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.
