Pluggable Disk Schedulers in GEOM

[Luigi Rizzo]

On Fri, Jan 05, 2007 at 01:25:43PM -0800, Paul Allen wrote:
> >From Luigi Rizzo <rizzo at icir.org>, Fri, Jan 05, 2007 at 10:29:05AM -0800:
> > Essentially, a process doing e.g. a 'cvs checkout' on a large
> > tree will still kill the performance of your disk no matter     
> > which scheduler you are using (elevator, hybrid, the dumb one that
> > we wrote as an example). The reasons, i suspect, are a mixture
> > of the ones described above.
> 
> Interaction of the disk scheduler and softupdates?  As I understand
> the softupdate code, the disk scheduler would be ignorant of I/O that 

this was also on 4.11 which probably does not use softupdates.

> depends on the completion of certain writes because issuing that I/O
> to the lower layers is delayed until the equiv of bio_done occurs.
> 
> I don't see what use a disk scheduler would be unless it is possible
> to push information about any partial ordering requirements down to
> the level at which the scheduler can see and enforce them.

that does not mean that a scheduler is useless in general.
in fact, what i find questionable is hardwiring unnecessary
(in the sense that they are done only for performance reasons)
ordering constraints in the layers above. I cannot comment for
the disk, but e.g. for the process scheduler there are priority
updates in many places depending on what the process is doing.

	cheers
	luigi