Hard Disk Performance

When you select a hard disk drive, one of the important features you should consider is the performance (speed) of the drive. Hard drives can have a wide range of performance capabilities. As is true of many things, one of the best indicators of a drive's relative performance is its price.

An old saying from the automobile-racing industry is appropriate here: "Speed costs money. How fast do you want to go?" The speed of a disk drive is typically measured in two ways:

  • Transfer rate

  • Average access time

Transfer Rate

The transfer rate is probably more important to overall system performance than any other statistic, but it is also one of the most misunderstood specifications. The problem stems from the fact that several transfer rates can be specified for a given drive; however, the most important of these is usually overlooked.

The confusion results from the fact that drive manufacturers can report up to seven different transfer rates for a given drive. Perhaps the least important (but one that people seem to focus on the most) is the raw interface transfer rate, which for most modern ATA drives is either 100MBps or 133MBps, or 150MBps for Serial ATA drives.

Unfortunately, few people seem to realize that the drives actually read and write data much more slowly than that. The more important transfer rate specifications are the media transfer rates, which express how fast a drive can actually read or write data.

Media transfer rates can be expressed as a raw maximum, a raw minimum, a formatted maximum, formatted minimum, or averages of either. Few report the averages, but they can be easily calculated.

The media transfer rate is far more important than the interface transfer rate because the media transfer rate is the true rate at which data can be read from (or written to) the disk.

In other words, it tells how fast data can be moved to and from the drive platters (media). It is the rate that any sustained transfer can hope to achieve. This rate is usually reported as a minimum and maximum figure, although many drive manufacturers report the maximum only.

Media transfer rates have minimum and maximum figures because drives today use zoned recording with fewer sectors per track on the inner cylinders than the outer cylinders. Typically, a drive is divided into 16 or more zones, with the inner zone having about half the sectors per track (and therefore about half the transfer rate) of the outer zone.

Because the drive spins at a constant rate, data can be read twice as fast from the outer cylinders than from the inner cylinders. Another issue is the raw transfer rate versus the formatted transfer rate. The raw rate refers to how fast bits can be read off the media.

Because not all bits represent data (some are intersector, servo, ECC, or ID bits), and because some time is lost when the heads have to move from track to track (latency), the formatted transfer rate represents the true rate at which user data can be read from or written to the drive.

Note that some manufacturers report only raw internal media transfer rates, but you usually can calculate that the formatted transfer rates are about three-fourths of the raw rates. This is because the user data on each track is only about three-fourths of the actual bits stored due to servo, ECC, ID, and other overhead that is stored.

Likewise, some manufacturers report only maximum transfer rates (either raw, formatted, or both); in that case, you generally can assume the minimum transfer rate is one-half of the maximum and that the average transfer rate is three-fourths of the maximum.

Average Seek Time

Average seek time, usually measured in milliseconds (ms), is the average amount of time it takes to move the heads from one cylinder to another a random distance away. One way to measure this specification is to run many random track-seek operations and then divide the timed results by the number of seeks performed.

This method provides an average time for a single seek. The standard method used by many drive manufacturers when reporting average seek times is to measure the time it takes the heads to move across one-third of the total cylinders.

Average seek time depends only on the drive itself; the type of interface or controller has little effect on this specification. The average seek rating is primarily a gauge of the capabilities of the head actuator mechanism.