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SubjectRe: disk head scheduling
On Fri, 19 Mar 1999, Richard B. Johnson wrote:
> On Fri, 19 Mar 1999, Arvind Sankar wrote:
[much useful snippage]
> > The real argument against doing all this is that it will save at most one seek
> > per two requests, which is only 5% timewise. Probably not worth all the effort.
> >
>
> The real argument against this is called "IDE". If you sorted reads
> and writes (scatter/gather) for SCSI devices, you could accomplish
> something.

A number of responses are all suggesting to me that what we really need is
several different well-chosen scheduling strategies, and some reasonably
good way to choose one for a particular drive, if we want to squeeze out
the last drop of disk performance. Maybe as simple as:

if (disk is SCSI)
{
use bidirectional elevator
merge reads and writes
}
else /* probably a dumb IDE drive */
{
use one-directional elevator
segregate reads and writes
}

--
Mark H. Wood, Lead System Programmer mwood@IUPUI.Edu
Remembering the day when disks were so primitive that each pack had a
slotted ring which would interrupt a beam of light to inform the drive of
the sector boundaries.


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