Pasi Kärkkäinen wrote:
>
> On Tue, Oct 02, 2007 at 09:39:09AM -0400, Ross S. W. Walker wrote:
> > Simon Banton wrote:
> > >
> > > At 12:30 +0200 2/10/07, matthias platzer wrote:
> > > >
> > > >What I did to work around them was basically switching
> to XFS for
> > > >everything except / (3ware say their cards are fast, but only on
> > > >XFS) AND using very low nr_requests for every blockdev
> on the 3ware
> > > >card.
> > >
> > > Hi Matthias,
> > >
> > > Thanks for this. In my CentOS 5 tests the nr_requests
> turned out by
> > > default to be 128, rather than the 8192 of CentOS 4.5.
> I'll have a go
> > > at reducing it still further.
> >
> > Yes, the nr_requests should be a realistic reflection of what the
> > card itself can handle. If too high you will see io_waits stack up
> > high.
> >
> > 64 or 128 are good numbers, rarely have I seen a card that
> can handle
> > a depth larger then 128 (some older scsi cards did 256 I think).
> >
>
> Hmm.. let's say you have a linux software md-raid array made of sata
> drives.. what kind of nr_request values you should use for
> that for optimal
> performance?
>
> Thanks!
Pasi,
Good to hear from you again.
I haven't done much testing with software RAID, but after googling
around I have found that there truly is no 1 nr_requests setting
that fits all pictures.
The nr_requests is the maximum number of requests that can be queued
before the queue is unplugged and the perfect # of requests queued
is a reflection of the workload and the hardware together. Also most
system func unplug after each request, so it isn't such a big issue
unless the system is under high load.
If the default 128 isn't working I would explore hardware or RAID
configuration problems first before trying to tweak this setting.
The old nr_requests = 8192 was definitely too high.
-Ross
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