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+.. _ceph-volume-systemd:
+
+systemd
+=======
+As part of the activation process (either with :ref:`ceph-volume-lvm-activate`
+or :ref:`ceph-volume-simple-activate`), systemd units will get enabled that
+will use the OSD id and uuid as part of their name. These units will be run
+when the system boots, and will proceed to activate their corresponding
+volumes via their sub-command implementation.
+
+The API for activation is a bit loose, it only requires two parts: the
+subcommand to use and any extra meta information separated by a dash. This
+convention makes the units look like::
+
+    ceph-volume@{command}-{extra metadata}
+
+The *extra metadata* can be anything needed that the subcommand implementing
+the processing might need. In the case of :ref:`ceph-volume-lvm` and
+:ref:`ceph-volume-simple`, both look to consume the :term:`OSD id` and :term:`OSD uuid`,
+but this is not a hard requirement, it is just how the sub-commands are
+implemented.
+
+Both the command and extra metadata gets persisted by systemd as part of the
+*"instance name"* of the unit.  For example an OSD with an ID of 0, for the
+``lvm`` sub-command would look like::
+
+    systemctl enable ceph-volume@lvm-0-0A3E1ED2-DA8A-4F0E-AA95-61DEC71768D6
+
+The enabled unit is a :term:`systemd oneshot` service, meant to start at boot
+after the local file system is ready to be used.
+
+
+Failure and Retries
+-------------------
+It is common to have failures when a system is coming up online. The devices
+are sometimes not fully available and this unpredictable behavior may cause an
+OSD to not be ready to be used.
+
+There are two configurable environment variables used to set the retry
+behavior:
+
+* ``CEPH_VOLUME_SYSTEMD_TRIES``: Defaults to 30
+* ``CEPH_VOLUME_SYSTEMD_INTERVAL``: Defaults to 5
+
+The *"tries"* is a number that sets the maximum number of times the unit will
+attempt to activate an OSD before giving up.
+
+The *"interval"* is a value in seconds that determines the waiting time before
+initiating another try at activating the OSD.