Hardware Reference
In-Depth Information
transfer rates. The more important transfer rate specifications are the media transfer rates,
which express howfast adrive can actually read orwrite data. Media transfer rates can be
expressed as a raw maximum, a raw minimum, a formatted maximum, a formatted min-
imum, or averages of either. Few report the averages, but they can be easily calculated.
The media transfer rate is far more important than the interface transfer rate because the
media transfer rate is the true rate at which data can be read from (or written to) the disk.
In other words, it tells how fast data can be moved to and from the drive platters (media).
It is the rate that any sustained transfer can hope to achieve. This rate is usually reported
as a minimum and maximum figure, although many drive manufacturers report the max-
imum only.
Mediatransferrateshaveminimumandmaximumfiguresbecausedrivestodayusezoned
recordingwithfewersectorspertrackontheinnercylindersthantheoutercylinders.Typ-
ically, a drive is divided into 16 or more zones, with the inner zone having about half the
sectors per track (and therefore about half the transfer rate) of the outer zone. Because the
drive spins at a constant rate, data can be read twice as fast from the outer cylinders than
from the inner cylinders.
Anotherissueistherawtransferrateversustheformattedtransferrate.The
raw
raterefers
to how fast bits can be read off the media. Because not all bits represent data (some are
intersector, servo, ECC, or ID bits), and because some time is lost when the heads have to
move from track to track (latency), the
formatted
transfer rate represents the true rate at
which user data can be read from or written to the drive.
Notethatsomemanufacturersreportonlyrawinternalmediatransferrates,buttheformat-
ted transfer rates are about three-fourths of the raw rates. This is because the user data
on each track is only about three-fourths of the actual bits stored due to servo, ECC, ID,
and other overhead that is stored. Likewise, some manufacturers report only maximum
transfer rates (raw, formatted, or both); in that case, you generally can assume that the
minimum transfer rate is one-half of the maximum and that the average transfer rate is
three-fourths of the maximum.
Let's look at some specific drives as an example.
Table 9.12
shows the performance spe-
cifications for several modern 3.5-inch form factor SATA drives.
Table 9.12 Drive Performance Specifications
