Hardware Reference
In-Depth Information
The Ultimate HDD Analogy
There is an old analogy that compares the interaction of the heads and the medium in a
typical HDD as being similar in scale to a 747 Jumbo Jet flying a few feet off the ground
at cruising speed (500+ mph). I have heard this analogy used repeatedly for years, and in
thepastIevenuseditmyselfwithoutcheckingtoseewhethertheanalogywastechnically
accurate with respect to modern hard drives. It isn't.
Perhapsthemostinaccurateaspectofthe747analogyistheuseofanairplaneofanytype
to describe the head-and-platter interaction. This analogy implies that the heads fly low
over the surface of the disk, but technically, this is not true. The heads do not fly at all
in the traditional aerodynamic sense; instead, they float or ski on a cushion of air that the
platters drag around.
A much better analogy would use a hovercraft instead of an airplane; the action of a hov-
ercraft much more closely emulates the action of the heads in an HDD. Like a hovercraft,
the drive heads rely somewhat on the shape of the bottom of the head to capture and con-
trol the cushion of air that keeps them floating over the disk. By nature, the cushion of air
on which the heads float forms only in close proximity to the platter and is often called an
air bearing
by those in the disk drive industry.
I thought it was time to come up with a new analogy that more correctly describes the di-
mensions and speeds at which an HDD operates today. I looked up the specifications on
a specific HDD and then magnified and rescaled all the dimensions involved by a factor
of more than 300,000. For my example, I use an IBM Deskstar 75GXP drive, which is a
75GB (formatted capacity), 3 1/2-inch ATA drive. The head sliders (called
pico
sliders)
in this drive are about 0.049 inches long, 0.039 inches wide, and 0.012 inches high. They
float on a cushion of air about 15 nanometers (nm or billionths of a meter) over the sur-
face of the disk while traveling at an average true speed of 53.55 miles per hour (figuring
an average track diameter of about 2 1/2 inches). These heads read and write individual
bits spaced only 2.56 micro-inches (millionths of an inch) apart, along tracks separated
by only 35.27 micro-inches. The heads can move from one track to another in 8.5 milli-
seconds during an average seek.
To create my analogy, I magnified the scale to make the head floating height equal to 5
millimeters (about 0.2 inches). Because 5 millimeters is about 333,333 times greater than
15 nanometers (nm), I scaled up everything else by the same amount.
Magnified to such a scale, the heads in this typical hard disk would be about 1,361 feet
long, 1,083 feet wide, and 333 feet high. (The length and height would be about equal
to the Sears Tower if it were tipped over sideways.) These skyscraper-sized heads would
float on a cushion of air that to scale would be only 5mm thick (about 0.2 inches) while
