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Seek Time

The seek time of a hard disk measures the amount of time required for the read/write heads to move between tracks over the surfaces of the platters. Seek time is one of the most commonly discussed metrics for hard disks, and it is one of the most important positioning performance specifications. However, using this number to compare drives can be somewhat fraught with danger. Alright, that's a bit melodramatic; nobody's going to get hurt or anything. :^) Still, to use seek time properly, we must figure out exactly what it means.

Switching between tracks requires the head actuator to move the head arms physically, which being a mechanical process, takes a specific amount of time. The amount of time required to switch between two tracks depends on the distance between the tracks. However, there is a certain amount of "overhead" involved in track switching, so the relationship is not linear. It does not take double the time to switch from track 1 to track 3 that it does to switch from track 1 to track 2, much as a trip to the drug store 2 miles away does not take double the time of a trip to the grocery store 1 mile away, when you include the overhead of getting into the car, starting it, etc.

Seek time is normally expressed in milliseconds (commonly abbreviated "msec" or "ms"), with average seek times for most modern drives today in a rather tight range of 8 to 10 ms. Of course, in the modern PC, a millisecond is an enormous amount of time: your system memory has speed measured in nanoseconds, for example (one million times smaller). A 1 GHz processor can (theoretically) execute over one million instructions in a millisecond! Obviously, even small reductions in seek times can result in improvements in overall system performance, because the rest of the system is often sitting and waiting for the hard disk during this time. It is for this reason that seek time is usually considered one of the most important hard disk performance specifications. Some consider it the most important.

At one point many years ago seek times were difficult to use because manufacturers wouldn't agree on a standardized way of reporting them. Today, this has largely been corrected. While seek time is usually given as a single number, in fact there are three different seek time specifications you should examine for a drive, as they  represent the drive's performance when doing different types of seeks:

• Average: As discussed, this is meant to represent an average seek time from one random track (cylinder) to any other. This is the most common seek time metric, and is usually 8 to 10 ms, though older drives had much higher numbers, and top-of-the-line SCSI drives are now down to as low as 4 ms!
• Track-to-Track: This is the amount of time that is required to seek between adjacent tracks. This is similar in concept (but not exactly the same as) the track switch time and is usually around 1 ms. (Incidentally, getting this figure without at least two significant digits is pretty meaningless; don't accept "1 ms" for an answer, get the number after the decimal point! Otherwise every drive will probably round off to "1 ms".)
• Full Stroke: This number is the amount of time to seek the entire width of the disk, from the innermost track to the outermost. This is of course the largest number, typically being in the 15 to 20 ms range. In some ways, combining this number with the average seek time represents the way the drive will behave when it is close to being full.

While I believe that seek time is a very important specification, I have become somewhat cynical in the last few years regarding the amount of attention paid to it. The reason is that there is so little difference between the seek time specs of most comparable drives in any given class or category. For example, almost all IDE/ATA 7200 RPM drives shipping in 2000 have an average seek time specification of 8.0, 8.5 or 9.0 milliseconds. This doesn't leave a lot to work with. However, at the same time, we must realize that of the four components that comprise the drive's access time, if you are comparing two drives of the same class and spindle speed, only seek time will differ much between them. So this small differential may be the only thing to distinguish drives; and small differences are what you are likely to see. (Larger discrepancies though, directly translate into often very noticeable differences in performance. A drive with a 5 ms seek time will generally blow the doors off one with a seek time of 8.0 to 9.0 ms in random positioning tasks, which is why these fast drives are preferred for servers and other multi-user environments.)

To really put seek time in proper context, it should be remembered that it is the largest component of access time, which is the composite metric that best represents positioning performance. However, it is only one component, and there is at one that is of at least equal importance (see the discussion of access time for more on seek time's role in overall positioning performance). Also, bear in mind that seek times are averages that make certain assumptions of how the disk will be used. For example, file system factors will always have an impact on seek performance in the real world.

A couple of additional caveats on seek times. First, unless you see two numbers, one for read performance and one for write, seek times always refer to reads; see here for more details. Ask for the write numbers if you are interested, or you can approximate by adding 1 ms to the average read numbers. Second, watch out for "less than X ms" specifications. Rather bogus, and fortunately not seen as often as in the past, I interpret "less than X ms" as "X ms" and you generally should do so as well--if the true average were under "X-1", they'd say "less than X-1 ms" instead of "less than X ms".

Seek time is almost entirely a function of the design and characteristics of the hard disk's actuator assembly. It is affected slightly by the read/write head design since the size of the heads affects movement speed.

Note: Some manufacturers include settle time as part of their seek time specification. Since settle time is relatively small this doesn't really change the seek time numbers much.

Next: Settle Time

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