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## Find the capacity and the rotational latency of the hard drive / How to calculate the hard disk capacity and its rotational latency?

#### Question:

1) Suppose that we have a hard drive with the following characteristics. It has 10,000 cylinders, 10 tracks per cylinder, 60 sectors per track, and 500 bytes per sector. The hard drive spins at 7200 rpm (rotations per minute) and has an average seek time of 10 ms. Which of the following combination is the right i) capacity and ii) rotational latency of the hard drive?

(a) 2 GB and 3.17 ms
(b) 3 GB and 4.17 ms
(c) 4 GB and 5.17 ms
(d) 1 GB and 2.17 ms

b) 3 GB and 4.17 ms

Discussion:

Given,
Number of cylinders                      -        10,000
Number of tracks per cylinder      -        10
Number of sectors per track          -        60
Number of bytes per sector            -        500
Hard drive spinning speed           -        7200 rpm

i) Capacity of the hard drive

Capacity = cylinders * tracks per cylinder * sectors per track * bytes per sector
= 10000 * 10 * 60 * 500 = 3,000,000,000 bytes
= 3 GB

ii) Rotational latency of the hard drive

Time taken for one rotation = 1/RPM of the hard drive
= 1/7200 per minute
= (1/7200) * 60 = 0.0083 seconds
= 8.33 ms
Rotational latency = Half of the time consumed for one full rotation
= ½ * 8.3 ms = 4.17 ms