Question

Suppose a disk system has 200 cylinders, numbered 0 to 199. Assume that the read / write head is at cylinder 63.
Determine the order of head movement for each algorithm to satisfy the following stream of requests received in the following order:
Queue:   100, 175, 51, 133, 8, 140, 73, 77
Head: 63

-Draw head movement for Shortest Seek Time First (SSTF) algorithm.

-Calculate the total head movement for SSTF algorithm

-Draw head movement for SCAN algorithm. *The disk arm is moving downward

-Calculate the total head movement for SCAN algorithm

Answer 1

Hi,

i am glad to help you with this question.

This question consists of four parts . The question contains following information.

Queue:   100, 175, 51, 133, 8, 140, 73, 77
Head: 63

First part ) we have to draw the sstf algorithm head movements.

In Shortest Seek Time First (SSTF) algorithm, the request which is the closest to the current head position is serviced first .

IN the image as shown follows sstf algorithm as per that the current head position is at 63.

so the head moves to 73 as the distance from 63 to 73(i.e 10) is less than the distance from 63 to 51(i.e 12) therefore the head moves to 73 first then to 77 and so on .i have attached an image to show the head movements .

Second part ) this part needs all the head movements in sstf algorithm.

this has all the head movements of the queue.

the image attached shows the total head movements i.e sum of all the head movements .

The total head movements is 279.

Third part) This part heads the execution of the jobs using scan algorithm.

According to the scan algorithm , the head moves in one direction which the nearest to down and gives services and then to the opposite direction .when it moves in the opposite direction service is given to the jobs which have not received the service before.

The image attached shows the head movements , the head moves to zero because it is closest to the down and services all the jobs in the path and then moves towards 175 and serves the jobs in between the path.

Fourth part) this part needs the total head movements of the scan algorithm. which the sum of head movements .

The total head movements is 238.

Thank you.

I hope i was able to help you .If you any queries please be comfortable to ask in the comments i will be happy to help.

:) .