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+
+Experimental Features
+=====================
+
+CephFS includes a number of experimental features which are not fully stabilized
+or qualified for users to turn on in real deployments. We generally do our best
+to clearly demarcate these and fence them off so they cannot be used by mistake.
+
+Some of these features are closer to being done than others, though. We describe
+each of them with an approximation of how risky they are and briefly describe
+what is required to enable them. Note that doing so will *irrevocably* flag maps
+in the monitor as having once enabled this flag to improve debugging and
+support processes.
+
+Inline data
+-----------
+By default, all CephFS file data is stored in RADOS objects. The inline data
+feature enables small files (generally <2KB) to be stored in the inode
+and served out of the MDS. This may improve small-file performance but increases
+load on the MDS. It is not sufficiently tested to support at this time, although
+failures within it are unlikely to make non-inlined data inaccessible
+
+Inline data has always been off by default and requires setting
+the "inline_data" flag.
+
+
+
+Mantle: Programmable Metadata Load Balancer
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Mantle is a programmable metadata balancer built into the MDS. The idea is to
+protect the mechanisms for balancing load (migration, replication,
+fragmentation) but stub out the balancing policies using Lua. For details, see
+:doc:`/cephfs/mantle`.
+
+Snapshots
+---------
+Like multiple active MDSes, CephFS is designed from the ground up to support
+snapshotting of arbitrary directories. There are no known bugs at the time of
+writing, but there is insufficient testing to provide stability guarantees and
+every expansion of testing has generally revealed new issues. If you do enable
+snapshots and experience failure, manual intervention will be needed.
+
+Snapshots are known not to work properly with multiple filesystems (below) in
+some cases. Specifically, if you share a pool for multiple FSes and delete
+a snapshot in one FS, expect to lose snapshotted file data in any other FS using
+snapshots. See the :doc:`/dev/cephfs-snapshots` page for more information.
+
+Snapshots are known not to work with multi-MDS filesystems.
+
+Snapshotting was blocked off with the "allow_new_snaps" flag prior to Firefly.
+
+Multiple filesystems within a Ceph cluster
+------------------------------------------
+Code was merged prior to the Jewel release which enables administrators
+to create multiple independent CephFS filesystems within a single Ceph cluster.
+These independent filesystems have their own set of active MDSes, cluster maps,
+and data. But the feature required extensive changes to data structures which
+are not yet fully qualified, and has security implications which are not all
+apparent nor resolved.
+
+There are no known bugs, but any failures which do result from having multiple
+active filesystems in your cluster will require manual intervention and, so far,
+will not have been experienced by anybody else -- knowledgeable help will be
+extremely limited. You also probably do not have the security or isolation
+guarantees you want or think you have upon doing so.
+
+Note that snapshots and multiple filesystems are *not* tested in combination
+and may not work together; see above.
+
+Multiple filesystems were available starting in the Jewel release candidates
+but were protected behind the "enable_multiple" flag before the final release.
+
+
+Previously experimental features
+================================
+
+Directory Fragmentation
+-----------------------
+
+Directory fragmentation was considered experimental prior to the *Luminous*
+(12.2.x).  It is now enabled by default on new filesystems.  To enable directory
+fragmentation on filesystems created with older versions of Ceph, set
+the ``allow_dirfrags`` flag on the filesystem:
+
+::
+
+    ceph fs set <filesystem name> allow_dirfrags
+
+Multiple active metadata servers
+--------------------------------
+
+Prior to the *Luminous* (12.2.x) release, running multiple active metadata
+servers within a single filesystem was considered experimental.  Creating
+multiple active metadata servers is now permitted by default on new
+filesystems.
+
+Filesystems created with older versions of Ceph still require explicitly
+enabling multiple active metadata servers as follows:
+
+::
+
+    ceph fs set <filesystem name> allow_multimds
+
+Note that the default size of the active mds cluster (``max_mds``) is
+still set to 1 initially.
+