On Jun 29, 2006, at 8:41 AM, David Robillard wrote:
Well, there are two issues here: access time (rpm) and storage capacity (GB). The access time deals with rotational speed of the drives (rpm) while storage capacity (GB) does not care how fast the drive spins.
There is a third and that is bit density. The reason that that is important is that it can compensate for a slower drive (rotational speed). If a fast drive with lower bit density has to rotate X rotation to get to the data, a higher bit density drive will usually have to rotate something less than X because the data is more dense. In simple terms (these numbers are made up to illustrate this and have no bearing on real numbers except that the concept holds: a fast RPM with lower bit density might have 1GB per cylinder and hence say 2/3 of a rotation might be needed to get data X. A higher density drive might have 6GB per cylinder so needs only, say 1/9 of a slower rotation to get to the same data). This was amply illustrated by some 500GB SATA benchmark I read that had it equaling some much faster RPM drives for access time with much lower bit density. Other factors play in here as well but hopefully you get the idea.
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