self dout

Question

Consider a disk system with 101(0 to 100) cylinders. The requests to access cylinders occurs in sequence as:
15, 75, 49, 65, 93, 80, 10
If the head is currently at 60. What is the additional distance that will be traversed by head when C-SCAN algorithm is used compared to SCAN algorithm is ________. (Assume SCAN algorithm head moves towards 100 when it starts execution).

in C-scan algorithm  what is distance traversed by head  (100-60)+(49-0) is it true or

(100-60)+100+(49-0) this is true 

Comments

@ ADITYA CHAURASIYA 5 Is it $39$ extras?

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ans is given 59

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Yes ! its 59 , Earlier I was not including the time of reach $0$ from last and discover that C_SCAN works like that here is my work :

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Read/Write head is initially at $60.$

Sequence of Cylinders for accessing : $\{10,15,49,65,75,80, 93\}$

For Scan : 

 $\large \Rightarrow$ $|60-65|+|65-75|+|75-80|+|80-93|+|93-100|+|100-49|+|49-15|+|15-10|$

 $\large \Rightarrow$ $ 130$

For C-scan :

 $\large \Rightarrow$ $|60-65|+|65-75|+|75-80|+|80-93|+|93-100|+|100-0|+|0-10|+|10-15|+|15-49|$

 $\large \Rightarrow $ $189$

So difference = $189-130=59$