Hardware Reference
In-Depth Information
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 cylinders. Average seek time
depends only on the drive itself; the type of interface or controller 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 a
scheme called sector translation, so any commands the drive receives to move the heads to a
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 more quickly. Latency
figures for most popular drive rotational speeds are shown in
Table 9.12
.
Table 9.12. Hard Disk Rotation Speeds and Their Latencies
Many drives today spin at 7,200 rpm, resulting in a latency time of only 4.17ms, whereas others spin
at 10,000 rpm or even 15,000 rpm, resulting in incredible 3.00ms or 2.00ms latency figures. In
addition to increasing performance where real-world access to data is concerned, spinning the
platters more quickly also increases the data-transfer rate after the heads arrive at the desired sectors.
Average Access Time
A measurement of a drive's average access time is the sum of its average seek time plus latency. The
average access time is usually expressed in milliseconds.
A measurement of a drive's average access time (average seek time plus latency) provides the
average total amount of time required for the drive to access a randomly requested sector.
Cache Programs and Caching Controllers
