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On Fri, Oct 23, 2015 at 10:45:29AM +0000, Tim Bell wrote:
> https://bugs.launchpad.net/nova/+bug/1509304 has been reported.

Thanks, I slightly reworded the description to this:

    "Add support for 'l2-cache-size' (a QCOW2 run time option for
    metadata cache size) for drives"

And, added some URLs with documentation.

> 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.

Yeah, this needs input from other Nova developers who're more familiar
with the intricacies of qcow2 cache configuration.


> > -----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