Hardware Reference
In-Depth Information
Notice that sector 9 is longer than the others; this is to enable rotational speed differences
betweendrives,sothatallthedatacanbewrittenbeforerunningintothestartofthetrack.
Also notice that a good portion of the disk surface isn't used because it is simply imprac-
tical 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 ini-
tially grew at a rate of about 25% per year (doubling every four years); then in the early
1990stherateacceleratedtoagrowthrateofabout60%peryear(doublingeveryyearand
ahalf).ThedevelopmentandintroductionofMRheadsin1991,GMRheadsin1997,and
AFCpixie dustmedia in2001(see the next section) subsequently droveafurther 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,
thearealdensityofmagnetic storagehasincreased morethan200million fold,from2Kb/
