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From
SubjectRe: elevator algorithm bug in ll_rw_blk.c
Date
Followup to:  <199811160539.XAA19590@dyheli.kwr>
By author: kwrohrer@ce.mediaone.net
In newsgroup: linux.dev.kernel
>
> The cylinder/head/sector numbers have been lies for years. With a SCSI
> disk you could read the true geometries, including sectors per track in
> each of the various zones, together with the defect map and know exactly
> where each block lies on the disk...but the angular position of the disc
> is a whopping 11 ms variable (5400 RPM), and you can't optimize for it
> in the OS without guessing at it, then hoping you aren't interrupted
> too badly before acting on that guess.
>

Actually u could measure the position once, measure the rate of
rotation and then just correlate to the system clock... of course
doesn't work for ZBR disks :)

> > i think it was rsx-11 which used to format the disk to take into account
> > retational delays
> I've seen DOS floppy formatting programs that do that. Meanwhile, since
> hard disk makers have been bragging about sustained transfer rate for the
> past several years, I can guarantee they've skewed the tracks properly to
> optimize sequential reads across track boundaries...and not even a SCSI
> drive will tell you the value of this skew, AFAIK...

Yes, these kinds of optimizations are useful, and beyond our
knowledge.

-hpa
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