A disk has cylinders 0 to 199. The head is at cylinder 70 and the request queue is 120, 35, 180, 10, 95, 150, 60. The head is moving towards higher cylinders. What is the total head movement under SCAN, which goes to the end of the disk before reversing?
Correct answer A. 318 cylinders
Explanation
SCAN serves 95, 120, 150 and 180 on the way up and continues to cylinder 199, a movement of 129. It then reverses and serves 60, 35 and 10, moving 199 - 10 = 189. The total is 129 + 189 = 318 cylinders. LOOK reverses at 180 instead and would need only 280. SCAN is also called the elevator algorithm.
Answer checked How we write and check questions
Discussion