Question

Suppose that a disk drive has 6,000 cylinders, numbered 0 to 5999. The drive is currently serving a request at cylinder 3150, and the previous request was at cylinder 1805 (Hint: this indicates the reading head’s moving direction).

The queue of pending requests, in FIFO order, is:

3511, 2332, 2800, 3192, 658, 1296, 1918, 1356, 5936, 2527

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

f. C-LOOK

Answer 1

Hi,

These all are disk-scheduling algorithms which will be helpful to process the requests coming.

1.FCFS: It is a very basic algorithm namely "First Come First Serve. "

Here the current request is 3150 and the next requests are: 3511, 2332, 2800, 3192, 658, 1296, 1918, 1356, 5936, 2527.

If we apply FCFS then the order in which requests will be served is same as the order in which requests are coming. 3150-->3511 --> 2332 --> 2800 --> 3192 --> 658  --> 1296  --> 1918 --> 1356  --> 5936  --> 2527.

Here total distance is calculated by the absolute difference between in each request.

total distance= (3511-3150)+(3511-2332)+(2800-2332)+(3192-2800)+(3192-658)+(1296-658)+(1918-1296)+(1918-1356)+(5936-1356)+(5936-2527)

=14745(verify calculations once)

2. SSTF : Stands for Shortest Seek Time First, the algorithm works like as follows: it checks for shortest seek time near by current request.

Currently request is at

3150 so the closest to this request is 392, continue like this and we end up like : 3150 -- > 3192 --> 2800 --> 2527 --> 2332 --> 1918 --> 1356 --> 1296 --> 658 --> 3511 --> 5936

and total distance= 42 + 392 + 273 + 195 + 414 + 562 + 60 + 638 + 2853 + 2425 =7854

3. SCAN: In this alogrithm disk moves into one direction first till and processes the requests coming in the way till the end of the disk then reverse the direction.

Here the order would be like : 3150 --> 3192(Since the previous request is 1805 so moving in high direction) --> 3511 --> 5936 --> 6000(End) || reverse() || --> 2800 --> 2527 --> 2332 --> 1918 --> 1356 --> 1296 --> 658. Calculate the difference and add them as mentioned above, that would the total distance

4.  C-SCAN : This is Circular SCAN, the only difference to SCAN algorithm is the requests will not be processed while reversing the direction. It'll reach the end then the requests will be processed. Below is the order. Please calculate the difference and add.

3150 --> 3192(Since the previous request is 1805 so moving in high direction) --> 3511 --> 5936 --> 6000(End) || reverse() || then go to starting from 6000 -->0 then 0 --> 658 --> 1296-->1356--> 1918 --> 2332 --> 2527 --> 2800

5. LOOK is just similar to SCAN but the only difference is the disk will not go till the end rather stops at last request then reverse the direction: Below is the order:

3150 --> 3192(Since the previous request is 1805 so moving in high direction) --> 3511 --> 5936 --> (Don't go till end rather stops here and reverse the direction) --> 2800 --> 2527 --> 2332 --> 1918 --> 1356 --> 1296 --> 658

6. C-LOOK: same as C-SCAN but not go to the end of the disk rather stops ar farthest request.

3150 --> 3192(Since the previous request is 1805 so moving in high direction) --> 3511 --> 5936 --> |stops| reverse() || then go to farthest request(658) not till 0|| 658 --> 1296-->1356--> 1918 --> 2332 --> 2527 --> 2800. Please calculate the differences and add as mentioned.

I Hope it is clear.

Thanks.