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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Performance | Hard Disk Performance Specifications ]

General Notes On Performance Specifications

Before diving into the specifications themselves, I want to discuss a couple of topics of relevance to hard disk performance specifications in general. Keep these in mind as you read the various pages describing the specifications themselves. Most of them are really caveats about the specifications and some of the little tricks some manufacturers use in providing them:

  • Look At All The Specifications: Some manufacturers publish two "sets" of specifications. For "public consumption" they put out a short list of specs, often highlighting the areas they feel presents the drive in the best possible light. There's really nothing wrong with that, but if you are contemplating a purchase you want the whole story. Check the manufacturer's web site for a downloadable product manual or comprehensive data sheet that contains more detail.
  • Watch For "Maximum": Some performance characteristics vary depending on what part of the disk is being used, or in what way the disk is being exercised. Therefore, it is common for some specifications to be listed with the word "maximum" next to them. Don't gloss over this very important word! :^) What it really is telling you is that the average value of whatever that specification is will be lower. Make sure that if one manufacturer provides an "average" value for something, you don't compare it to a "maximum" value provided by another manufacturer. (Fortunately the specifications are mostly standardized between hard disk makers.)
  • A Matter Of Trust: Bear in mind when you read the specifications for a device that the manufacturer is providing them. Usually companies are mostly honest about the numbers, but exaggeration is certainly not unheard of. Mistakes also occur on occasion; if a number seems too good to be true, verify it against another source before accepting it.
  • Don't Overvalue A Single Performance Metric: Hard disk manufacturers often play the "magic number" game where they try to convince you that one or maybe two specifications show how great their product is. The culprit for this used to be seek time, which manufacturers overemphasized to the point of laughability--seek time is important, but it was still being incredibly "oversold". Today, the worst specifications in this regard are probably interface speed and cache (buffer) size. The latter in particular has been shown to have a rather insignificant impact on overall performance, but some companies act like a hard disk with a 2 MiB buffer will be twice as fast as one with a 1 MiB buffer.
  • Some Specifications Are Derived: Some of the specifications I discuss here are actually derived from other ones, so you won't find them listed on most spec sheets. They sometimes do a better job of expressing performance than the "official" specs upon which they are based. I explain these in detail in the appropriate sections.
  • Read Vs. Write Performance: Some performance specifications that manufacturers provide are accurate only when the drive is reading, not when it is writing; sometimes this is stated explicitly, and sometimes it is not! See here for more on this.

Next: Positioning Plus Transfer Performance Specifications

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