doc/dev/ceph-volume add systemd reference

Signed-off-by: Alfredo Deza <adeza@redhat.com>

diff --git a/doc/dev/ceph-volume/systemd.rst b/doc/dev/ceph-volume/systemd.rst
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+.. _ceph-volume-systemd:
+
+systemd
+=======
+The workflow to *"activate"* an OSD is by relying on systemd unit files and its
+ability to persist information as a suffix to the instance name.
+
+``ceph-volume`` exposes the following convention for unit files::
+
+    ceph-volume@<osd id>-<osd uuid>-<device type>
+
+For example, this is how enabling an OSD could look like using the :doc:`lvm
+<lvm/lvm>` sub command::
+
+    systemctl enable ceph-volume@0-8715BEB4-15C5-49DE-BA6F-401086EC7B41-lvm
+
+
+These 3 pieces of persisted information are needed by the systemd unit so that
+it understands what OSD it needs to activate along with the related sub
+command. Since ``lvm`` is not the only subcommand that will be supported, this
+is how it will allow other device types to be defined.
+
+At some point for example, for plain disks, it could be::
+
+    systemctl enable ceph-volume@0-8715BEB4-15C5-49DE-BA6F-401086EC7B41-disk
+
+
+At startup, the systemd unit will execute a helper script that will parse the
+suffix and will end up calling ``ceph-volume`` back. Using the previous
+example for lvm, that call will look like::
+
+    ceph-volume lvm activate 0 8715BEB4-15C5-49DE-BA6F-401086EC7B41
+
+
+.. warning:: These workflows are not meant to be public, but are documented so that
+             it is clear what the tool is doing behind the scenes. Do not alter
+             any of these values.