Hardware Reference
In-Depth Information
the achievements of researchers and engineers from a variety of disciplines [81].
Imagine an airplane flying at 5M miles per hour but only
1
16
inch above the
ground on a highway with 100,000 lanes where the width of each lane is only
fraction of an inch. The challenge of the problem is further intensified by the
factthattheairplaneisexpectedtoswitchlanesfrequentlyandthenfollowthe
new lane with the same precision. A scaled down version of this scenario is
what one finds in the head positioning servomechanism of an HDD.
We live in an era of information technology where every aspect of our
life is affected by some kind of information processing or information storage.
Modern computing systems use different technologies to store information,
either temporarily or permanently. These are semiconductor memories such
as ROM (Read Only Memory ), RAM (Random Access Memory ) etc, magnetic
storage such as hard disk,
fl
oppy disk, tape etc, and optical storage such as CD-
ROM (Compact Disk - ROM), DVD (Digital Versatile Disk)etc. Important
attributes of a storage device considered by users include cost, rate of data
transfer, access time, and reliability. If low cost is the main consideration
while selecting the storage device for a speci
fi
c application then one must
accept less desirable features such as slower response, lower transfer rate and
poorer reliability.
It is more efficient for the processor to access and store information in
semiconductor RAM. The average access time (time taken by the process of
recording or retrieval of data) is the shortest for this type of memory. How-
ever they are the most expensive, constitute the least of the storage volume
associated with an information processing system, and form the highest level
of the storage pyramid shown in Figure 1.1. The lowest level of this pyramid
consists of removable storage devices such as magnetic tapes, zip-disks and
fl
oppy disks using magnetic recording, and CD-ROM, DVD etc employing op-
tical recording technology. Removability is the main advantage offered by this
class of storage devices. Magnetic tapes and
fl
oppy disks are cheap but very
slow and, therefore, not suitable for on-line direct access of data or programs.
Optical disks are widely used for applications like program distribution, library
and archive, entertainment systems etc, but they are not suitable for on-line
storage due to their slow performance and high cost per read/write element.
This segment of storage market was dominated by magnetic tapes in the early
days of computing, but the emergence of optical recording technology caused
the tapes to be replaced gradually by more cost-effective CD-ROM, DVD etc.
Hard disk drives sit in the middle of the storage pyramid, between the
semiconductor memories and removable drives, and occupy the non-removable
on-line data storage niche. They provide direct access to large amounts of
non-volatile storage (no power is required to preserve the data ). Speed of data
access in HDD is much higher than the removable, non-volatile storage, and
its cost per gigabyte is only a fraction of that of non-volatile, direct access
semiconductor memory such as “
fl
ash”. Hard disk drives are also known as
