Suppose a disk has 201 cylinders, numbered from 0 to 200. At some time the disk arm is at cylinder 100, and there is a queue of disk access requests for cylinders 30, 85, 90, 100, 105, 110, 135 and 145. If Shortest-Seek Time First (SSTF) is being used for scheduling the disk access, the request for cylinder 90 is serviced after servicing ________ number of requests.

In Shortest-Seek-Time-First (SSTF) scheduling, the request closest to the current head position is served next.

• Initial head position: $100$

• Request queue: $\{30, 85, 90, 100, 105, 110, 135, 145\}$

Step-by-step servicing:

1. At head position $100$, there is a request at cylinder $100$ (distance $= |100 - 100| = 0$). Cylinder 100 is serviced first.

2. Head is at $100$. Remaining requests: $\{30, 85, 90, 105, 110, 135, 145\}$.

   • Distance to $105 = 5$

   • Distance to $90 = 10$

   • Distance to $110 = 10$

   • Distance to $85 = 15$

   • Nearest is $105$. Cylinder 105 is serviced second.

3. Head is at $105$. Remaining requests: $\{30, 85, 90, 110, 135, 145\}$.

   • Distance to $110 = 5$

   • Distance to $90 = 15$

   • Distance to $85 = 20$

   • Nearest is $110$. Cylinder 110 is serviced third.

4. Head is at $110$. Remaining requests: $\{30, 85, 90, 135, 145\}$.

   • Distance to $90 = 20$

   • Distance to $135 = 25$

   • Distance to $85 = 25$

   • Nearest is $90$. Cylinder 90 is serviced fourth.

Thus, cylinder $90$ is serviced after servicing 3 requests (namely cylinders 100, 105, and 110).