Ceph

http://ceph.com/docs/master/start/quick-ceph-deploy/#add-a-monitor says

Tip

In production environments, we recommend running Ceph Monitors on nodes that do not run OSDs.

Two different customers have read that as "don't put mons and osds on the same host." We should
lighten that warning and/or explain the motivation for the explanation as it's needlessly
confusing users.