Bill Lancaster writes:

>The issue isn't so much putting the data on the outer edges as much as it
>is making sure the data is a "compacted" as it can be...
> My example is that if you have a drive 50 percent full and it's
>completely fragmented (1 page permanent space, 1 page free space, 1 page
>perm, page free etc) the average head travel distance is r/2 where "r" is
>the radius of the platter.  If you completely condense the drive, the
>average head travel becomes less than r/4.  Why less than???  Because the
>outer tracks are "denser" than the inner tracks (e.g. more sectors per
>track).

On the other hand, the average seek time may not be reduced by nearly as
much. The reason is that seek time isn't a linear function of seek
distance. When seeking a new track, the drive must first accelerate the
heads to full speed, drive the heads close to the target position, then
decelerate them, allow them to settle, and reacquire the servo signal.
Only the middle portion, where the heads are travelling at full speed,
behaves linearly with respect to distance.

The result is that a fast disc drive may perform a track-to-track seek in
2ms, but an end-to-end seek of several thousand tracks in only 16ms. Most
of the time, especially with today's drive geometry, is spent in initial
acceleration, terminal deceleration, and servo reacquisition. This
reduces the advantage to clustering data unless the cluster is very tight
and very well-maintained. In particular, if the "cluster" is half the
drive, as in the example above, an average seek might go from 10ms to
9ms, not to 5ms as would be expected from simply comparing average
distance.

-- Bruce


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