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Positioning and Transfer Performance Factors

The process of reading or writing to the hard disk really comprises two large steps, the work done by the drive itself (measured by internal performance factors) and the interface (external factors). The internal work of the drive can itself be thought of as two functions: finding the correct location on the drive, and then reading or writing the data. These are very different jobs, and two drives can be very similar in one regard but very different in another. They also depend on different design characteristics. I call these tasks positioning and transfer.

Both of these factors are important to overall performance, although if you read the literature and the numbers that people talk about, positioning metrics are probably more commonly discussed than transfer measurements. You might be fooled by this into thinking they are more important, but often they are not--they are just simpler to explain in many cases, or people are used to using them to compare drives.

Which influences on performance are most important also depends on how you are using the device. If you are running a file server, the hard disk will be doing a lot of random accesses to files all over the disk, and positioning performance will be very important. If you are a single user doing multimedia editing where you need to read multi-megabyte consecutive files as fast as possible, data transfer is far more important than positioning speed.

Positioning performance factors (or characteristics with a direct and significant influence on positioning performance) discussed in this chapter include:

• Seek time.
• Latency.
• Access time (a derived metric combining seek time and latency).
• Track switch time.
• Head switch time.
• Spindle speed.
• Areal density (both components of the measure).

Transfer performance factors (or characteristics with a direct and significant influence on transfer performance) discussed in this chapter include:

• Internal data transfer rate.
• Spindle speed.
• Areal density (primarily the bit density per track component).

Every performance factor is also directly influenced by a myriad of other design decisions that go into the creation of the drive.

Next: Seek Time

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