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+.. _ceph-volume-lvm-activate:
+
+``activate``
+============
+Once :ref:`ceph-volume-lvm-prepare` is completed, and all the various steps
+that entails are done, the volume is ready to get "activated".
+
+This activation process enables a systemd unit that persists the OSD ID and its
+UUID (also called ``fsid`` in Ceph CLI tools), so that at boot time it can
+understand what OSD is enabled and needs to be mounted.
+
+.. note:: The execution of this call is fully idempotent, and there is no
+          side-effects when running multiple times
+
+New OSDs
+--------
+To activate newly prepared OSDs both the :term:`OSD id` and :term:`OSD uuid`
+need to be supplied. For example::
+
+    ceph-volume lvm activate --bluestore 0 0263644D-0BF1-4D6D-BC34-28BD98AE3BC8
+
+.. note:: The UUID is stored in the ``fsid`` file in the OSD path, which is
+          generated when :ref:`ceph-volume-lvm-prepare` is used.
+
+Activating all OSDs
+-------------------
+It is possible to activate all existing OSDs at once by using the ``--all``
+flag. For example::
+
+    ceph-volume lvm activate --all
+
+This call will inspect all the OSDs created by ceph-volume that are inactive
+and will activate them one by one. If any of the OSDs are already running, it
+will report them in the command output and skip them, making it safe to rerun
+(idempotent).
+
+requiring uuids
+^^^^^^^^^^^^^^^
+The :term:`OSD uuid` is being required as an extra step to ensure that the
+right OSD is being activated. It is entirely possible that a previous OSD with
+the same id exists and would end up activating the incorrect one.
+
+
+dmcrypt
+^^^^^^^
+If the OSD was prepared with dmcrypt by ceph-volume, there is no need to
+specify ``--dmcrypt`` on the command line again (that flag is not available for
+the ``activate`` subcommand). An encrypted OSD will be automatically detected.
+
+
+Discovery
+---------
+With OSDs previously created by ``ceph-volume``, a *discovery* process is
+performed using :term:`LVM tags` to enable the systemd units.
+
+The systemd unit will capture the :term:`OSD id` and :term:`OSD uuid` and
+persist it. Internally, the activation will enable it like::
+
+    systemctl enable ceph-volume@lvm-$id-$uuid
+
+For example::
+
+    systemctl enable ceph-volume@lvm-0-8715BEB4-15C5-49DE-BA6F-401086EC7B41
+
+Would start the discovery process for the OSD with an id of ``0`` and a UUID of
+``8715BEB4-15C5-49DE-BA6F-401086EC7B41``.
+
+.. note:: for more details on the systemd workflow see :ref:`ceph-volume-lvm-systemd`
+
+The systemd unit will look for the matching OSD device, and by looking at its
+:term:`LVM tags` will proceed to:
+
+# mount the device in the corresponding location (by convention this is
+  ``/var/lib/ceph/osd/<cluster name>-<osd id>/``)
+
+# ensure that all required devices are ready for that OSD. In the case of
+a journal (when ``--filestore`` is selected) the device will be queried (with
+``blkid`` for partitions, and lvm for logical volumes) to ensure that the
+correct device is being linked. The symbolic link will *always* be re-done to
+ensure that the correct device is linked.
+
+# start the ``ceph-osd@0`` systemd unit
+
+.. note:: The system infers the objectstore type (filestore or bluestore) by
+          inspecting the LVM tags applied to the OSD devices
+
+Existing OSDs
+-------------
+For existing OSDs that have been deployed with ``ceph-disk``, they need to be
+scanned and activated :ref:`using the simple sub-command <ceph-volume-simple>`.
+If a different tooling was used then the only way to port them over to the new
+mechanism is to prepare them again (losing data). See
+:ref:`ceph-volume-lvm-existing-osds` for details on how to proceed.
+
+Summary
+-------
+To recap the ``activate`` process for :term:`bluestore`:
+
+#. require both :term:`OSD id` and :term:`OSD uuid`
+#. enable the system unit with matching id and uuid
+#. Create the ``tmpfs`` mount at the OSD directory in
+   ``/var/lib/ceph/osd/$cluster-$id/``
+#. Recreate all the files needed with ``ceph-bluestore-tool prime-osd-dir`` by
+   pointing it to the OSD ``block`` device.
+#. the systemd unit will ensure all devices are ready and linked
+#. the matching ``ceph-osd`` systemd unit will get started
+
+And for :term:`filestore`:
+
+#. require both :term:`OSD id` and :term:`OSD uuid`
+#. enable the system unit with matching id and uuid
+#. the systemd unit will ensure all devices are ready and mounted (if needed)
+#. the matching ``ceph-osd`` systemd unit will get started