Hardware Reference
In-Depth Information
relatively meaningless, and although the rotational speed is important, some drives have
a slower media transfer rate than others even though they spin faster. Be careful with
simplistic comparisons. With hard drives, the bottom line is that the media transfer rate
is probably the most important specification you can know about a drive—and faster is
better.
Tofindthetransferspecificationsforagivendrive,lookinthedatasheetorpreferablythe
documentation or manual for the drive. These can usually be downloaded from the drive
manufacturer's website.
Average Seek Time
Average seek time
, usually measured in milliseconds (ms), is the average amount of time
it takes to move the heads from one cylinder to another a random distance away. One way
to measure this specification is to run many random track-seek operations and then divide
thetimedresultsbythenumberofseeksperformed.Thismethodprovidesanaveragetime
for a single seek.
The standard method used by many drive manufacturers when reporting average seek
times is to measure the time it takes the heads to move across one-third of the total cylin-
ders.Averageseektimedependsonlyonthedriveitself;thetypeofinterfaceorcontroller
has little effect on this specification. The average seek rating is primarily a gauge of the
capabilities of the head actuator mechanism.
Note
Be wary of benchmarks that claim to measure drive seek performance. Most ATA drives use
aschemecalledsectortranslation,soanycommandsthedrivereceivestomovetheheadstoa
specific cylinder might not actually result in the intended physical movement. This situation
renders some benchmarks meaningless for those types of drives. SCSI drives also require
an additional step because the commands first must be sent to the drive over the SCSI bus.
These drives might seem to have the fastest access times because most benchmarks don't
factor in the command overhead. However, when this overhead is factored in by benchmark
programs, these drives receive poor performance figures.
Latency
Latency
is the average time (in milliseconds) it takes for a sector to be available after the
heads have reached a track. On average, this figure is half the time it takes for the disk to
rotate once. A drive that spins twice as fast would have half the latency.
Latency is a factor in disk read and write performance. Decreasing the latency increases
the speed of access to data or files and is accomplished only by spinning the drive platters
