In at least one case, this is a problem that is blocking launching a ceph buster VM (cloudvirt1028). It appears that when an instance launches, the image is cloned to the /var/lib/nova/instances/_base path, which is the root disk. Since they are quite large sometimes (and possibly not cleaned up?), this seems to be a problem.
[bstorm@cloudvirt1028]:instances $ du -sh _base/* 20G _base/4dfdfe374763360d37aa1d930157602452d8c8dd 20G _base/612031ed4f080b36c1841be7b0a227577fcf7935 20G _base/da3e2add64b5dd579131564236e799ff768fc7a7