As discussed here, the process of reading
or writing to the hard disk really comprises two large parts: the work done by the drive
itself (measured by internal performance factors) and the rest of the system (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 two
different tasks positioning and transfer, and their performance positioning
performance and transfer performance.
Both of these 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 extremely
important. If you are a single user doing multimedia editing where you need to read
multi-gigabyte consecutive files as fast as possible, data transfer is far more important
than positioning speed.
Most of the performance specifications that hard disk manufacturers provide to describe
their products can be broken down into categories by which aspect of performance they
measure. I have designed the section on performance
specifications with this in mind: there are sections discussing positioning
performance specifications and transfer performance
specifications. In addition, there are two key specifications that reflect aspects of both positioning and transfer. There are also some
specifications that don't really fit into these categories.
Next: Read vs. Write Performance
