Command overhead refers to the time that elapses from when a command is given
to the hard disk until something actually starts happening to fulfill the command. In a
way, it's sort of like a "reaction time" for the disk. Consider when you're
driving a car and a streetlight suddenly turns red; your "command overhead" is
the time that elapses from when the light changes, until your foot starts to move toward
the brake pedal. (Or if you live in the greater Boston area, the accelerator. ;^)
Like settle time, command overhead is a component of access time and thus part of the
overall equation of random positioning performance. Also like settle time, it is generally
very small and not highly variable between drive designs; it is generally around 0.5 ms
for pretty much all modern drives and therefore not something that requires a lot of
attention. Also like settle time, it is sometimes not even specified separately from seek
time but rather "lumped in" with it. It is dominated by seek time and rotational latency
in the overall positioning performance picture.
Command overhead is influenced primarily by the
design of the disk's integrated controller, and to some extent, the nature of the interface used (which of course is a major
influence on the design of the controller!)
Next: Latency
