# GATE - 2018 | OS | Consider a storage disk with 4 platters (numbered as 0, 1, 2 and 3)

## Question:

Consider a storage disk with 4 platters (numbered as 0, 1, 2 and 3), 200 cylinders (numbered as 0, 1, … , 199), and 256 sectors per track (numbered as 0, 1, … 255). The following 6 disk requests of the form [sector number, cylinder number, platter number] are received by the disk controller at the same time:

[120, 72, 2], [180, 134, 1], [60, 20, 0], [212, 86, 3], [56, 116, 2], [118, 16, 1]

Currently head is positioned at sector number 100 of cylinder 80, and is moving towards higher cylinder numbers. The average power dissipation in moving the head over 100 cylinders is 20 milliwatts and for reversing the direction of the head movement once is 15 milliwatts. Power dissipation associated with rotational latency and switching of head between different platters is negligible.

The total power consumption in milliwatts to satisfy all of the above disk requests using the Shortest Seek Time First disk scheduling algorithm is ______ .

 A 85 B 86 C 87 D 88

## Solution:

### Option (A) is Correct

From the question we are getting
→ A storage disk - 4 platters(0, 1, 2 & 3), Cylinder - 200 (0, 1, …, 199) , 256 sector per track.
In the above question the disk requests are given in the form of ⟨sector no, cylinder no, platter no⟩.
In SSTF (Shortest Seek Time First) Disk Scheduling algo, requests having shortest seek time are executed first.
So, the seek time of every request is calculated in advance in queue and then they are scheduled according to their calculated seek time. Head is positioned at 80 and moving towards higher cylinder no.

Head Movement in SSTF = (86 – 80) + (86 – 72) + (134 – 72) + (134 – 16) = 200
P1: Power dissipated by 200 movements = 0.2 * 200 = 40 mW
Power dissipated in reversing Head direction once = 15 mW
No. of times Head changes its direction = 3 P2: Power dissipated in reversing Head direction = 3 * 15 = 45 mW
Total power consumption is P1 + P2 = 85 mW

Thank You