A disk has 4000 cylinders, each with 8 tracks of 512 blocks. A seek takes 1 msec

ID: 3579188 • Letter: A

Question

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?

Firstly, the access time per block is calculated as follows:

5 msec + (10 msec + 0.02 msec) = 15.02 msec

Thus for 100 blocks the access time will be as :

15.02 msec * 100 = 1.502 sec

For the second part of the question, the access time per block is calculated as follows:

(0.1 msec * 2) + (10 msec + 0.02 msec) = 10.22 msec

For 100 blocks, the access time will be:

10.22 msec * 100 = 1.022 sec

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