Hardware Reference
In-Depth Information
Figure 1.11: Trend in HDD areal density (From Grochowski, E., IBM Systems
Journal, 42(2), 2003. With permission.).
to be able to read and write smaller bits. Improved technologies have enabled
new generation media to reliably hold magnetic domains of smaller size. Track
density has been increased by reducing component disturbances that move the
head off track and by improving the servo controller's ability to regulate the
head on narrower tracks. Smoother disk surface, better quality of lubricant
and better air bearing technologies allow the slider to
fl
yincloserproximity
of the disk so that the bit size is reduced. Typical
fl
y heights in 1997 were
25 nanometers (nm). Today they are about 5 nm. The read-write electronics
and data encoding schemes have played their part in improving bit density
by enabling detection of information reliably from ever smaller data signals
contaminated by the surrounding noise. Excellence in design and production
maintained the steady growth in the areal density which was accelerated time
to time by availability of new technologies. Introduction of new technologies
has always made an impact on the strive for improving areal density. Cumu-
lative average growth rate of the areal density was 25% in the seventies and
eighties. Drives in those days used inductive heads for both reading and writ-
ing. MR heads, introduced in early 1990s, boosted the growth rate to 60%.
The giant MR heads (GMR) appeared in the drives in later half of 1990s, and
the industry experienced a growth rate of more than 100%.
The ever-increasing areal density is sustained by increasing both track den-
sity and linear density. The industry has been experiencing 100% per year
