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+.. _cephfs-multimds:
+
+Configuring multiple active MDS daemons
+---------------------------------------
+
+*Also known as: multi-mds, active-active MDS*
+
+Each CephFS filesystem is configured for a single active MDS daemon
+by default.  To scale metadata performance for large scale systems, you
+may enable multiple active MDS daemons, which will share the metadata
+workload with one another.
+
+When should I use multiple active MDS daemons?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You should configure multiple active MDS daemons when your metadata performance
+is bottlenecked on the single MDS that runs by default.
+
+Adding more daemons may not increase performance on all workloads.  Typically,
+a single application running on a single client will not benefit from an
+increased number of MDS daemons unless the application is doing a lot of
+metadata operations in parallel.
+
+Workloads that typically benefit from a larger number of active MDS daemons
+are those with many clients, perhaps working on many separate directories.
+
+
+Increasing the MDS active cluster size
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Each CephFS filesystem has a *max_mds* setting, which controls how many ranks
+will be created.  The actual number of ranks in the filesystem will only be
+increased if a spare daemon is available to take on the new rank. For example,
+if there is only one MDS daemon running, and max_mds is set to two, no second
+rank will be created. (Note that such a configuration is not Highly Available
+(HA) because no standby is available to take over for a failed rank. The
+cluster will complain via health warnings when configured this way.)
+
+Set ``max_mds`` to the desired number of ranks.  In the following examples
+the "fsmap" line of "ceph status" is shown to illustrate the expected
+result of commands.
+
+::
+
+    # fsmap e5: 1/1/1 up {0=a=up:active}, 2 up:standby
+
+    ceph fs set <fs_name> max_mds 2
+
+    # fsmap e8: 2/2/2 up {0=a=up:active,1=c=up:creating}, 1 up:standby
+    # fsmap e9: 2/2/2 up {0=a=up:active,1=c=up:active}, 1 up:standby
+
+The newly created rank (1) will pass through the 'creating' state
+and then enter this 'active state'.
+
+Standby daemons
+~~~~~~~~~~~~~~~
+
+Even with multiple active MDS daemons, a highly available system **still
+requires standby daemons** to take over if any of the servers running
+an active daemon fail.
+
+Consequently, the practical maximum of ``max_mds`` for highly available systems
+is at most one less than the total number of MDS servers in your system.
+
+To remain available in the event of multiple server failures, increase the
+number of standby daemons in the system to match the number of server failures
+you wish to withstand.
+
+Decreasing the number of ranks
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Reducing the number of ranks is as simple as reducing ``max_mds``:
+
+::
+    
+    # fsmap e9: 2/2/2 up {0=a=up:active,1=c=up:active}, 1 up:standby
+    ceph fs set <fs_name> max_mds 1
+    # fsmap e10: 2/2/1 up {0=a=up:active,1=c=up:stopping}, 1 up:standby
+    # fsmap e10: 2/2/1 up {0=a=up:active,1=c=up:stopping}, 1 up:standby
+    ...
+    # fsmap e10: 1/1/1 up {0=a=up:active}, 2 up:standby
+
+The cluster will automatically stop extra ranks incrementally until ``max_mds``
+is reached.
+
+See :doc:`/cephfs/administration` for more details which forms ``<role>`` can
+take.
+
+Note: stopped ranks will first enter the stopping state for a period of
+time while it hands off its share of the metadata to the remaining active
+daemons.  This phase can take from seconds to minutes.  If the MDS appears to
+be stuck in the stopping state then that should be investigated as a possible
+bug.
+
+If an MDS daemon crashes or is killed while in the ``up:stopping`` state, a
+standby will take over and the cluster monitors will against try to stop
+the daemon.
+
+When a daemon finishes stopping, it will respawn itself and go back to being a
+standby.
+
+
+Manually pinning directory trees to a particular rank
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In multiple active metadata server configurations, a balancer runs which works
+to spread metadata load evenly across the cluster. This usually works well
+enough for most users but sometimes it is desirable to override the dynamic
+balancer with explicit mappings of metadata to particular ranks. This can allow
+the administrator or users to evenly spread application load or limit impact of
+users' metadata requests on the entire cluster.
+
+The mechanism provided for this purpose is called an ``export pin``, an
+extended attribute of directories. The name of this extended attribute is
+``ceph.dir.pin``.  Users can set this attribute using standard commands:
+
+::
+
+    setfattr -n ceph.dir.pin -v 2 path/to/dir
+
+The value of the extended attribute is the rank to assign the directory subtree
+to. A default value of ``-1`` indicates the directory is not pinned.
+
+A directory's export pin is inherited from its closest parent with a set export
+pin.  In this way, setting the export pin on a directory affects all of its
+children. However, the parents pin can be overridden by setting the child
+directory's export pin. For example:
+
+::
+
+    mkdir -p a/b
+    # "a" and "a/b" both start without an export pin set
+    setfattr -n ceph.dir.pin -v 1 a/
+    # a and b are now pinned to rank 1
+    setfattr -n ceph.dir.pin -v 0 a/b
+    # a/b is now pinned to rank 0 and a/ and the rest of its children are still pinned to rank 1
+