Operating System Concepts, 7th Edition, Q12.2

Question

Suppose that a disk drive has 5,000 cylinders, numbered 0 to 4999. The
drive is currently serving a request at cylinder 143, and the previous
request was at cylinder 125. The queue of pending requests, in FIFO
order, is:

86,1470, 913, 1774, 948, 1509, 1022,1750,130

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 C-SCAN?

Comments

9985?

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Yes I think that is right considering the jump.

Answer 1

9985.....

Comments

https://gateoverflow.in/109286/virtual-gate-operating-system-scan-algorithm
read this for scan

Answer 2

The requests in sorted order are:

83, 130, 913, 948, 942, 1022, 1470, 1509, 1750, 1771

The head is currently position at 143 and previous head position was 125. So, its moving to right extreme.

So, it moves to 4999. That counts to 4999 - 143 = 4856

Then it jumps to 0 and starts serving in same direction as before.

So, 130 seeks after jumping.

So, total head movements = 4856 + 130 = 4986

REF: http://www.cs.iit.edu/~cs561/cs450/disksched/disksched.html

Comments

What part you don't understand? I think it is totally reasonable to count that head movement as it is actual movement of head.

Anways a cool video! :D

https://youtu.be/BlB49F6ExkQ

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Does it lift up the head and wraps around or travels serially in reverse ordr?

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Are you confusing cylinder with sectors? Disk will rotate only in one direction we just move head from inner track/cylinder track to outer cylinder or vice versa.