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[Samuel Winchenbach <swinchen@xxxxxxxxx>]

N
o harm will come to existing VMs if I set "use_cow_images=False" and
restart nova-api correct?  Just all new VMs will be created with a raw disk
backend?


Thanks,
Sam


On Mon, Mar 4, 2013 at 2:54 PM, Pádraig Brady <P@xxxxxxxxxxxxxx> wrote:

> On 03/04/2013 07:19 PM, Samuel Winchenbach wrote:
> > Hi All,
> >
> > I have a cluster of three nodes set up.  Live migration is accomplished
> via an NFS mount over 10GigE shared by all three nodes.
> >
> > I have noticed that when a VM is brand new I am getting close to 60MB/s
> disk write when using "dd if=/dev/zero of=file bs=1M count=1k
> conv=fdatasync". after doing this 10s of times the perfomance of the disk
> seems to drop  to around 4 - 12 MB/s.
> >
> > I have also noticed that doing a live-migration causes a similar effect
> immediately.
> >
> > Here is the virsh xml output of one of my VMs:
> https://gist.github.com/swinchen/397fbe3bb74305064944
> >
> > I have read several "tuning" guides and most of the suggestions seems to
> be configured already (cache='none', virtio for network, disk and
> memballoon).
> >
> > Do you think qcow2 is causing my issue and if so is there a way to boot
> an instance and override the disk format?
>
> qcow may possibly be the issue.
>
> You could use raw disk images by setting the
> use_cow_images=False nova config option.
> The tradeoff there is slower instance start and
> increased (but more guaranteed) disk usage.
>
> Alternatively you could try to fallocate the disk
> image before running the perf test like:
>   # get virt_size
>   qemu-img info /var/lib/nova/instances/instance-0000002f/disk
>   fallocate -n -l $virt_size /var/lib/nova/instances/instance-0000002f/disk
>
> There is also the possibility of adjusting nova
> to use preallocation=metadata on the qcow images
> (forfeiting CoW in the process), as discussed in "future work" at:
> https://blueprints.launchpad.net/nova/+spec/preallocated-images
>
> thanks,
> Pádraig.
>