Consider a disk queue with I/O requests on the following cylinders in their…

Solution: Compute total head movement using the SCAN (elevator) algorithm. Assume cylinders are numbered 0 to 149 (total 150). The head starts at 23 and is moving toward decreasing cylinder numbers (toward 0). SCAN goes to the end before reversing.

• Serve requests below 23 in descending head path order: 15, 12, 10, 6.

• Distances for this pass: 23 → 15 = 8, 15 → 12 = 3, 12 → 10 = 2, 10 → 6 = 4. Sum = 8 + 3 + 2 + 4 = 17.

• Continue to the start of the disk (cylinder 0): 6 → 0 = 6. Total distance for the downward pass = 17 + 6 = 23.

• Reverse direction and serve remaining requests in increasing order: 34, 44, 45, 54, 73, 97, 110, 128.

• Distances on the upward pass: 0 → 34 = 34, 34 → 44 = 10, 44 → 45 = 1, 45 → 54 = 9, 54 → 73 = 19, 73 → 97 = 24, 97 → 110 = 13, 110 → 128 = 18. Sum = 34 + 10 + 1 + 9 + 19 + 24 + 13 + 18 = 128.

• SCAN continues to the far end of the disk (cylinder 149) on this pass: 128 → 149 = 21.

Total head movement calculation:

Downward pass total = 23; upward pass total = 128; final move to cylinder 149 = 21. Grand total = 23 + 128 + 21 = 172 cylinders.

Answer: 172