Under normal circumstances--meaning, unless special reliability features are
implemented--when a hard disk drive is idle, the head-actuator assembly just "sits
there" wherever it happened to be when the last read or write occurred. In theory,
there should not be any problem that results from this since the heads are floating on air
and not touching the surface. However, at least one manufacturer (Western Digital)
theorized that it might not be a good idea to keep the heads sitting in one spot for long
periods of time even if in theory this was OK. After all, head slaps can occur, and under normal usage the location of the
heads is not likely to be completely random: some files and areas of the disk are more
often accessed than others, especially those where the file
system structures are located.
To eliminate what they considered the potential for wear, Western Digital incorporated
onto its drives a feature called wear leveling. Every fifteen seconds of idle
time, the drive controller moves the heads by one track, traversing the entire disk area,
so that no area of the disk has the heads sitting over it for very long. It takes about
three hours for the entire surface of the disk to be traversed.
The importance of this feature seems to still be under debate. In theory it improves
drive reliability, but it also causes the drive to make more noise. It also makes some
people think that there is something wrong with their drive. (I did when I first
encountered these strange clicking sounds on my drive even when it was idle; I later
realized the noises were exactly 15 seconds apart every time, so I realized it must have
been something that had been done intentionally.) The fact that other manufacturers have
apparently not rushed to implement wear leveling says something, considering that most
technologies that are universally considered "good ideas" end up on all
manufacturers' drives within a few years of one company inventing them. It may be that the
other manufacturers don't consider wear leveling useful enough to bother, or they may want
to avoid the noise complaints that plagued some of WD's early efforts with this feature.
Certainly, it would be hard to argue that wear leveling is a better way to spend idle time
than say, error checking during idle time,
which also moves the heads but does a lot more at the same time.
Tip: After some
initial complaints about the noise caused by wear leveling on early implementations of the
feature, Western Digital made available a firmware
patch to reduce the noise caused by this feature. It can be found here on their web site.
It is only necessary for drives 5.1 GB in size or below that employ wear leveling, as the
patch is built into the firmware in later drives.
Next: Head Load/Unload Technology
