You got your math wrong. The heads are not moved by step-by-step
motors anymore. They accelarate (more or less linearly)
for half the time, than decelerate for the other half. The result is that time
taken to move by N tracks is proportinal to sqrt(N) rather than to N.
Then there is the settling time (once you're `more or less on the track')
and this is independent of the actual distance.
What this all means is that going across the whole disk is very cheap.
A `10ms' drive will typically take 15ms for this `return scan', which should be
compared to the average rotational latency (around 5ms) over which operating
system have very little control.
> I had a look in the three OS textbooks I have access to. One followed
> your comments pretty much to the letter, one explained that a one-way
> scan is normally used because it's easier to program than a two-way
> scan, and the third states that a two-way scan is normally used "in
> professional systems" because of its higher efficiency.
What about the age of those textbooks ?
Stefan
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