Question

Suppose that a disk drive has 5000 cylinders, numbered 0 to 4999. The drive is currently serving a request at cylinder 143, and the previous request was at cylinder 125. The queue of pending requests, in FIFO order, is 86, 1470, 913, 1774, 948, 1509, 1022, 1750, 130 Starting from the current head position, what is the total distance (in cylinders) that the disk arm moves to satisfy all the pending requests, for each of the following disk-scheduling algorithms? a. FCFS b. SSTF c. SCAN d. LOOK e. C-SCAN

Answer 1

a. The FCFS schedule is 143, 86, 1470, 913, 1774, 948, 1509, 1022, 1750,130. The total seek distance is 7081.

This is the first come, first serve schedule

b. The SSTF schedule is 143, 130, 86, 913, 948, 1022, 1470, 1509, 1750,1774. The total seek distance is 1745.

Shortest seek time first scheduling

c. The SCAN schedule is 143, 913, 948, 1022, 1470, 1509, 1750, 1774,4999, 130, 86. The total seek distance is 9769.

d. The LOOK schedule is 143, 913, 948, 1022, 1470, 1509, 1750, 1774, 130, 86. The total seek distance is 3319.

e.The C-SCAN schedule is 143, 913, 948, 1022, 1470, 1509, 1750, 1774,4999, 86, 130. The total seek distance is 9813