The hard disk's job is to store data from the system, or get data to the system, as
fast as possible. When considering performance, it is this ability to move data into or
out of the hard disk that we are looking to measure. There are two separate parts of this
data movement job. For a write, the data must be fetched from the system, and then written
to the correct sector(s) on the disk. For a read, the process is reversed; data must be
read from the disk, and then transmitted over the hard disk interface to the system.
Clearly, the hard disk itself is only responsible for some portion of the overall
performance we attribute to the hard disk subsystem. Some of the factors that affect
performance are related to characteristics of the PC that are not specific to the
particular hard drive being used. Performance characteristics that are largely a matter of
how the hard disk itself is designed and implemented I call internal performance
factors; those that are mostly affected by the rest of the system, or how the hard
disk is used (and hence are largely independent of the particular hard disk model) are external
performance factors.
The distinction between internal and external is a very important one! In any system,
the bottleneck to high performance can reside either within the disk, or within the rest
of the system (the interface, the system bus, CPU, drivers, file system, and so on.) It's
usually not in both at the same time. If the main limiting factor in a particular system
is, say, the system bus being used for the hard disk interface, putting a faster hard disk
into that system will have very little impact on performance. Similarly, if the hard disk
itself is slow, putting it on a faster interface will yield little improvement.
In some cases, performance bottlenecks can change from being primarily affected by
internal factors, to being more influenced by external factors, depending on what type of
work is being done on the PC. In addition, making a change to a system can cause the
bottleneck to "shift". Let's say you have a high-speed hard disk that is on a
slow interface. The interface may be the bottleneck to high performance; if you move the
disk onto a faster interface, the disk itself may become the bottleneck. In many PCs,
external performance can be enhanced simply by making no-cost performance enhancements to
the system.
In this discussion of hard disk performance, the important distinction between internal
and external factors is the reason why they are in separate sections; internal performance factors are discussed here; and external performance factors are here.
Next: Positioning vs. Transfer Performance
