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