Hardware Reference
In-Depth Information
Figure 1.1: Storage hierarchy pyramid in a computing system
Direct Access Storage Devices (DASD, pronounced as daz-dee).
In hard disk drives, a binary bit is stored in a tiny segment of the surface
of a circular disk by magnetizing the medium coated on the surface with the
help of an inductive head. In a majority of hard disk drives, the disk is spun
at constant angular velocity by a spindle motor when the bits are written,
and the head traces a circular path (Track ) on the spinning disk. Saturated
magnetization of the media is used and it is magnetized in one of the two
possible polarizations. The transitions between two opposite polarizations in
the magnetic medium can be sensed by a sensor held over the track of a spinning
disk. The disks are spun at the same speed during both writing and reading.
The read head and write head are fabricated on a single slider whose surface
facing the disk is pro
fi
led such that an air bearing surface (ABS) is produced
between the spinning disk and the slider. As a result, the slider is lifted
and is not in contact with the disk. The separation between the slider and
the spinning disk, known as the flying height, is maintained as constant as
possible. Characteristics of the
fl
ying height depend on many factors such as
pro
fi
le of the slider surface, smoothness of the disk surface, rotating speed of
the disk etc. Flying height has direct effect on the achievable areal density -a
key parameter de
fi
ning the storage capacity and is equal to the number of bits
recorded in unit area of the disk surface. Demand for higher areal density has
always been and still is the driving force behind the dramatic growth of the
magnetic storage technology. Areal density in magnetic recording has grown
by a factor of 5, 000, 000 over last four decades.
Some Commonly used Terminologies:
• Data Track- Concentric circular (not perfectly) tracks on the disk where
binary bits are stored sequentially.
• Track Pi tch - Distance between two adjacent tracks.
• Track Dens i ty - Inverse of track pitch, i.e., the number of tracks in unit
length of radius of the disk. It is usually de
fi
ned in units of Tracks per
