Ceph Charm Migration documentation needs a caveat for lp#1452641

[Drew Freiberger]

During a recent incident, we discovered that in the transition of ceph to ceph-mon, the new 3 ceph-mon units get different IPs from the old mons (necessarily, and understood). This gets communicated through all of the relation updates as well as the live ceph clients updating their monmap. However, there is a bug that we uncovered in Nova when performing this ceph->ceph-mon+ceph-osd migration on a trusty/mitaka cloud.

http://pad.lv/1452641 - Static Ceph mon IP addresses in connection_info can prevent VM startup

I believe it should be noted in this document that any previously deployed nova instances using cinder to interface with ceph, or using nova's libvirt-image-backend=rbd settings will necessitate a manual database update to change the old 3 ceph-mon IPs to the new IPs.

The stale data for cinder will need to be updated in the block_device_mapping table.

- [ ] This doc is inaccurate in this way: ______
- [X] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

-----------------------------------
Release: 0.1 on 2018-05-23 21:16
SHA: 94a2231947c4a55fc80b7253b1106e78ceb003cb
Source: Can't derive source file URL
URL: https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/latest/app-ceph-migration.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/693600

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/693600
Committed: https://git.openstack.org/cgit/openstack/charm-deployment-guide/commit/?id=c6b135270da6c5cc1e63271914eb73de6ea6302e
Submitter: Zuul
Branch: master

commit c6b135270da6c5cc1e63271914eb73de6ea6302e
Author: Peter Matulis <email address hidden>
Date: Fri Nov 8 18:02:57 2019 -0500

    Add note re nova and cinder for migrating to ceph-mon

    Closes-Bug: #1776780

    Change-Id: Ib97a1c81c09c69c501c3c999726fea8eff34e60d