Low quality (swaminath-bera “Wrong”)
i think ans c is correct
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Why @saykatamkeen ? Any reason supporting your comment?

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