Question

Consider the disk request sequence for a disk with 100 tracks (range 0-99) 82, 36, 13, 23, 64, 9, 78. Head
pointer starting at 19 and moving in left direction. Find the number of head movements in cylinders using
a) SSTF scheduling (5 marks)
b) SCAN (5 marks)​

Answer 1

Answer:

Shortest Seek Time First Representation. Total Head Movement = (100-90) + (90-60) + (60-50) + (50-25) + (135-25) + (190-135) = 240 tracks. The major disadvantage of SSTF disk scheduling algorithm is that some requests have to wait for a long time if new requests with shorter seek time keep arriving but the advantage is that the throughput is higher as compared to FIFO disk scheduling algorithm and produces less head movements. Scan Disk Scheduling Algorithm (SCAN): In Scan disk scheduling algorithm, the read/write head starts from one end and move towards the other end and servicing requests as it reaches each track until it reaches to other end of the disk. The direction of the read/write head reverses after reaching at the other end and servicing continues. In this way, the read/write head continuously swings from end to end [6]. Let us consider the same track requests in the disk queue to compute the total head movement of the read/write head and let us again assume that the current read/write head position is at location 100 for a 200 track disk (0-199). Track Requests: 25, 90, 135, 50, 190 and 60. In Scan disk scheduling algorithm, the read/write head initially move from current location 100 to 90, then 90 to 60, then 60 to 50, then 50 to 25, then 25 to 0 and then to the other direction from 0 to 135 and at last 135 to 190. It services all the requests at either side firstly then moves the other way that's why it is sometimes called as elevator algorithm