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Positioning vs. Transfer Performance
As discussed here, the process of reading or writing to the hard disk really comprises two large parts: the work done by the drive itself (measured by internal performance factors) and the rest of the system (external factors). The internal work of the drive can itself be thought of as two functions: finding the correct location on the drive, and then reading or writing the data. These are very different jobs, and two drives can be very similar in one regard but very different in another. They also depend on different design characteristics. I call these two different tasks positioning and transfer, and their performance positioning performance and transfer performance.
Both of these are important to overall performance, although if you read the literature and the numbers that people talk about, positioning metrics are probably more commonly discussed than transfer measurements. You might be fooled by this into thinking they are more important, but often they are not--they are just simpler to explain in many cases, or people are used to using them to compare drives.
Which influences on performance are most important also depends on how you are using the device. If you are running a file server, the hard disk will be doing a lot of random accesses to files all over the disk, and positioning performance will be extremely important. If you are a single user doing multimedia editing where you need to read multi-gigabyte consecutive files as fast as possible, data transfer is far more important than positioning speed.
Most of the performance specifications that hard disk manufacturers provide to describe their products can be broken down into categories by which aspect of performance they measure. I have designed the section on performance specifications with this in mind: there are sections discussing positioning performance specifications and transfer performance specifications. In addition, there are two key specifications that reflect aspects of both positioning and transfer. There are also some specifications that don't really fit into these categories.