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Areal Density

The areal density of the drive, or more correctly of its platters, measures how many bits can be packed into each square inch of surface area. Areal density is discussed in detail here. Like spindle speed, areal density is an important characteristic that determines overall performance. It is, however, somewhat more difficult to determine. Most manufacturers don't talk a great deal about areal density, and usually only mention it (if at all) in their detailed specifications for a drive.

Areal density is important because, much like spindle speed, it impacts on both positioning time and data transfer. Areal density consists of two components, and each of these impacts on one of these areas:

• Track Density: The first component of areal density measures how closely spaced the tracks are. This impacts on positioning because the closer together the tracks, the less time that is required for seeks and cylinder switches. This is a relatively minor factor compared to the other factors below.
• Bit Density: The second component of areal density is bit density per track. This impacts directly on both data transfer, which is proportional to bit density, and also on positioning. The impact on positioning in this case is that fewer head switches and cylinder switches per megabyte must be done on a large read of a disk with a higher density, compared to a lower one. This is because there are more bytes per cylinder that can be read between switches--a higher density drive requires fewer cylinders to store a 1 MB file than a lower density drive does.

Note that some drives improve areal density by increasing only one of these factors, while others increase them both. Determining actual areal density figures for drives can be difficult, but you can use the readily available statistics of various drives to do rough sketch comparisons quite easily. One useful measurement that I have employed to compare the areal density of drives is capacity per surface. It is calculated as follows:

Capacity per surface = Total formatted capacity / Number of read-write heads

The number of read-write heads is usually equal to double the number of platters, but not always, since some drives use only a single head on one of the platters in the drive. Note that you need to know the real physical number of heads for the drive, not the logical geometry (which will usually state 16 heads regardless of the real number).

It is easy to compare the areal density of drives using this technique, and this tells you a lot about the expected performance of the drives. Let's take as an example a series of Western Digital Caviar hard drives, which have a convenient model numbering that tells us their approximate capacity and number of platters instantly (although not the number of read/write heads, which is double the number of platters with two exceptions):

As you can see, the density per surface increases with the newer, larger capacity drives. But you can also see that some drives have the same capacity, but much higher density. This is because they are newer generation drives, and the technology has advanced to allow them to fit the same capacity in a smaller number of surfaces. An AC21000 is going to provide faster data transfer than an AC31000 for this reason.

There is one other factor here as well. Generally speaking--not always, but usually--drives with higher density are newer, and therefore they generally have design improvements in other areas: newer controller electronics, etc., so they are even more likely to be better than the older drive overall (assuming all else is equal of course).

Next: External or System Performance Factors

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