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

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

On Tue, 2007-08-14 at 02:37 +0100, Mark Williamson wrote:
> > Based on some tests we ran, it seems the biggest performance
> > hit you get from running within domU is from disk I/O. We
> > did some mysql read and write tests, and for both, our
> > performance is about half that compared to native. Has that
> > been others' experience?
> >
> > Is there any way to make this better? We are using physical
> > partitions. In contrast, cpu/memory tests appear to be near
> > native.
> 
> Sounds a bit weird.  How many CPUs in this box?  My memories of the 
> benchmarks 
> suggest this should be better than you're seeing, but maybe your workload is 
> tickling some bad cases or something...
> 
> I know this sounds really weird but when comparing to native performance you 
> do need to test on the same area of the disk, have you done this?  Portions 
> of the disk nearer to the outside edge of the platter can have significantly 
> higher transfer rates due to moving at a higher linear velocity.
> 
> > Does buying commercial Xen (XenSource) help in anyway? Do
> > they have optimized disk drivers?
> 
> XenEnterprise and friends add optimised disk drivers for Windows, but if 
> you're running paravirtualised Linux then you've already got optimised disk 
> drivers, so the commercial product probably won't help.
> 
> Cheers,
> Mark
> 
> 
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.

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
-- 
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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