Hardware Reference
In-Depth Information
Notice that sector 9 is longer than the others; this is to enable rotational speed differences between
drives, so that all the data can be written before running into the start of the track. Also notice that a
good portion of the disk surface isn't used because it is simply impractical to have the heads travel in
and out that far, and the difference in length between the sectors on the inner and outer tracks becomes
more of a problem.
Areal density has been rising steadily since the first magnetic storage drive (the IBM RAMAC,
featuring an areal density of 2Kb/sq. inch) was introduced in 1956. Density initially grew at a rate of
about 25% per year (doubling every four years); then in the early 1990s the rate accelerated to a
growth rate of about 60% per year (doubling every year and a half). The development and
introduction of MR heads in 1991, GMR heads in 1997, and AFC pixie dust media in 2001 (see the
next section) subsequently drove a further increase in the areal density growth rate to 100% annually.
The result of all this growth in density is amazing. In just slightly more than 50 years since the
RAMAC drive was introduced, the areal density of magnetic storage has increased more than 200
million fold, from 2Kb/sq. inch in the 1956 RAMAC (5MB of storage on 50 24-inch platters) to
beyond 500Gb/sq. inch in 3TB drives in 2010.
Current drives have used perpendicular recording techniques to go well past what was previously
considered the point at which the superparamagnetic effect takes place. This is an effect in which the
magnetic domains become so small that they are intrinsically unstable at room temperature.
Techniques such as perpendicular recording combined with extremely high coercivity media are
being employed to enable future magnetic storage densities of up to 1000Gb/sq. inch or more, but
beyond that, scientists and engineers might have to look toward other technologies. One such
technology being considered for the future is patterned media, in which a disk is preformatted with
magnetic domains that can be more tightly packed without interfering with each other. Another
possible future technology is holographic storage, in which a laser writes data three-dimensionally in
a crystal plate or cube.
Figure 8.13
shows how areal density has increased by a factor of more than 200 million times from
when magnetic storage was developed (1956 RAMAC) to the present.
