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Suppose that a disk drive has $5000$ 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 each of the following disk – scheduling algorithms?

FCFS
SSTF
SCAN

  1. $7081, \ 1745, \ 9769$
  2. $7081, \ 1745, \ 7081$
  3. $1745, \ 1004, \ 9600$
  4. $9769, \ 1745, \ 7081$
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The total seek time is sum of difference between successive head positions.
 A  The FCFS schedule is 143, 86, 1470, 913, 1774, 948, 1509, 1022, 1750, 130. The total seek distance  is 7081.

 B  The SSTF schedule is 143, 130, 86, 913, 948, 1022, 1470, 1509, 1750, 1774. The total seek distance is 1745.

 C  The SCAN schedule is 143, 913, 948, 1022, 1470, 1509, 1750, 1774, 4999, 130, 86. The total seek distance is 9769 .
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4 Comments

Ok so it means, to decide the direction in the question they will directly give the direction or

If ( previous request < current req)  then right direction

If ( previous request > current req)  then left direction

Right sir??
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Yes I think so @Shubhanshu
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if there is a senior around,should we attempt questions this big in GATE examination ?.It took me 8-9 min to solve it
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