A disk has 4000 cylinders, each with 8 tracks of 512 blocks. A seek takes 1 msec per cylinder moved. If no attempt is made to put the blocks of a file close to each other, two blocks that are logically consecutive (i. e. follow one another in the file) will require an average seek, which takes 5 msec. If, however, the operating system makes an attempt to cluster related blocks, the mean interblock distance can be reduced to 2 cylinders and the seek time reduced to 100 microsec. How long does it take to read a 100 block file in both cases, if the rotational latency is 10 msec and the transfer time is 20 microsec per block

The answer is below

Explanation:

a)

If the blocks of a file are not put together, that is for nonadjacent block of files to each other, the time (t) taken to read a 100 block file is:

t = (average seek + rotational latency + transfer time) * 100 block

Average seek = 5 msec, rotational latency = 10 msec and transfer time = 20 microsec = 0.02 msec

t = (5 + 10 + 0.02) * 100 = 1502 msec

b)

If the blocks of a file are put together, that is for adjacent block of files to each other, the time (t) taken to read a 100 block file is:

t = (seek time * mean interblock distance + rotational latency + transfer time) * 100 block

seek time = 100 microsec = 0.1 msec, rotational latency = 10 msec and transfer time = 20 microsec = 0.02 msec, mean interblock distance = 2 cylinders

t = (0.1*2 + 10 + 0.02) * 100 = 1022 msec

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