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https://bugs.launchpad.net/nova/+bug/1509304 has been reported.

Not sure the best place to ask for the option... something in nova.conf may allocate too much memory if not many VMs need it but adding support in flavors/images would be more complex. This could be done in stages.

Tim

> -----Original Message-----
> From: Kashyap Chamarthy [mailto:kchamart at redhat.com]
> Sent: 23 October 2015 12:37
> To: Tim Bell <Tim.Bell at cern.ch>
> Cc: Marc Heckmann <marc.heckmann at ubisoft.com>; openstack-
> operators at lists.openstack.org
> Subject: Re: [Openstack-operators] raw ephemeral disks and qcow2 images
> 
> On Fri, Oct 23, 2015 at 06:21:23AM +0000, Tim Bell wrote:
> >
> > On 22/10/15 20:01, "Marc Heckmann" <marc.heckmann at ubisoft.com>
> wrote:
> >
> > >Hi,
> > >
> > >On Thu, 2015-10-22 at 08:17 -0700, Abel Lopez wrote:
> > >> I've actually looked for this for our RBD backed ephemeral
> > >> instances, but found the options lacking. I last looked in Juno.
> > >>
> > >> On Thursday, October 22, 2015, Tim Bell <Tim.Bell at cern.ch> wrote:
> > >>
> > >>
> > >>         Has anyone had experience with setting up Nova with KVM so it
> > >>         has raw ephemeral disks but qcow2 images for the VMs ? We’ve
> > >>         got very large ephemeral disks and could benefit from the
> > >>         performance of raw volumes for this.
> > >
> > >We looked into this for the very same reasons and it doesn't seem to
> > >be supported.
> > >
> > >That being said, I'm fearful of the boot time performance impact of
> > >using RAW for ephemeral.
> > >
> > >I suggest you check out the following presentation about qcow2
> > >performance if you haven't already done so:
> > >
> > >http://events.linuxfoundation.org/sites/events/files/slides/p0.pp_.pd
> > >f
> 
> I was about to recommend this (/me was present in this session in person).
> 
> Also, slightly off-topic: I'd recommend the "Incremental Backups"
> session:
> 
> 
> http://events.linuxfoundation.org/sites/events/files/slides/kvm2015_rh_lig
> ht_44_vfinal.pdf
> 
> > >I think it would be worthwhile for Openstack (and libvirt if
> > >required) to support the "l2-cache-size" option for qcow2.
> >
> > +1 for l2-cache-size. This presentation was the basis under which I
> > was looking for a temporary approach until we get the latest qcow2
> > support.
> 
> Would you or Marc like to file a bug report to track this?
> 
> The 'l2-cache-size' runtime pption to '-drive' command-line argument is a
> recent addition (AUG-2015):
> 
>     [...]-drive file=hd.qcow2,l2-cache-size=2097152[...]
> 
> --
> /kashyap
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