doc/mgr: flesh out introductory text

Signed-off-by: John Spray <john.spray@redhat.com>

diff --git a/doc/mgr/index.rst b/doc/mgr/index.rst
index 965a8b020183e56284f0e29f0cd17d7d7c279843..2f88aa295bc7ccd743c6eb9efdd782593b3f9a5f 100644 (file)
--- a/doc/mgr/index.rst
+++ b/doc/mgr/index.rst
@@ -8,6 +8,20 @@ The :term:`Ceph Manager` daemon (ceph-mgr) runs alongside monitor daemons,
 to provide additional monitoring and interfaces to external monitoring
 and management systems.
 
+Since the 12.x (*luminous*) Ceph release, the ceph-mgr daemon is required for
+normal operations.  The ceph-mgr daemon is an optional component in
+the 11.x (*kraken*) Ceph release.
+
+By default, the manager daemon requires no additional configuration, beyond
+ensuring it is running.  If there is no mgr daemon running, you will
+see a health warning to that effect, and some of the other information
+in the output of `ceph status` will be missing or stale until a mgr is started.
+
+Use your normal deployment tools, such as ceph-ansible or ceph-deploy, to
+set up ceph-mgr daemons on each of your mon nodes.  It is not mandatory
+to place mgr daemons on the same nodes as mons, but it is almost always
+sensible.
+
 .. toctree::
     :maxdepth: 1