Comment 1 for bug 1904062

Train external ceph docs: https://docs.openstack.org/kolla-ansible/train/reference/storage/external-ceph-guide.html#cinder

[rbd-1]
rbd_ceph_conf=/etc/ceph/ceph.conf
rbd_user=cinder
backend_host=rbd:volumes
rbd_pool=volumes
volume_backend_name=rbd-1
volume_driver=cinder.volume.drivers.rbd.RBDDriver
rbd_secret_uuid = {{ cinder_rbd_secret_uuid }}

Ussuri made the integration simpler, adding the following to ceph.conf:

{% if cinder_backend_ceph | bool %}
[rbd-1]
volume_driver = cinder.volume.drivers.rbd.RBDDriver
volume_backend_name = rbd-1
rbd_pool = {{ ceph_cinder_pool_name }}
rbd_ceph_conf = /etc/ceph/ceph.conf
rbd_flatten_volume_from_snapshot = false
rbd_max_clone_depth = 5
rbd_store_chunk_size = 4
rados_connect_timeout = 5
rbd_user = {{ ceph_cinder_user }}
rbd_secret_uuid = {{ cinder_rbd_secret_uuid }}
report_discard_supported = True
image_upload_use_cinder_backend = True
{% endif %}

This is missing backend_host=rbd:volumes. There is a related Tripleo bug [1], which explains that this option is used to set the same host for all backends in an environment with multiple cinder-volume services representing a single storage cluster.

[1] https://bugs.launchpad.net/bugs/1753596