On Thu, Oct 22, 2015 at 06:01:42PM +0000, Marc Heckmann 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.b...@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.
There should be no performance impact of using fully pre-allocated raw images. Any decent modern filesystem (ext4, xfs) supports fallocate which allows you to pre-allocate an arbitrary sizes plain file in constant time. By default Nova does *not* preallocate images - so both raw & qcow2 will grow-on-demand as guest writes to sectors. If the "preallocate_images" nova.conf option is set to "space", then Nova will call fallocate for both raw & qcow2 images to fully allocate the maximum space they require. There's no appreciable time overhead for this - it just prevents you overcommitting disk obviously. If you fully preallocate a qcow2 image its performance should pretty much match raw images (modulo the l2-cache-size item mentioned below), unfortunately, the way Nova is preallocating qcow2 images is wrong - it preallocates the space on disk, but does not pre-initialize the internal qcow2 data structures to match :-( So we need to fix that for qcow2 in Nova. > 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_.pdf > > I think it would be worthwhile for Openstack (and libvirt if required) > to support the "l2-cache-size" option for qcow2. Yep, we should look at supporting that. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :| _______________________________________________ OpenStack-operators mailing list OpenStack-operators@lists.openstack.org http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-operators