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Previous Year Question (PYQs)
Consider a disk system with $100$ cylinders. The requests to access the cylinders occur in the following sequence:
$4,34,10,7,19,73,2,15,6,20$
Assuming that the head is currently at cylinder $50$, what is the time taken to satisfy all requests if it takes $1$ ms to move from one cylinder to adjacent one and the shortest seek time first policy is used?
Solution
Using shortest seek time first policy, the head always moves to the nearest request.
Initial head position is
$50$
Request sequence in SSTF order will be:
$50 \to 34 \to 20 \to 19 \to 15 \to 10 \to 7 \to 6 \to 4 \to 2 \to 73$
Now, total head movement is
$|50-34|+|34-20|+|20-19|+|19-15|+|15-10|+|10-7|+|7-6|+|6-4|+|4-2|+|2-73|$
$=16+14+1+4+5+3+1+2+2+71$
$=119$
Since $1$ cylinder movement takes $1$ ms,
Total time taken is
$119$ ms
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