Volume-backed instance cannot backup

It is said that "This API is not supported for volume-backed instances." in "Create Server Back Up" API in the compute API reference.

https://developer.openstack.org/api-ref/compute/#create-server-back-up-createbackup-action

So it is by design.
If you want to back up cinder volumes, you can utilize "Create a backup" API in cinder.

https://developer.openstack.org/api-ref/block-storage/v3/index.html#create-a-backup

Firstly,"Create a backup" API in cinder can back up a vlolume, but it's a operation of volume and not a operation of iinstance.
What's more important is that "nova backup" keep specified number backups of instance. But cinder "Create backup" API cannot do this.

Might want to talk with Fei Long Wang about this, I think he tried adding this support to nova a few years ago.

Yeah see: https://review.openstack.org/#/c/164494/

I'm going to mark this as invalid since you have a blueprint for it:

https://blueprints.launchpad.net/nova/+spec/volume-backed-instance-support-backup