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+.. _ceph-volume-simple-activate:
+
+``activate``
+============
+Once :ref:`ceph-volume-simple-scan` has been completed, and all the metadata
+captured for an OSD has been persisted to ``/etc/ceph/osd/{id}-{uuid}.json``
+the OSD is now ready to get "activated".
+
+This activation process **disables** all ``ceph-disk`` systemd units by masking
+them, to prevent the UDEV/ceph-disk interaction that will attempt to start them
+up at boot time.
+
+The disabling of ``ceph-disk`` units is done only when calling ``ceph-volume
+simple activate`` directly, but is avoided when being called by systemd when
+the system is booting up.
+
+The activation process requires using both the :term:`OSD id` and :term:`OSD uuid`
+To activate parsed OSDs::
+
+    ceph-volume simple activate 0 6cc43680-4f6e-4feb-92ff-9c7ba204120e
+
+The above command will assume that a JSON configuration will be found in::
+
+    /etc/ceph/osd/0-6cc43680-4f6e-4feb-92ff-9c7ba204120e.json
+
+Alternatively, using a path to a JSON file directly is also possible::
+
+    ceph-volume simple activate --file /etc/ceph/osd/0-6cc43680-4f6e-4feb-92ff-9c7ba204120e.json
+
+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.
+
+
+Discovery
+---------
+With OSDs previously scanned by ``ceph-volume``, a *discovery* process is
+performed using ``blkid`` and ``lvm``. There is currently support only for
+devices with GPT partitions and LVM logical volumes.
+
+The GPT partitions will have a ``PARTUUID`` that can be queried by calling out
+to ``blkid``, and the logical volumes will have a ``lv_uuid`` that can be
+queried against ``lvs`` (the LVM tool to list logical volumes).
+
+This discovery process ensures that devices can be correctly detected even if
+they are repurposed into another system or if their name changes (as in the
+case of non-persisting names like ``/dev/sda1``)
+
+The JSON configuration file used to map what devices go to what OSD will then
+coordinate the mounting and symlinking as part of activation.
+
+To ensure that the symlinks are always correct, if they exist in the OSD
+directory, the symlinks will be re-done.
+
+A 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@simple-$id-$uuid
+
+For example::
+
+    systemctl enable ceph-volume@simple-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``.
+
+
+The systemd process will call out to activate passing the information needed to
+identify the OSD and its devices, and it 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 and properly linked,
+regardless of objectstore used (filestore or bluestore). The symbolic link will
+**always** be re-done to ensure that the correct device is linked.
+
+# start the ``ceph-osd@0`` systemd unit