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Re: [Xen-users] disk speed

To: Sebastian Reitenbach <sebastia@xxxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] disk speed
From: Christian Horn <chorn@xxxxxxxxxxxx>
Date: Mon, 22 Oct 2007 19:31:20 +0200
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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).


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