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Re: [Xen-users] disk speed

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] disk speed
From: "Sebastian Reitenbach" <sebastia@xxxxxxxxxxxxxxxxxxxx>
Date: Tue, 23 Oct 2007 14:07:16 +0200
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Reply-to: Sebastian Reitenbach <sebastia@xxxxxxxxxxxxxxxxxxxx>
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Sebastian Reitenbach <sebastia@xxxxxxxxxxxxxxxxxxxx> wrote: 
> Hi all,
> Dylan Martin <dmartin@xxxxxxxxxxxx> wrote: 
> > 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
> > in..
> yes, it has been done with these files. In my usecase I have to handle a 
> of files of that size. So I do not really care how fast I can handle a 
> million 1k sized files.
> > 
> > > 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 
> > > 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 
> > > 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 
> 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 
> > > pci-device like a san- or scsi-card over to the domU (with this 
> the
> > > better performance for features like live-migration).
> Trying iSCSI sounds interesting. Also I did now know yet, that I can hand 
> over the SAN device to the virtual node. 
> I want to use xen in a HA cluster, as long as everything is in a good 
> condition each virtual machine will be on a separate physical machine, but 
> if one of the physical nodes dies, two or more of the xen instances have 
> share a physical node. Do I can hand over one physical device to more than 
> one virtual instance in that case? If not, then I have to use iSCSI.
I just tried to use iscsi, but it does not seem to be faster than the 
physical disk. 

I tried to figure out whether there are some parameters that I can set to 
influence the disk speed. In the end I am now more confused than before.
I created the virtual machines with virt-manager GUI. 
It created a file /etc/xen/vm/sles10, there the disk is configured like 
disk=[ 'phy:/dev/sdv1,sda,w', 'phy:/dev/sdv2,xvdb,w', ]
in the xen manual example disk configurations look like this:
disk = [ &#8217;phy:hda1,sda1,w&#8217; ]
especially I am wondering about the differences here between xvdb and sda1, 
I tried to edit /etc/xen/vm/sles10[.xml] manually and restarted xend, but 
the virtual machine still has the xvda and xvdb devices. I also changed the 
file in /var/lib/xend/domains/.../config.sxp, but they were overwritten on 
Does this configuration makes a difference? How can I tell xen to use sda 
instead of xvdb.

I have a physical partition, where the virtual host creates its own 
partitions in it. in case I understand the example in the xen manual 
correctly, a physical partition is mapped one to one to a virtual partition.
Could that possibly speed up the disk access?

I searched the wiki for disk, and I found the 
http://wiki.xensource.com/xenwiki/XenStoreReference, but it did not helped 
me that much.

kind regards

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