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Areal Density

Areal density, sometimes also (imprecisely) called bit density or even just density, refers to the amount of data that can be stored in a given amount of hard disk platter space. It is one of the most important indicators of overall hard disk performance, though one that outside the PC enthusiast community is sadly under-discussed. If you do not understand what areal density is about, I would advise that you read this operation page discussing it in detail before continuing with this page.

Areal density is a two-dimensional measure calculated by multiplying two linear measures: recording density (bit density) and track density. The result is measured in bits per square inch (BPSI). Since densities today are in the billions of bits per square inch, the most commonly seen unit is "Gbits/in2". Sometimes the two measures that comprise areal density, are specified separately; other data sheets don't show these components individually. It's much better to be able to evaluate the numbers separately, since they are very different in terms of how they reflect aspects of performance.

Areal density is strongly correlated to the transfer rate specifications of a drive. The higher the drive's areal density, in general, the higher its transfer rates will be, however, most of the improvement in transfer rate is due to increases in bit density, not track density. (When more bits are in a given length of track, the heads will read more data in a unit of time, assuming the spindle speed is constant.) If drive "A" has an areal density 5% lower than that of drive "B", but its bit density is 10% higher, it will have a higher transfer rate than drive "B".

Both bit density and track density have an impact on positioning performance. Increases in either one allow the data on the hard disk to be stored physically closer together on the disk. This reduces the distance that the read/write heads must seek to find different files on the disk, slightly improving seek time. Do keep in mind though that the improvements here are relatively small compared to the impact areal density has on transfer rates. Also, improvements only in track density don't do a lot to improve performance.

Areal density specifications are usually maximum specifications; look for the magic "M word" near the spec. The areal density will only be this high in certain regions of the disk. Modern drives use zoned bit recording to allow the areal density not to vary too greatly over the surface of the platter, but density will still be higher or lower in different parts of the disk. See the full discussion of areal density for more on this.

There's also a "rough cut" areal density measure commonly used when talking about hard drives or comparing one generation of drives to another. Often, the total formatted capacity of the disk will be divided by the number of platters, and the density of the drive discussed in terms of "GB per platter". For example, the 30 GB Maxtor DiamondMax Plus 40 is a three-platter drive; it's rough density then is 10 GB/platter, and that applies to all the members of that family. The IBM GXP75 family is 15 GB/platter, and so on.

This is a convenient short-hand and is useful when discussing drives, just keep in mind its limitations. For starters, it's rather crude, so it's only good for contrasting different generations of drives with big differences in density. Second, implied in the "GB/platter" measure is the size of each platter. A 10 GB/platter drive with 2.5" platters has much higher density than a 10 GB/platter drive using 3.5" platters. Also, some drives use only one side of one of their platters; the 15 GB DiamondMax Plus 40 for example uses two platters but only three of the four surfaces, so it is still a 10 GB/platter drive, not 7.5 GB/platter. (A better measure would be "GB per surface, but nobody seems to use that since most drives use both sides of each platter.)

The primary factors that influence areal density specifications are those that relate to data and recording: this means that all the factors discussed in this section are relevant. It is also influenced by the design and speed of the spindle motor; faster motors may require density to be reduced for reliability reasons.

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