I'm puzzled by nova instance rebuilds.

When I run 'openstack server rebuild --image <whatever>' on a new test
VM, it does what I expect:

- throws away the old server image

- boots up a brand new server using the requested image with the old
user/vendor data

- attaches new server to the old network port and cinder volumes.

That's just what I want! Alas, when I try this with the actual old VMs
in need of update, usually it does a totally different thing:

- reboots the server

- changes the nova db record to reflect the requested VM (so that the
server /looks/ like it was rebuilt in 'openstack server show')

- That's it!

It absolutely does not rebuild the actual server disk image. So, for
instance, any files that were present in '/' of the old server are still
present post-rebuild. lsb_release still shows the OS of the original
base image, disagreeing with the the OS version displayed by 'openstack
server show'.

I've been reading logfiles for days but I don't see nova ever complain
(in either case) that it's doing something unexpected and in both cases
the servers wind up in state 'Active'.

Am I misunderstanding what this is supposed to do?

If not, does anyone have thoughts about what might be going wrong? I
assume nova is encountering a roadbump during the rebuild and doing a
graceful revert, but if so it sure isn't telling me.

Bobcat, kvm, VM images are hosted on ceph/rbd.