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SubjectRe: disk head scheduling
On Fri, 19 Mar 1999, Mark H. Wood wrote:
> On Fri, 19 Mar 1999, Ingo Molnar wrote:
> > On Thu, 18 Mar 1999, Yasushi Saito wrote:
> > > What I tried to implement was two-way elevator seeking (SCAN). In my
> > > tiny benchmark that let many threads write on random files, SCAN
> > > showed a throughput improvement of anywhere between 0 to 20%. But I
> > > also noticed benefits in the original algorithm (it's fairer), so I
> > > don't know if my change makes sense.
> >
> > the bigger problem is that dumber devices will just execute non-forwards
> > ordered requests. Most modern harddisks will either cache a full track, or
> > will reorder the request per-track anyway, but eg. a floppy disk or a
> > CD-ROM will execute the requests as given, and the 'downwards' queue will
> > perform badly. Would you mind doing the seek benchmark on your CDROM too,
> > just to test this theory?
>
> Well of course a 2-way elevator should sort by *ascending* sector within
> descending track. I take it this is difficult?
Since all modern harddiscs lie faster than a politician in an election
year when asked about their geometry, the very idea of knowing enough
about the track boundaries to switch sort direction within each track is
absurd.

--
Henrik Olsen, Dawn Solutions I/S
URL=http://www.iaeste.dk/~henrik/
Get the rest there.


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