I saw the question bellow in a Computer Organization Practice Book for University exams.

Its given that, a magnetic disk has the following specifications:

Number of tracks = 1024
Number of sectors/track = 512
Bytes/Sector = 512
Disk Rotation Speed = 7200 RPM
Seek time = 9ms

The question given: What is the data transfer rate?
The answer given: 30GBPS
My answer: 30MBPS

My method of doing it:

Rotation speed: 7200RPM
So, 7200 rotations in 60 sec
1 rotation in 60/7200 sec.

Now, in 1 rotation only 1 track is read
1 track = 512 sectors = 512*512 bytes.

So, in 1 rotation 512*512 bytes is read.
i.e. In 1 Rotation 256 KB is read [ since, 512*512 bytes =256*1024 bytes ]

Thus, In 60/7200 sec 256 KB is read
In 1 sec 256*7200/60 KB is read
i.e 256*120 KB = 30 * 1024 KB = 30 MB

So, data transfer rate according to me: 30 MBPS.

But the explanation given in the book considered that in 1 rotation all tracks are read,
i.e In 1 rotation 1024*512*512 bytes is read and this is how they are getting the answer as 30 GBPS.

Kindly help me out here. Am I right? Or is the book correct?

  • Wow, 30 GB in 1 second. That's even faster than PCI-Express connected SSD cards. – ZippyV Jan 12 '14 at 17:07
  • hahaha, fortunately it is just a university question :P But, am I correct? – user2738133 Jan 12 '14 at 17:13
  • 30 MB/sec sounds about right – ZippyV Jan 12 '14 at 17:22

As a technical point, a hard drive is a record shaped. The outer track has significantly more data in it than the inner track. The outer track of a hard drive maybe 80mb/s but the inner track could be 40.

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