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Internal vs. External Performance
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.