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Ranking Performance Specifications and Factors
Amongst those who are concerned with hard disk performance, there has been considerable attention given lately to ranking the various performance specifications and factors by how important they are for a hard disk. The reason for this is usually to assist in choosing hardware: if you are comparing two hard disks that have similar specifications, how do you decide which one is likely to be better? Of course, reading reviews and benchmarks is one part of the answer. It's still useful at times to have a way to rank the various specifications in terms of which is most important to overall performance.
There's just one little problem with this: it's virtually impossible to do. Which specification is most critical depends entirely on how you use your hard disk and your PC as a whole. People disagree greatly on the matter of which specification or performance factor translates most directly into improved overall performance. There is no one "correct answer".
The biggest argument seems to be over which is more important, positioning or transfer. The answer is: it depends. If you are doing a lot of work with large files, for example editing video or audio streams, transfer is typically more important. Few people fall into this category, however. If you are running a server or other application where many people are accessing smallish files on a regular basis, positioning is definitely more important. Even fewer people fall into this category. :^)
Beyond the special cases, things get much more complex. While transfer rates have typically been under-emphasized in recent years, lately some tests have shown that they are now often being over-emphasized. Since most operating systems now use a large number of different files and drivers, and since effects such as fragmentation cause parts of files to be spread over the surface of the disk, many users access their disks less often in a sequential manner than was previously thought.
In my opinion, it is best to value both and not worry excessively over the issue. :^) To this end, when selecting a hard disk I believe the most important things to look at are spindle speed, areal density, and seek time. The first two are important because they have an influence on both positioning and transfer performance: other specifications generally do not. Higher spindle speed in particular is the most obvious indicator of a faster hard disk; a 7200 RPM hard disk of the same age and interface as a 5400 RPM drive will almost always be faster, not just due to the faster spindle but also due to other improvements that appear first on faster-spindled drives. Seek time is important because of the issue of reading large numbers of scattered small files mentioned above.
Another key point to keep in mind: if two hard disks are very close in specifications, then most likely the two drives will perform similarly. It's not worth spending a lot of energy worrying about small discrepancies between them. See here for more on this subject.