.. _cephfs-multimds: Configuring multiple active MDS daemons --------------------------------------- *Also known as: multi-mds, active-active MDS* Each CephFS file system 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 file system has a *max_mds* setting, which controls how many ranks will be created. The actual number of ranks in the file system 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 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 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 ```` 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. .. _cephfs-pinning: 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 .. _cephfs-ephemeral-pinning: Setting subtree partitioning policies ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It is also possible to setup **automatic** static partitioning of subtrees via a set of **policies**. In CephFS, this automatic static partitioning is referred to as **ephemeral pinning**. Any directory (inode) which is ephemerally pinned will be automatically assigned to a particular rank according to a consistent hash of its inode number. The set of all ephemerally pinned directories should be uniformly distributed across all ranks. Ephemerally pinned directories are so named because the pin may not persist once the directory inode is dropped from cache. However, an MDS failover does not affect the ephemeral nature of the pinned directory. The MDS records what subtrees are ephemerally pinned in its journal so MDS failovers do not drop this information. A directory is either ephemerally pinned or not. Which rank it is pinned to is derived from its inode number and a consistent hash. This means that ephemerally pinned directories are somewhat evenly spread across the MDS cluster. The **consistent hash** also minimizes redistribution when the MDS cluster grows or shrinks. So, growing an MDS cluster may automatically increase your metadata throughput with no other administrative intervention. Presently, there are two types of ephemeral pinning: **Distributed Ephemeral Pins**: This policy causes a directory to fragment (even well below the normal fragmentation thresholds) and distribute its fragments as ephemerally pinned subtrees. This has the effect of distributing immediate children across a range of MDS ranks. The canonical example use-case would be the ``/home`` directory: we want every user's home directory to be spread across the entire MDS cluster. This can be set via: :: setfattr -n ceph.dir.pin.distributed -v 1 /cephfs/home **Random Ephemeral Pins**: This policy indicates any descendent sub-directory may be ephemerally pinned. This is set through the extended attribute ``ceph.dir.pin.random`` with the value set to the percentage of directories that should be pinned. For example: :: setfattr -n ceph.dir.pin.random -v 0.5 /cephfs/tmp Would cause any directory loaded into cache or created under ``/tmp`` to be ephemerally pinned 50 percent of the time. It is recommended to only set this to small values, like ``.001`` or ``0.1%``. Having too many subtrees may degrade performance. For this reason, the config ``mds_export_ephemeral_random_max`` enforces a cap on the maximum of this percentage (default: ``.01``). The MDS returns ``EINVAL`` when attempting to set a value beyond this config. Both random and distributed ephemeral pin policies are off by default in Octopus. The features may be enabled via the ``mds_export_ephemeral_random`` and ``mds_export_ephemeral_distributed`` configuration options. Ephemeral pins may override parent export pins and vice versa. What determines which policy is followed is the rule of the closest parent: if a closer parent directory has a conflicting policy, use that one instead. For example: :: mkdir -p foo/bar1/baz foo/bar2 setfattr -n ceph.dir.pin -v 0 foo setfattr -n ceph.dir.pin.distributed -v 1 foo/bar1 The ``foo/bar1/baz`` directory will be ephemerally pinned because the ``foo/bar1`` policy overrides the export pin on ``foo``. The ``foo/bar2`` directory will obey the pin on ``foo`` normally. For the reverse situation: :: mkdir -p home/{patrick,john} setfattr -n ceph.dir.pin.distributed -v 1 home setfattr -n ceph.dir.pin -v 2 home/patrick The ``home/patrick`` directory and its children will be pinned to rank 2 because its export pin overrides the policy on ``home``.