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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 _____
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68 votes
68 votes

Shortest Seek Time First (SSTF), selects the request with minimum to seek time first from the current head position.

In the given question disk requests are given in the form of $\langle \text{sectorNo, cylinderNo, platterNo}\rangle$. 

Cylinder Queue $: 72, 134, 20, 86, 116, 16$

Head starts at $:80$

Total head movements in SSTF $= (86-80)+ (86-72)+ (134-72)+ (134-16)=200$

  • Power dissipated in moving $100$ cylinder $= 20\;\text{mW}$
  • Power dissipated by $200$ movements $($say $P1) = 0.2\ast 200= 40\;\text{mW}$
  • Power dissipated in reversing head direction once $= 15\;\text{mW}$
  • Number of times head changes its direction $= 3$
  • Power dissipated in reversing head direction $($say $P2)= 3\ast 15= 45\;\text{mW}$

Total Power Consumption is $P1+P2 = 85\;\text{mW}$

Hence, $85\;\text{mW}$ is the correct answer.

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7 votes

I have written the answer in best way possible in this image

As we reverse the direction of the head 3 times so 15mW power will be used thrice i.e. 45mW

20mW per 100 head movements i.e. 0.2 mW or 200kW for 1 head movement.

I hope the answer

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