Chapter 3 of Integrating an Overcloud with an Existing Red Hat Ceph Cluster [1] has a reference to an environment file which has been removed. Please replace all instances of: /usr/share/openstack-tripleo-heat-templates/environments/storage/external-ceph.yaml with: /usr/share/openstack-tripleo-heat-templates/environments/ceph-ansible/ceph-ansible-external.yaml [1] https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/12/html-single/integrating_an_overcloud_with_an_existing_red_hat_ceph_cluster/#integrate-cluster
While updating this section of the document you could also mention that by using these steps one can make OSP12 talk to an external Ceph Luminous (RHCS3) cluster. So in OSP12, if RHCS is external, its version can be 1.3, 2, and 3. Follow the same steps for RHCS 2 and 3.
*** This bug has been marked as a duplicate of bug 1544536 ***