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SubjectRe: elevator algorithm bug in ll_rw_blk.c
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> On Sun, 15 Nov 1998, Vojtech Pavlik wrote:
> > You could apply a two way elevator well if you knew about the physical
> > characteristics of the drive. Unfortunately there is no way to find that out
> > for most modern drives.
>
> Actually, by analyzing timings on carefully selected request sequences, you
> can figure out pretty much all of the physical characteristics of most
> drives

This would be trivial if the real disk geometry was simple, without
variations in the number of sectors per cylinder. It isn't so though.
Manufacturers can sometimes fit more sectors on the outer cylinders,
this gives faster transfers and more data in those areas. Figuring
out the real geometry with timing tests is not simple in this case, there may
be lots of different sector counts and you have no idea where they change.

You would need to do timing tests on every physical cylinder (not
knowing where they start in the first place) and build a big table.
Not something we want during bootup. And timing could go wrong
if anything else happen with the machine in the meantime.

Helge Hafting



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