Open Stack

Hi Martin:
     Which os version on your host machines? I tested kvm on rhel 6.1 have
this issue,  guest os is freebsd & virtio, on rhel 6.2 performance is well.


2012/3/26 Pádraig Brady <P at draigbrady.com>

> On 03/26/2012 01:59 PM, Martin van Wilderen - JDN BV wrote:
> > Hi List,
> >
> > I have a question about KVM disk performance. We are using Openstack
> Nova on three machines. These machines have a SSD drive with a dd write
> performance of about 130mb/s
> >
> > Within the instance the write performance is down to about 5 mb/s. When
> using the allocate trick (dd zero to disk before newfs) we get a
> performance of 20 mb/s.
>
> Interesting. I presume this performance difference is when growing the
> allocation.
> I.E. without the "dd allocate trick", writes to new files = 5MB/s, but old
> files = 20MB/s ?
>
> What file did you do the `dd` on exactly and what size was it?
>
> I could do a patch to nova to do something like:
>
> fallocate --version || exit 0
> fallocate -l $SIZE "$DISKIMG" || { rm -f "$DISKIMG"; exit 1; }
>
> But only when I get a better understand of your setup.
> Note it would need to be optional too, akin to a memory
> overcommit like flag.
>
> Note the above change would also give the benefit of
> immediate feedback of ENOSPC
>
> > Things a have tried but don't give any extra results are:
> > - Settings the disklayout from qcow2 to raw
>
> I'm a bit surprised that had no effect.
>
> > - Settings the cache type in libvirt.xml (writeback, writethrough, none)
> > - Switching KSM on and off.
> > - Tested with different guests OS, Linux, FreeBSD, Windows.
> >
> > Is there someone who had some extra info i can check? Or are there more
> people with this issue?
> >
> > Snippet from libvirt.xml
> > <driver type='qcow2'/ cache='writeback'/>
> > <source file='/var/lib/nova/instances/instance-00000113/disk'/>
> > <target dev='vda' bus='virtio'/>
>
> I'll let others more knowledgeable comment on
> general virt IO overheads.
>
> cheers,
> Pádraig.
>
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