I think the problem is not really about computers or hard disks, if it is asked like in the exam question. It is more about basics.
- If you have 7200 RPM, that means the disk will rotate
(7200/60) = 120
times per second
- Each time the disk does a whole rotation, you are theoretically able to read 1 complete track. That means reading 1 track takes
(1/120)s
So, now the question of the 1ms seek time:
- If you can do a continuous read, then you have to add the seek time only once. So you have
(1/120)s
for each track and the one-time addition of seek time. This seems like an oversimplification.
- More realistically: Assuming the question implies that you need an additional
1ms (1/1000)s
to switch from one track to the next, you can not read 120 tracks each second as the rotation alone would suggest.
- Taking the rotation and the seek time into account, each track will take
(1/1000 + 1/120)s
to read.
Note:
As @sawdust correctly points out in the comments, this answer uses multiple simplifications. This is due to the fact that no more information was given in the question and I tried to work with the basics. If the question is more complex (or dealing with actual hardware, instead of being an exam question), these factors have to be accounted for:
- The disk might have multiple (say,
N
) platters, so the 20000 tracks can be distributed over the N
platters. In general, a platter can be used on both sides.[1] So the 20000 tracks each of the 2N
Read-/Write-Heads in the drive are not 20000 cylinders, but rather 20000/(2N)
cylinders. That might enable to read 2N
tracks at the same time. Then, reading 2N
tracks takes (1/120)s
.
- The seek time should not be fixed to
1ms
. It is usually a combination of head seek time, where the head is moved to the correct cylinder (which might not be always constant depending on where you are on the drive), PLUS the rotational latency, which means the disk itself has to rotate until the head is above the correct sector. On average, the rotational latency might be simplified to be half of the time it takes the disk to do 1 whole revolution. So the rotational latency might be around (1/240)s
for each new cylinder.
- Real drives can have certain latency issues when facing problems to read the data correctly. This happens when blocks are damaged or otherwise difficult to read and by the drive tries to read the sectors with different head positionings and parameters to increase the chance of retrieving any data. The resulting latencies can sometimes even be observed with new retail drives and must be taken into account when planning a real system that depends on timely disk reads. So you can not always assume that a track will be completely read after 1 single revolution. The problem can become very bad, so more enterprise drives offer features such as TLER to limit the allowed time to retry reading a sector.[2]
[1]: Wikipedia: Cylinder-Head-Sector
[2]: Western Digital: Difference between Desktop edition (WD Blue, WD Green and WD Black) and RAID (Enterprise) edition drives