Question

We have a disk drive with 1250 cylinders, which are numbered 0 to 1249. The drive is currently servicing a request at cylinder 1050, and the previous request was at cylinder 800. The queue of pending requests, in FIFO order, is:

900, 1090, 4, 55, 37, 1200, 56, 890, 450, 1000, 1100

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) SSTF

(b) Scan

(c) Look

(d) C-Scan

Answer 1

The total distance that the disk arm moves to satisfy all the pending requests depends on the disk scheduling algorithm used.

For this particular scenario, the total distances for the SSTF, Scan, Look, and C-Scan algorithms are 3078, 2883, 2848, and 2836 cylinders, respectively.

Step-by-step explanation:

The total distance that the disk arm moves to satisfy all the pending requests depends on the disk scheduling algorithm used.

Let's calculate the total distance for each algorithm:

(a) SSTF:

1. Start at cylinder 1050 and move to cylinder 1000 (distance = 50)
2. Move to cylinder 1090 (distance = 90)
3. Move to cylinder 1200 (distance = 110)
4. Move to cylinder 1100 (distance = 100)
5. Move to cylinder 890 (distance = 210)
6. Move to cylinder 900 (distance = 10)
7. Move to cylinder 800 (distance = 100)
8. Move to cylinder 55 (distance = 745)
9. Move to cylinder 56 (distance = 1)
10. Move to cylinder 450 (distance = 394)
11. Move to cylinder 4 (distance = 446)
12. Move to cylinder 37 (distance = 33)
13. Move to cylinder 5 (distance = 32)
14. Move to cylinder 1050 (distance = 1045)

The total distance for SSTF is 3078 cylinders.

(b) Scan:

1. Start at cylinder 1050 and move to the highest numbered cylinder (1249) (distance = 199)
2. Move to cylinder 0 (distance = 1249)
3. Move to cylinder 4 (distance = 4)
4. Move to cylinder 5 (distance = 1)
5. Move to cylinder 37 (distance = 32)
6. Move to cylinder 55 (distance = 18)
7. Move to cylinder 56 (distance = 1)
8. Move to cylinder 450 (distance = 394)
9. Move to cylinder 800 (distance = 350)
10. Move to cylinder 890 (distance = 90)
11. Move to cylinder 900 (distance = 10)
12. Move to cylinder 1000 (distance = 100)
13. Move to cylinder 1090 (distance = 90)
14. Move to cylinder 1100 (distance = 10)
15. Move to cylinder 1200 (distance = 100)
16. Move to cylinder 1050 (distance = 150)

The total distance for Scan is 2883 cylinders.

(c) Look:

1. Start at cylinder 1050 and move to the highest numbered cylinder lower than the previous request, which is 900 (distance = 150)
2. Move to cylinder 890 (distance = 10)
3. Move to cylinder 800 (distance = 90)
4. Move to cylinder 55 (distance = 745)
5. Move to cylinder 56 (distance = 1)
6. Move to cylinder 450 (distance = 394)
7. Move to cylinder 4 (distance = 446)
8. Move to cylinder 37 (distance = 33)
9. Move to cylinder 5 (distance = 32)
10. Move to cylinder 1050 (distance = 1045)

The total distance for Look is 2848 cylinders.

(d) C-Scan:

1. Start at cylinder 1050 and move to the highest numbered cylinder (1249) (distance = 199)
2. Move to cylinder 0 (distance = 1249)
3. Move to cylinder 4 (distance = 4)
4. Move to cylinder 5 (distance = 1)
5. Move to cylinder 37 (distance = 32)
6. Move to cylinder 55 (distance = 18)
7. Move to cylinder 56 (distance = 1)
8. Move to cylinder 450 (distance = 394)
9. Move to cylinder 800 (distance = 350)
10. Move to cylinder 890 (distance = 90)
11. Move to cylinder 900 (distance = 10)
12. Move to cylinder 1000 (distance = 100)
13. Move to cylinder 1050 (distance = 50)
14. Move to cylinder 1200 (distance = 150)
15. Move to cylinder 1100 (distance = 100)
16. Move to cylinder 1090 (distance = 10)
17. Move to cylinder 1050 (distance = 40)

The total distance for C-Scan is 2836 cylinders.