Suppose the following disk request sequence (track numbers) for a disk with 100 tracks is given: 45, 20, 90, 10, 50, 60, 80, 25, 70. Assume that the initial position of the R/W head is on track 50. The additional distance that will be traversed by the R/W head when the Shortest Seek Time First (SSTF) algorithm is used compared to the SCAN (Elevator) algorithm (assuming that SCAN algorithm moves towards 100 when it starts execution) is _____________ tracks.

Given requests: $45, 20, 90, 10, 50, 60, 80, 25, 70$ Initial head position: $50$

1. SSTF (Shortest Seek Time First):

• From $50$, request $50$ is serviced first (distance $= 0$).

• From $50$, the closest pending request is $45$ (distance $= |50-45| = 5$).

• From $45$, candidates are $25$ (distance $20$) and $60$ (distance $15$). The head moves to $60$ (distance $= 15$).

• From $60$, closest is $70$ (distance $= 10$).

• From $70$, closest is $80$ (distance $= 10$).

• From $80$, closest is $90$ (distance $= 10$).

• From $90$, remaining requests are $25, 20, 10$. Head moves to $25$ (distance $= |90-25| = 65$).

• From $25$, moves to $20$ (distance $= 5$).

• From $20$, moves to $10$ (distance $= 10$).

Total SSTF seek distance:

2. SCAN (Elevator) moving towards 100:

• Head starts at $50$ and moves upwards servicing $50, 60, 70, 80, 90$.

• Distance moving upwards to the highest request ($90$): $|90 - 50| = 40$ tracks.

• Head then reverses direction and services remaining requests downward: $45, 25, 20, 10$.

• Distance moving downwards from $90$ to $10$: $|90 - 10| = 80$ tracks.

Total SCAN seek distance:

3. Additional distance: