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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:
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
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
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.
Increasing the areal density of disks is a difficult task that requires many technological advances and changes to various components of the hard disk. As the data is packed closer and closer together, problems result with interference between bits. This is often dealt with by reducing the strength of the magnetic signals stored on the disk, but then this creates other problems such as ensuring that the signals are stable on the disk and that the read/write heads are sensitive and close enough to the surface to pick them up. In some cases the heads must be made to fly closer to the disk, which causes other engineering challenges, such as ensuring that the disks are flat enough to reduce the chance of a head crash. Changes to the media layer on the platters, actuators, control electronics and other components are made to continually improve areal density. This is especially true of the read/write heads. Every few years a read/write head technology breakthrough enables a significant jump in density, which is why hard disks have been doubling in size so frequently--in some cases it takes less than a year for the leading drives on the market to double in size.