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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. This is one of the most commonly stated metrics for hard disks, and it is one of the most important positioning performance factors. However, using this metric to compare drives can be somewhat dangerous. To use it properly, we must figure out exactly what seek time 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 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, with average seek times for most modern drives in the 8 to 12 ms range. 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 200 MHz processor can (theoretically) execute 200,000 instructions in a millisecond. Cutting the seek time of an average read instruction down from 12 ms to 8 ms then, can result in great system performance improvement, because the rest of the system is often sitting and waiting for the hard disk during this time.

There are problems, however, with using seek time to compare hard disks. The first one is that there is no standardized way of reporting them. Most companies only specify a single seek time for their drive. This is obviously problematic, because there is no single number that expresses the seek time for an entire drive. The time to seek from one track to another one depends on the distance between them. Many manufacturers seek to avoid this problem by defining and quoting an "average" seek time. In fairness, this is a good way of representing the seek time for overall use, but the definition of what seek patterns represent "average" is left basically up to each manufacturer to decide. In many ways a truly random access pattern is not realistic, since most users don't do truly random access to their disks.

Other manufacturers provide more information about the seek time of their drives than just the average. Here are some more detailed seek time specifications that you can use to do a much more complete comparison of two drives:

• 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 12 ms.
• Track-to-Track: This is the amount of time that is required to seek between adjacent tracks. This is the same as the track switch time and is usually 2 to 4 ms, but can be as low as 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 20 ms range.

A second problem is that so many drives have near-identical seek times. At any given time, the leading-edge drives are usually within 1 ms or less of each other in terms of average seek time, which doesn't leave much to distinguish them. To make matters worse, some manufacturers don't even quote a true average seek time, saying just "less than 11 ms" for example. This doesn't help you terribly, because in some ways this really means "we're not entirely sure". (At least it tells you what they think of seek time as a metric.)

Finally, while seek time is an important component of overall hard disk performance, it is only one component. There are situations in which it is very unimportant as an overall factor. If you are doing work that involves reading large blocks of contiguous data from the disk, average seek time is much less important to you than the drive's transfer rate and cylinder switch time, for example. Don't be fooled by drive manufacturers and others that place so much importance on seek time that they even refer to their drives as a "10 ms hard disk" when their average seek time is this value.

Next: Latency

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