Question 8208 – Disk-scheduling

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.

Correct Answer: A

Question 10 Explanation:

SCAN:

∴ Total head movements,
= 10 + 10 + 10 + 10 + 10 + 55 + 20 + 5 + 10
= 140
SSTF:

∴ Total head movements
= 5 + 15 + 10 + 10 + 65 + 5 + 10 + 10
= 130
∴ Additional distance that will be traversed by R/W head is
= 140 – 130
= 10