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
