If there is one thing that takes up the majority of a system
administrator's day, it would have to be storage management. It seems
that disks are always running out of free space, becoming overloaded with
too much I/O activity, or failing unexpectedly. Therefore, it is vital to
have a solid working knowledge of disk storage in order to be a successful
system administrator.
Before managing storage, it is first necessary to understand the
hardware on which data is stored. Unless you have at least some
knowledge about mass storage device operation, you may find yourself in
a situation where you have a storage-related problem, but you lack the
background knowledge necessary to interpret what you are seeing. By
gaining some insight into how the underlying hardware operates, you
should be able to more easily determine whether your computer's storage
subsystem is operating properly.
The vast majority of all mass-storage devices use some sort of
rotating media and supports the random access of data on that media.
This means that the following components are present in some form within
nearly every mass storage device:
The following sections explore each of these components in more
detail.
The rotating media used by nearly all mass storage devices are in
the form of one or more flat, circularly-shaped platters. The platter
may be composed of any number of different materials, such aluminum,
glass, and polycarbonate.
The surface of each platter is treated in such a way as to enable
data storage. The exact nature of the treatment depends on the data
storage technology to be used. The most common data storage
technology is based on the property of magnetism; in these cases the
platters are covered with a compound that exhibits good magnetic
characteristics.
Another common data storage technology is based on optical
principles; in these cases, the platters are covered with materials
whose optical properties can be modified, thereby allowing data to be
stored optically.
No matter what data storage technology is in use, the disk
platters are spun, causing their entire surface to sweep past another
component — the data reading/writing device.
The data reading/writing device is the component that takes the
bits and bytes on which a computer system operates and turns them into
the magnetic or optical variations necessary to interact with the
materials coating the surface of the disk platters.
Sometimes the conditions under which these devices must operate
are challenging. For instance, in magnetically-based mass storage the
read/write devices (known as heads) must be
very close to the surface of the platter. However, if the head and
the surface of the disk platter were to touch, the resulting friction
would do severe damage to both the head and the platter. Therefore,
the surfaces of both the head and the platter are carefully polished,
and the head uses air pressure developed by the spinning platters to
float over the platter's surface, "flying" at an altitude less than
the thickness of a human hair. This is why magnetic disk drives are
sensitive to shock, sudden temperature changes, and any airborne
contamination.
The challenges faced by optical heads are somewhat different than
for magnetic heads — here, the head assembly must remain at a
relatively constant distance from the surface of the platter.
Otherwise, the lenses used to focus on the platter does not produce a
sufficiently sharp image.
In either case, the heads use a very small amount of the platter's
surface area for data storage. As the platter spins below the heads,
this surface area takes the form of a very thin circular line.
If this was how mass storage devices worked, it would mean that
over 99% of the platter's surface area would be wasted. Additional
heads could be mounted over the platter, but to fully utilize the
platter's surface area more than a thousand heads would be necessary.
What is required is some method of moving the head over the surface of
the platter.
By using a head attached to an arm that is capable of sweeping
over the platter's entire surface, it is possible to fully utilize the
platter for data storage. However, the access arm must be capable of
two things:
Moving very quickly
Moving very precisely
The access arm must move as quickly as possible, because the time
spent moving the head from one position to another is wasted time.
That is because no data can be read or written until the access arm
stops moving.
The access arm must be able to move with great precision because,
as stated earlier, the surface area used by the heads is very small.
Therefore, to efficiently use the platter's storage capacity, it is
necessary to move the heads only enough to ensure that any data
written in the new position does not overwrite data written at a
previous position. This has the affect of conceptually dividing the
platter's surface into a thousand or more concentric "rings" or
tracks. Movement of the access arm from one
track to another is often referred to as
seeking, and the time it takes the access arms
to move from one track to another is known as the seek
time.
Where there are multiple platters (or one platter with both
surfaces used for data storage), the arms for each surface are
stacked, allowing the same track on each surface to be accessed
simultaneously. If the tracks for each surface could be visualized
with the access stationary over a given track, they would appear to be
stacked one on top of another, making up a cylindrical shape;
therefore, the set of tracks accessible at a certain position of the
access arms are known as a cylinder.
