[CentOS] 3Ware 9550SX and latency/system responsiveness

[Ross S. W. Walker]

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.

-Ross

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