[Openstack-operators] [Glance] [Nova] Multiple backends / Qcow Derived Images

Blair Bethwaite blair.bethwaite at gmail.com
Wed Jan 4 22:22:28 UTC 2017

Hi Adam,

On 5 January 2017 at 08:48, Adam Lawson <alawson at aqorn.com> wrote:
> Just a friendly bump. To clarify, the ideas being tossed around are to host
> QCOW images on each Compute node so the provisioning is faster (i.e. less
> dependency on network connectivity to a shared back-end). I need to know if
> this is possible or not. So far, I've seen nothing that suggests that it is
> but i want to confirm that.

If you are using qcow based ephemeral disks then you will have
something like a /var/lib/nova/instances/_base/ dir on each
nova-compute. That directory holds the backing files for each
"derived" disk file in /var/lib/nova/instances/<UUID>/. It also acts
as a local image cache - their are corresponding nova-compute config
options controlling whether and how often unused base files get
removed. See e.g.
http://www.pixelbeat.org/docs/openstack_libvirt_images/ for a great
dive into the possibilities.

> Also, derived images is a QCOW thing[1], I'm wondering if creating these
> dynamically is supported by Nova and/or Glance.

The default (typical?) configuration does use qcow layering, but you
can also change your nova-compute settings to e.g. force all instances
to run off raw disk files.


More information about the OpenStack-operators mailing list