Consider the following five disk five disk access requests of the form…
2020
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 ?
- A.
T is serviced before P
- B.
Q is serviced after S, but before T
- C.
The head reverses its direction of movement between servicing of Q and P
- D.
R is serviced before P
Attempted by 100 students.
Show answer & explanation
Correct answer: B
Key insight: SSTF always picks the pending request with the smallest absolute distance from the current head position.
Step 1: Start at 100. Distances: R(110)=10, Q(85)=15, T(115)=15, P(155)=55, S(30)=70. Nearest is R(110) → service R.
Step 2: Head at 110. Distances: T(115)=5, Q(85)=25, P(155)=45, S(30)=80. Nearest is T(115) → service T.
Step 3: Head at 115. Distances: Q(85)=30, P(155)=40, S(30)=85. Nearest is Q(85) → service Q.
Step 4: Head at 85. Distances: S(30)=55, P(155)=70. Nearest is S(30) → service S.
Step 5: Remaining request P(155) → service P.
Final SSTF service order: R (110) → T (115) → Q (85) → S (30) → P (155).
Using this order:
The statement 'T is serviced before P' is true (T appears before P).
The statement 'Q is serviced after S, but before T' is false — Q is actually serviced after T and before S.
The statement 'The head reverses its direction of movement between servicing of Q and P' is true because after Q the head moves left to S and then reverses to move right to P.
The statement 'R is serviced before P' is true (R is first, P is last).
Therefore the false statement is: 'Q is serviced after S, but before T'.