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Areal Density

Areal density, also sometimes called bit density, refers to the amount of data that can be stored in a given amount of hard disk platter "real estate". Since disk platters surfaces are of course two-dimensional, areal density is a measure of the number of bits that can be stored in a unit of area. It is usually expressed in bits per square inch (BPSI).

Being a two-dimensional measure, areal density is computed as the product of two other one-dimensional density measures:

• Track Density: This is a measure of how tightly the concentric tracks on the disk are packed: how many tracks can be placed down in inch of radius on the platters. For example, if we have a platter that is 3.74" in diameter, that's about 1.87 inches. Of course the inner portion of the platter is where the spindle is, and the very outside of the platter can't be used either.  Let's say about 1.2 inches of length along the radius is usable for storage. If in that amount of space the hard disk has 22,000 tracks, the track density of the drive would be approximately 18,333 tracks per inch (TPI).
• Linear or Recording Density: This is a measure of how tightly the bits are packed within a length of track. If in a given inch of a track we can record 200,000 bits of information, then the linear density for that track is 200,000 bits per inch per track (BPI). Every track on the surface of a platter is a different length (because they are concentric circles), and not every track is written with the same density. Manufacturers usually quote the maximum linear density used on each drive.

Taking the product of these two values yields the drive's areal density, measured in bits per square inch. If the maximum linear density of the drive above is 300,000 bits per inch of track, its maximum areal density would be 5,500,000,000 bits per square inch, or in more convenient notation, 5.5 Gbits/in2. The newest drives have areal densities exceeding 10 Gbits/in2, and in the lab IBM in 1999 reached 35.3 Gbits/in2--524,000 BPI linear density, and 67,300 TPI track density! In contrast, the first PC hard disk had an areal density of about 0.004 Gbits/in2!

Note: Sometimes you will see areal density expressed in a different way: gigabytes per platter (GB/platter). This unit is often used when comparing drives, and is really a different way of saying the same thing--as long as you are always clear about what the platter size is. It's easier conceptually for many people to contrast two units by saying, for example: "Drive A has 10 GB/platter and drive B has 6 GB/platter, so A has higher density". Also, it's generally easier to compute when the true areal density numbers are not easy to find. As long as they both have the same platter size, the comparison is valid. Otherwise you are comparing apples and oranges.

The linear density of a disk is not constant over its entire surface--bear this in mind when reading density specifications, which usually list only the maximum density of the disk. The reason that density varies is because the lengths of the tracks increase as you move from the inside tracks to the outside tracks, so outer tracks can hold more data than inner ones. This would mean that if you stored the same amount of data on the outer tracks as the inner ones, the linear density of the outer tracks would be much lower than the linear density of the inner tracks. This is in fact how drives used to be, until the creation of zoned bit recording, which packs more data onto the outer tracks to exploit their length. However, the inner tracks still generally have higher density than the outer ones, with density gradually decreasing from the inner tracks to the outer ones.

Tip: Here's how you can figure this out yourself. A typical 3.5" hard disk drive has an innermost track circumference of about 4.75" and an outermost track circumference of about 11". The ratio of these two numbers is about 2.32. What this means is that there is 2.32 times as much "room" on the outer tracks. If you look at the number of sectors per track on the outside zone of the disk, and it is less than 2.32 times the number of sectors per track of the inside zone, then you know the outer tracks have lower density than the inner ones. Consider the IBM 40GV drive, whose zones are shown in a table on this page on zoned bit recording. Its outermost tracks have 792 sectors; its innermost tracks 370. This is a ratio of 2.14, so assuming this drive uses tracks with the lengths I mentioned, it has its highest density on the innermost tracks.

There are two ways to increase areal density: increase the linear density by packing the bits on each track closer together so that each track holds more data; or increase the track density so that each platter holds more tracks. Typically new generation drives improve both measures. It's important to realize that increasing areal density leads to drives that are not just bigger, but also faster, all else being equal. The reason is that the areal density of the disk impacts both of the key hard disk performance factors: both positioning speed and data transfer rate. See this section for a full discussion of the impact of areal density on hard disk performance.

 This illustration shows how areal density works. First, divide the circle in your mind into a left and right half, and then a top and bottom half. The left half shows low track density and the right half high track density. The upper half shows low linear density, and the bottom half high linear density. Combining them, the upper left quadrant has the lowest areal density; the upper right and lower left have greater density, and the bottom right quadrant of course has the highest density. (I hope you like this illustration, because you don't want to know what I went through trying to make it... :^) )