Comment 4 for bug 1661772

Sorry for lag in response. Logs not handy, but to reproduce:

Set libvirt-image-backend for nova to rbd (ceph)

Create raw bootable image in glance of size 100GB (ensuring glance is ceph rbd backed)

Fill all available hypervisors such that /var/lib/nova/instances/_base has less than 100GB free

set disk_allocation_ratio = 1.0 or less

recycle nova-scheduler/nova-conductor services and attempt to boot image as new server and you'll find that it can't find a suitable hypervisor to deploy.

If you kick disk_allocation_ratio to something that would allow the 100GB image to fit within the free-space with overcommit, then the same image will boot and glance rbd and nova rbd backend work together to never move data to /var/lib/nova/instances/_base for conversion (as it just creates an rbd snapshot+COW for the new nova rbd object.

If glance and nova libvirt backend both == rbd and show_image_direct_url == true and image-format == raw, disk_allocation_ratio check should not be made