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Re: [Xen-users] disk speed

To: Christian Horn <chorn@xxxxxxxxxxxx>
Subject: Re: [Xen-users] disk speed
From: Dylan Martin <dmartin@xxxxxxxxxxxx>
Date: Mon, 22 Oct 2007 10:45:32 -0700
Cc: Sebastian Reitenbach <sebastia@xxxxxxxxxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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Has all the testing that shows this slowness been done with large
files?  I'd be interested to see if the same is true under more normal
use.  E.G. copy 10 medium files 10 times each and 100 medium files 1
time each.  Caching could make it faster on domU and seeking around
could make it slower... Or who knows what other variables might kick

> On Mon, Oct 22, 2007 at 02:12:39PM +0200, Sebastian Reitenbach wrote:
> > 
> > I measured the disk speed, created a 1gb file with dd. 
> > copying that file on the dom0 always took about 5 seconds, on the domU, it 
> > took about 15-20seconds. I used "time cp large_file large_file2" to measure 
> > the speed. I only expected a small time difference, but not factor 3-4.
> We also did some testing like this, writing inside a domU sitting on lvm
> on local discs took 3.5 times as long as dom0 writes to a filesystem
> there. Some values here: http://fluxcoil.net/doku.php/xen/docs - but i
> cant explain some numbers myself and should redo the testing.
> Also the values vary when testing different xen-packages from suse.
> > As far as I know, using the physical partitions as the virtual disk, should 
> > be the fastest solution for virtual disks, compared to files.
> Files when loopbackmounted showed good values, but shouldnt be used for
> known reasons. Just that using tap:aio still makes trouble for us on those
> sles10sp1 amd64 boxes.
> > Are there different ways to present a physical partition from dom0 to a 
> > domU, that would influence the speed? Or is the speed factor I have seen 
> > above the one to expect?
> When dom0 is involved i dont know of a different way. One could still look
> into performance of space available via iscsi to the domU, or handing a
> pci-device like a san- or scsi-card over to the domU (with this trading the
> better performance for features like live-migration).
> Christian
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