Re: [Xen-users] disk performance about half in domU? + question about XenSource

["John A. Sullivan III" <jsullivan@xxxxxxxxxxxxxxxxxxx>]

On Tue, 2007-08-14 at 15:21 +0100, Mark Williamson wrote:
> > I don't know how relevant our experience is since we are still on Xen
> > 2.0.7 and just starting to test 3.1, however, we have noticed the same
> > thing.
> 
> Yay - it's always somewhat satisfying to see how stable the 2.x series was 
> for 
> people and that they're still using it.  ISTR that 2.x (or maybe it was the 
> early 3.x?) might have had some weird block performance regression that crept 
> in.  The fact you're seeing it for 3.1 is a little weird though.
> 
> > We assumed it was simply I/O bottleneck - we have a single RAID
> > controller trying to service several virtual servers.  We did dedicate a
> > hyperthread to dom0 and changed the scheduler (I do not recall offhand
> > to what) and those made significant improvements but we still cringe
> > when we see how many CPU cycles are simply spent waiting for disk I/O.
> > This is especially scary because, although it is a nasty mix of email,
> > web and database servers along with intensive network I/O on two of the
> > domUs which serve as VPN gateways, the actual usage is quite low - a
> > handful of users coming across an Internet connection.  I don't know how
> > these would fare under LAN load with a few hundred users.
> >
> > Not complaining - just sharing.  Thanks for a great product - John
> 
> Well, it sounds slower than I'd expect things to be from what I remember of 
> the benchmarks...  Have you tried just doing a big dd (e.g. dd a big file 
> into /dev/null using a decent block size) and seeing what raw bandwidth you 
> can get?
> 
> Could you check what scheduler this is?  Credit is the most well supported at 
> the moment.  Is it an SMP or a UP box?
> 
> Cheers,
> Mark
> 
We haven't tested 3.1 enough to know if this is still a problem.  If it
is important to gather this information for 2.0.7, I'll do so and submit
it.  Please let me know so I don't spend the time unnecessarily.  If it
will be helpful, I most gladly try the dd and figure out what scheduler
we ultimately used.  Thanks - John
-- 
John A. Sullivan III
Open Source Development Corporation
+1 207-985-7880
jsullivan@xxxxxxxxxxxxxxxxxxx

Financially sustainable open source development
http://www.opensourcedevel.com


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