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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

Henrik Olsen, Dawn Solutions I/S
Get the rest there.

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