Hard Disks

Nearly every desktop computer and server in use today
contains one or more hard-disk drives. Every mainframe and
supercomputer is normally connected to hundreds of them. You
can even find VCR-type devices and camcorders that use hard
disks instead of tape. These billions of hard disks do one
thing well -- they store changing digital information in
a relatively permanent form. They give computers the ability
to remember things when the power goes out.

We'll take apart a hard disk so that you can see what's
inside, and also discuss how they organize the gigabytes of
information they hold in files!

Hard Disk Basics

Hard disks were invented in the 1950s. They started as large
disks up to 20 inches in diameter holding just a few
megabytes. They were originally called "fixed disks" or
"Winchesters" (a code name used for a popular IBM product).
They later became known as "hard disks" to distinguish them
from "floppy disks." Hard disks have a hard platter that
holds the magnetic medium, as opposed to the flexible plastic
film found in tapes and floppies.

At the simplest level, a hard disk is not that different from
a cassette tape. Both hard disks and cassette tapes use the
same magnetic recording techniques described in How Tape
Recorders Work. Hard disks and cassette tapes also share the
major benefits of magnetic storage -- the magnetic medium can
be easily erased and rewritten, and it will "remember" the
magnetic flux patterns stored onto the medium for many years.

Cassette Tape vs. Hard Disk

Let's look at the big differences between cassette tapes and
hard disks:

* The magnetic recording material on a cassette tape is
coated onto a thin plastic strip. In a hard disk, the
magnetic recording material is layered onto a high-precision
aluminum or glass disk. The hard-disk platter is then
polished to mirror-type smoothness.

* With a tape, you have to fast-forward or reverse to get
to any particular point on the tape. This can take several
minutes with a long tape. On a hard disk, you can move to any
point on the surface of the disk almost instantly.

* In a cassette-tape deck, the read/write head touches
the tape directly. In a hard disk, the read/write head
"flies" over the disk, never actually touching it.

* The tape in a cassette-tape deck moves over the head at
about 2 inches (about 5.08 cm) per second. A hard-disk
platter can spin underneath its head at speeds up to 3,000
inches per second (about 170 mph or 272 kph)!

* The information on a hard disk is stored in extremely
small magnetic domains compared to a cassette tape's. The
size of these domains is made possible by the precision of
the platter and the speed of the medium.

Because of these differences, a modern hard disk is able to
store an amazing amount of information in a small space.
A hard disk can also access any of its information in
a fraction of a second.

Capacity and Performance

A typical desktop machine will have a hard disk with
a capacity of between 10 and 40 gigabytes. Data is stored
onto the disk in the form of files. A file is simply a named
collection of bytes. The bytes might be the ASCII codes for
the characters of a text file, or they could be the
instructions of a software application for the computer to
execute, or they could be the records of a data base, or they
could be the pixel colors for a GIF image. No matter what it
contains, however, a file is simply a string of bytes. When
a program running on the computer requests a file, the hard
disk retrieves its bytes and sends them to the CPU one at
a time.

There are two ways to measure the performance of a hard disk:

* Data rate - The data rate is the number of bytes per
second that the drive can deliver to the CPU. Rates between
5 and 40 megabytes per second are common.

* Seek time - The seek time is the amount of time between
when the CPU requests a file and when the first byte of the
file is sent to the CPU. Times between 10 and 20 milliseconds
are common.

The other important parameter is the capacity of the drive,
which is the number of bytes it can hold.