Question

Disk requests come in to the disk driver for cylinders 10, 22, 20, 2, 40, 6, and 38, in that order. A seek takes 6 msec per cylinder. How much seek time is needed for (a) First-come, first served. (b) Closest cylinder next. (c) Elevator algorithm (initially moving upward). In all cases, the arm is initially at cylinder 20.

Answer 1

Explanation:

The order for FCFS is: 20->10->22->20->2->40->6->38.

Distance is

10+12+2+18+38+34+32 = 146

cylinders, so time is

146* 6 = 876 sec.

The order for elevator is:

20->20->22->38->40->10->6->2.

Distance is

0+2+16+2+30+4+4 = 58

cylinders, so time is

58 * 6 =348 msec.

The order for CCN is:

20->20->22->10->6->2->38->40.

Distance is

0+2+12+4+4+36+2 = 60

cylinders, so time is 60 * 6 =360 msec.