Hard Disk Drive Analogy

There is an old analogy that compares the interaction of the heads and the medium in a typical hard disk drive 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 the past I even used it myself without checking to see whether the analogy is technically accurate with respect to modern hard drives. It isn't.

Perhaps the most inaccurate aspect of the 747 analogy is the use of an airplane of any type to describe the head-and-platter interaction. This analogy implies that the heads fly very 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 is being dragged around by the platters. A much better analogy would use a hovercraft instead of an airplane; the action of a hovercraft much more closely emulates the action of the heads in a hard disk drive.

Like a hovercraft, the drive heads rely somewhat on the shape of the bottom of the head to capture and control 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 very 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 dimensions and speeds at which a hard disk drive operates today. I looked up the specifications on a specific modern hard disk drive 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'' ATA (AT Attachment interface) drive. The head sliders (called pico sliders) in this drive are about 0.049'' long, 0.039'' wide, and 0.012'' high.

They float on a cushion of air about 15 nanometers (nm or billionths of a meter) over the surface 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'').

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 milliseconds 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''). 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'') while traveling at a speed of 17.8 million miles per hour (4,958 miles per second), all while reading data bits spaced a mere 0.85'' apart on tracks separated by only 0.98 feet!

The proportionate forward speed of this imaginary head is difficult to comprehend, so I'll elaborate. The diameter of the Earth at the equator is 7,926 miles, which means a circumference of about 24,900 miles.

At 4,958 miles per second, this imaginary skyscraper-sized head would circle the earth once every 5 seconds (at only two-tenths of an inch over the surface)! It would also read 231.33MB in one lap around this equatorial track.

There is also sideways velocity to consider. Because the average seek time of 8.5 milliseconds is defined as the time it takes to move the heads over one-third of the total tracks (about 9,241 tracks in this example), the heads could move sideways within a scale distance of 1.71 miles in that short time.

This results in a scale seek velocity of more than 726,321mph, or 202 miles per second! This analogy should give you a new appreciation of the technological marvel that the modern hard disk drive actually represents. It makes the old Jumbo Jet analogy look rather pathetic (not to mention grossly inaccurate), doesn't it?