On Tue, Oct 02, 2007 at 03:56:17PM -0400, Ross S. W. Walker wrote:
> Pasi Kärkkäinen wrote:
> >
> > On Tue, Oct 02, 2007 at 08:57:28PM +0300, 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?
> > >
> >
> > Or let's put it this way:
> >
> > You have a md-raid array on dom0. What kind of nr_requests
> > values should you
> > use for normal 7200 rpm sata-ncq disks on intel ich8 (ncq)
> > controller?
> >
> > And then this md-array is seen as xvdb by domU.. what kind of
> > nr_requests
> > values should you use in domU?
> >
> > io-scheduler/elevator should be deadline in domU I assume..
> > how about in
> > dom0? deadline there too?
>
> Arrr, where thou go thar be monsters...
>
> You got me Pasi, with Xen as the workload it adds a whole new
> dimension.
>
> Unless you have hardware RAID, stick to the default setting
> and when you see a bottleneck double check your hardware
> drivers and RAID config first and only twiddle the queue
> settings if everything else has been twiddled first.
>
OK.
I'm seeing quite high io-wait times on domU, but hardly any io-wait in
dom0.. so I was wondering if I have too high nr_requests on domU. I think it
is 256 atm.
Maybe I'll have to do some benchmarking.
What's the best multi-threaded / multi-process io-benchmark utility that
works with filesystems instead of raw devices? and can read/write multiple
files at once..
-- Pasi
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