GATE CS 2014 Set 1 Q19 — I/O Scheduling

Under the Shortest-Seek Time First (SSTF) scheduling algorithm, the disk arm always services the pending request closest to its current position.

The initial position of the disk arm is 100, and the queue of pending requests is: {30, 85, 90, 100, 105, 110, 135, 145}.

We trace the servicing order step-by-step:

1. At cylinder 100: The request at 100 is at distance $0$.
   $ightarrow$ 1st request serviced: 100 (Current position: 100)
2. At cylinder 100: The closest remaining requests are 105 (distance $5$) and 90 (distance $10$).
   $ightarrow$ 2nd request serviced: 105 (Current position: 105)
3. At cylinder 105: The closest remaining requests are 110 (distance $5$) and 90 (distance $15$).
   $ightarrow$ 3rd request serviced: 110 (Current position: 110)
4. At cylinder 110: The closest remaining requests are 90 (distance $20$) and 135 (distance $25$).
   $ightarrow$ 4th request serviced: 90 (Current position: 90)

Thus, the request for cylinder 90 is serviced after servicing exactly 3 requests (namely 100, 105, and 110).