Hardware Reference
In-Depth Information
densities, such as ceramic/glass platters, GMR heads, pseudo-contact recording, and
PRML electronics, as discussed earlier in this chapter. The primary challenge in achiev-
inghigherdensitiesismanufacturingdriveheadsanddiskstooperateatclosertolerances.
Improvements in tolerances and the use of more platters in a given form factor continue
tofuelimprovements indrivecapacity,butdrivemakers continue toseekevengreater ca-
pacity increases, both by improving current technologies and by developing new ones.
Tofitmoredataonaplatterofagivensize,youmustplacethetracksmorecloselytogeth-
er, and the heads must be capable of achieving greater precision in their placements over
the tracks. This also means that, as hard disk capacities increase, heads must float ever
closer to the disk surface during operation. The gap between the head and disk is as close
as 10 nanometers (0.01 microns) in some drives, which is approximately the thickness of
a cell membrane. By comparison, a human hair is typically 80microns in diameter,which
is8,000timesthickerthanthegapbetweentheheadanddiskinsomedrives.Theprospect
of actual contact or near contact recording is being considered for future drives to further
increase density.
PMR
Originally all hard drives and other types of magnetic media recorded data using longit-
udinal recording, which stores magnetic bits horizontally across the surface of the media.
Today, however, perpendicular recording, which aligns magnetic signals vertically on the
media surface, is being incorporated in modern drives to achieve higher data densities.
Thisprocessworksbecause vertically orientedmagnetic bitsuselessspacethanlongitud-
inally stored bits (see
Figure 8.14
). Virtually all the major drive vendors are working with
perpendicular recording as a way to achieve signal density surpassing that is achievable
even with AFC pixie dust media.
Figure 8.14
Perpendicular versus longitudinal recording.
