Hardware Reference
In-Depth Information
actually more than 2.6 times faster in actual transfer rate performance, even though it spins over 33%
more slowly. If you are looking for performance, be sure to check the true
media
transfer rates of any
drives you are comparing.
The confusion results from the fact that drive manufacturers can report up to seven different transfer
rates for a given drive. Perhaps the least important (but one that people seem to focus on the most) is
the raw interface transfer rate, which for SATA drives is up to 600MBps, and up to 100MBps or
133MBps for older PATA drives. Unfortunately, the drives actually read and write data much more
slowly than that; you must use a high-performance SSD instead of a traditional hard disk if you want
to fully exploit SATA transfer rates. The more important transfer rate specifications are the media
transfer rates, which express how fast a drive can actually read or write data. Media transfer rates
can be expressed as a raw maximum, a raw minimum, a formatted maximum, a formatted minimum, 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 maximum only.
Media transfer rates have minimum and maximum figures because drives today use zoned recording
with fewer sectors per track on the inner cylinders than the outer cylinders. Typically, 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.
Another issue is the raw transfer rate versus the formatted transfer rate. The
raw
rate refers 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.
Note that some manufacturers report only raw internal media transfer rates, but the formatted 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.
Finally, one more issue affecting drive performance is size, with physically larger drives having an
advantage. For example, assuming equal areal density and rotational speed, 3.5-inch drives offer
transfer rates that are about 46% faster than 2.5-inch drives. This is due to the relative difference in
track length (circumference) between the two drives. 3.5-inch drives have platters that are 3.74
inches in diameter, while 2.5-inch drives have platters that are 2.56 inches in diameter. This means
that each track on a 3.5-inch drive is about 46% longer than those on 2.5-inch drives, and when
spinning at the same speed each drive reads a track in the same amount of time.
Let's look at some specific drives as an example.
Table 9.11
shows the performance specifications
and formatted media transfer rates for several modern 3.5-inch form factor SATA drives.
Table 9.11. Drive Performance Specifications
