Charm does not support multiple backends of the same type

[Thobias Trevisan]

The multiple backend support for charm-glance allows only one backend per storage type, i.e., it does not permit multiple rbd backends or multiple s3 backends.

Is it possible to add that support in charm-glance? This feature is useful when there is more than one ceph cluster in the same openstack installation.

[Felipe Reyes (freyes)]

On Fri, 2021-09-03 at 17:37 +0000, Thobias Trevisan wrote:
> Is it possible to add that support in charm-glance? This feature is
> useful when there is more than one ceph cluster in the same openstack
> installation.

It is possible using different application names for each cinder-ceph
deployment.

From the Overview:

"Through the use of multiple application names (e.g. cinder-ceph-1,
cinder-ceph-2), multiple Ceph clusters can be associated with a single
Cinder deployment." - https://jaas.ai/cinder-ceph

juju deploy cs:cinder-ceph cinder-ceph-1
juju add-relation cinder-ceph-1:storage-backend cinder:storage-backend
juju add-relation cinder-ceph-1:ceph ceph-mon-1:client

juju deploy cs:cinder-ceph cinder-ceph-2
juju add-relation cinder-ceph-2:storage-backend cinder:storage-backend
juju add-relation cinder-ceph-2:ceph ceph-mon-2:client

This is how you would be able to relate 2 diffent ceph clusters (ceph-
mon-1 and ceph-mon-2).

Let me invite you to our discourse at https://discourse.charmhub.io/ ,
it's a good place where users gather to discuss about how to achieve
certain tasks with juju, charms, openstack, k8s and other workloads.

[Felipe Reyes (freyes)]

Hi Thobias,

Sorry, I confused glance with cinder when I wrote my reply.

Re: glance charm and multiple storage support, the glance charm gained support for multiple storage backends in 21.01 and you are right that this doesn't support to be related to more than 1 ceph backends.

The name of the backend is hardcoded to "ceph:rbd"[0], this could probably be extended to use the remote application's name instead.

Best,

[0] https://opendev.org/openstack/charm-glance/src/branch/master/hooks/glance_contexts.py#L286

https://docs.openstack.org/glance/train/admin/multistores.html

[Billy Olsen (billy-olsen)]

Hi Thobias,

Can you describe your use case a bit more in how you want to use multiple Ceph backends here? You mention multiple Ceph clusters but I'd be interested to know a bit more about how you intend to use the multiple ceph backends, etc.

[Thobias Trevisan (thobiastrevisan)]

Hi Billy,

We have an OpenStack cloud running with 3 availability zones. Each AZ has it own Ceph cluster. In this scenario, if I have images stored only on one of those
Ceph clusters and that AZ goes down, other AZs won't be able to start new instances, as the glance image backend is pointing only to the unavailable Ceph cluster.
That said, I would like to be able to use more than one Ceph backend for high availability and to speed up instance creation on edge sites.

Additional use cases can be found at:
https://docs.openstack.org/glance/latest/admin/multistores.html
https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.1/html/creating_and_managing_images/using-image-service-with-mulitple-stores

Thanks