Libvirt RBD image backend support for glance multistore

Currently, Nova does not natively support a deployment where there are multiple Ceph RBD backends that are known to glance. If there is only one, Nova and Glance collaborate for fast-and-light image-to-VM cloning behaviors. If there is more than one, Nova generally does not handle the situation well, resulting in silent slow-and-heavy behavior in the worst case, and a failed instance boot failsafe condition in the best case. We can do better.

Problem description

There are certain situations where it is desirable to have multiple independent Ceph clusters in a single openstack deployment. The most common would be a multi-site or edge deployment where it is important that the Ceph cluster is physically close to the compute nodes that it serves. Glance already has the ability to address multiple ceph clusters, but Nova is so naive about this that such a configuration will result in highly undesirable behavior.

Normally when Glance and Nova collaborate on a single Ceph deployment, images are stored in Ceph by Glance when uploaded by the operator or the user. When Nova starts to boot an instance, it asks Ceph to make a Copy-on-Write clone of that image, which extremely fast and efficient, resulting in not only reduced time to boot and lower network traffic, but a shared base image across all compute nodes.

If, on the other hand, you have two groups of compute nodes, each with their own Ceph deployment, extreme care must be taken currently to ensure that an image stored in one is not booted on a compute node assigned to the other. Glance can represent that a single logical image is stored in one or both of those Ceph stores and Nova looks at this during instance boot. However, if the image is not in its local Ceph cluster, it will quietly download the image from Glance and then upload it to its local Ceph as a raw flat image each time an instance from that image is booted. This results in more network traffic and disk usage than is expected. We merged a workaround to make Nova refuse to do this antithetical behavior, but it just causes a failed instance boot.

Use Cases

  • As an operator I want to be able to have a multi-site single Nova deployment with one Ceph cluster per site and retain the high-performance copy-on-write behavior that I get with a single one.

  • As a power user which currently has to pre-copy images to a remote-site ceph backend with glance before being able to boot an instance, I want to not have to worry about such things and just have Nova do that for me.

Proposed change

Glance can already represent that a single logical image is stored in multiple locations. Recently, it gained an API to facilitate copying images between backend stores. This means that an API consumer can request that it copy an image from one store to another by doing an “import” operation where the method is “copy-image”.

The change proposed in this spec is to augment the existing libvirt RBD imagebackend code so that it can use this image copying API when needed. Currently, we already look at all the image locations to find which one matches our Ceph cluster, and then use that to do the clone. After this spec is implemented, that code will still examine all the current locations, and if none match, ask Glance to copy the image to the appropriate backend store so we can continue without failure or other undesirable behavior.

In the case where we do need Glance to copy the image to our store, Nova can monitor the progress of the operation through special image properties that Glance maintains on the image. These indicate that the process is in-progress (via os_glance_importing_to_stores) and also provide notice when an import has failed (via os_glance_failed_import). Nova will need to poll the image, waiting for the process to complete, and some configuration knobs will be needed to allow for appropriate tuning.


One alternative is always to do nothing. This is enhanced behavior on top of what we already support. We could just tell people not to use multiple Ceph deployments or add further checks to make sure we do not do something stupid if they do.

We could teach nova about multiple RBD stores in a more comprehensive way, which would basically require either pulling ceph information out of Glance, or configuring Nova with all the same RBD backends that Glance has. However, we would need to teach Nova about the topology and configure it to not do stupid things like use a remote Ceph just because the image is there.

Data model impact


REST API impact


Security impact

Users can already use the image import mechanism in Glance, so Nova using it on their behalf does not result in privilege escalation.

Notifications impact


Other end user impact

This removes the need for users to know details about the deployment configuration and topology, as well as eliminates the need to manually pre-place images in stores.

Performance Impact

Image boot time will be impacted in the case when a copy needs to happen, of course. Performance overall will be much better because operators will be able to utilize more Ceph clusters if they wish, and locate them closer to the compute nodes they serve.

Other deployer impact

Some additional configuration will be needed in order to make this work. Specifically, Nova will need to know the Glance store name that represents the RBD backend it is configured to use. Additionally, there will be some timeout tunables related to how often we poll the Glance server for status on the copy, as well as an overall timeout for how long we are willing to wait.

One other deployer consideration is that Glance requires an API setup capable of doing background tasks in order to support the image_import API. That means mod_wsgi or similar, as uwsgi does not provide reliable background task support. This is just a Glance requirement, but worth noting here.

Developer impact

The actual impact to the imagebackend code is not large as we are just using a new mechanism in Glance’s API to do the complex work of copying images between backends.

Upgrade impact

In order to utilize this new functionality, at least Glance from Ussuri will be required for a Victoria Nova. Individual nova-compute services can utilize this new functionality immediately during a partial upgrade scenario so no minimum service version checks are required. The control plane does not know which RBD backend each compute node is connected to, and thus there is no need for control-plane-level upgrade sensitivity to this feature.



Primary assignee:


Feature Liaison

Feature liaison:


Work Items

  • Plumb the image_import function through the nova.image.glance modules

  • Teach the libvirt RBD imagebackend module how to use the new API to copy images to its own backend when necessary and appropriate.

  • Document the proper setup requirements for administrators


  • Glance requirements are already landed and available


  • Unit testing, obviously.

  • Functional testing turns out to be quite difficult, as we stub out massive amounts of the underlying image handling code underneath our fake libvirt implementation. Adding functional tests for this would require substantial refactoring of all that test infrastructure, dwarfing the actual code in this change.

  • Devstack testing turns out to be relatively easy. I think we can get a solid test of this feature on every run, by altering that job to:

  • Enable Glance and Nova multistore support.

  • Enable Glance image conversion support, to auto-convert the default QCOW Cirros image to raw when we upload it.

  • Create two stores, one file-backed (like other jobs) and one RBD-backed (like the current Ceph job).

  • Default the Cirros upload to the file-backed store.

  • The first use of the Cirros image in a tempest test will cause Nova to ask Glance to copy the image from the file-backed store to the RBD-backed store. Subsequent tests will see it as already in the RBD store and proceed as normal.

    The real-world goal of this is to facilitate RBD-to-RBD backend store copying, but from Nova’s perspective file-to-RBD is an identical process, so it’s a good analog without having to bootstrap two independent Ceph clusters in a devstack job.

Documentation Impact

This is largely admin-focused. Users that are currently aware of this limitation already have admin-level knowledge if they are working around it. Successful implementation will just eliminate the need to care about multiple Ceph deployments going forward. Thus admin and configuration documentation should be sufficient.




Release Name