Re: [Xen-users] disk speed

Sebastian Reitenbach
Tue, 23 Oct 2007 05:25:54 -0000

Hi all,

Dylan Martin <dmartin@sccd.ctc.edu> 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 lot
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
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).
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 to
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.

kind regards
Sebastian