# How to calculate disk access rates?

Discussion in 'Hardware' started by dmurfitt, May 30, 2006.

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Can anyone help me with calculating disk access rates please?

Basically I've got that it's Seek time + Latency, but I can't seem to get to the access rate from the spin speed and the speed in which the heads can tranverse the cylenders.

Any help appreciated

2. ### JohnWillRetired Moderator

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The classic way it to simply benchmark them with an application like HDTune. There are a number of factors to consider, and you probably don't have all the data required.

You need to consider the time to read the data from all the cylinders, and you need to know where the number of sectors/track changes, typically there are a number of zones on the platters with different numbers of sectors/track. That's why speeds increase at the outside edge of the platters. You also have to consider the effect of the buffering, and the quality of the firmware managing the reading/writing.

What exactly is the reason for trying to calculate it?

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Thanks John, I wish it was that easy but I've got an exam tomorrow and all I will have it a calculator and pen and paper lol (sorry I should have mentioned that).

It's stuff like the disk spins at 10,000rpm and the heads can traverse 5 cylenders per microsecond, calculate the access time in milliseconds. The disk has 30800 cylenders.

4. ### JohnWillRetired Moderator

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Yes, you should have mentioned that.

Now you're getting into what is "access time" considered to be. Generally, it's 1/3 of a full seek, plus the rotational delay to read one or more sectors. However, in the academic world, it could be defined as almost anything. In the real world it's not as simple as 5cyl/uS, since in the real world there is acceleration and de-acceleration of the heads. They don't start and stop instantaneously.

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