Comment 12 for bug 1539253

Sorry for a delay with an answer.
We were able to reproduce the issue in our lab so we investigated it, here is what we’ve found:

* First of all, libvirt doesn’t support RBD pools for backing volume disks out of the box [1]. There is a patch that adds it but it wasn’t merged [2] so it’s not an option here.

* CEPH volumes attached to libvirt according to CEPH official documentation [3], please note that it requires type=‘network’ in ‘disk definition.

* Actual failure occurs in libvirt in ‘virStorageFileBackendForTypeInternal’ function [4] because there is no item in ‘fileBackends’ that is of type ‘network’ and protocol ‘rbd’. Looking at definition of ‘fileBackends’ [5] it becomes clear that there is no support for that case too, since the only backend with type ‘network’ is ‘virStorageFileBackendGluster’.

So there is no way to gather disk usage data if RBD is used as backend.

[1] https://www.redhat.com/archives/libvirt-users/2015-April/msg00001.html
[2] https://www.redhat.com/archives/libvir-list/2014-March/msg00107.html
[3] http://docs.ceph.com/docs/hammer/rbd/libvirt/#configuring-the-vm
[4] http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/storage/storage_backend.c;h=70d2a59cc857db1d4147e3d976374cb6b7ed37eb;hb=09661a8e2910d4f71ff0d55d0e86f8699c369bb9#l1209
[5] http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/storage/storage_backend.c;h=70d2a59cc857db1d4147e3d976374cb6b7ed37eb;hb=09661a8e2910d4f71ff0d55d0e86f8699c369bb9#l138