2. Disk scheduler fun

This question examines a disks internal scheduler as a black box, to see if we can learn anything about its behavior. The inputs we give to the disk are a bunch of requests; the outputs we observe are the order in which the requests are serviced. For example, we might issue requests to blocks 0, 100, and 200 at the same time. The disk might decide to service 100, then 200, and then 0, depending on its internal scheduling algorithm.

We also know some details of the disk. It has one surface with 100 tracks, each of which has 100 sectors. The outer track (track=0) contains sectors 0...99, the next track (track=1) contains sectors 100...199, and so forth.

In this problem, each question specifies a workload sent to a particular disk; you are then asked to determine the possible disk scheduling policy based on the order in which the requests are completed. In other words, what can you say about the algorithm in use? What about the initial state of the disk, before the requests arrived?

(a) Disk Model A Requests: 0, 1, 2, 3, 4, 5 Completed in order: 0, 1, 2, 3, 4, 5 What do you think Disk As scheduler is doing? What might the initial state of the disk have been?

(b) Disk Model B Requests: 0, 500, 200, 400, 300, 100 Completed in order: 0, 100, 200, 300, 400, 500 What do you think Disk Bs scheduler is doing? What might the initial state of the disk have been?

(c) Disk Model C Requests: 0, 500, 200, 400, 300, 100 Completed in order: 200, 300, 400, 500, 100, 0 What do you think Disk Cs scheduler is doing? What might the initial state of the disk have been?

(d) Disk Model D Requests: 0, 50, 110, 600 Completed in order: 0, 110, 50, 600 What do you think Disk Ds scheduler is doing? What might the initial state of the disk have been?

(e) Could these disks (A through D) actually be using the same scheduler? If so, why? If not, why not?