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diff --git a/doc/cephfs/standby.rst b/doc/cephfs/standby.rst new file mode 100644 index 000000000..22216c36f --- /dev/null +++ b/doc/cephfs/standby.rst @@ -0,0 +1,189 @@ +.. _mds-standby: + +Terminology +----------- + +A Ceph cluster may have zero or more CephFS *file systems*. CephFS +file systems have a human readable name (set in ``fs new``) +and an integer ID. The ID is called the file system cluster ID, +or *FSCID*. + +Each CephFS file system has a number of *ranks*, one by default, +which start at zero. A rank may be thought of as a metadata shard. +Controlling the number of ranks in a file system is described +in :doc:`/cephfs/multimds` + +Each CephFS ceph-mds process (a *daemon*) initially starts up +without a rank. It may be assigned one by the monitor cluster. +A daemon may only hold one rank at a time. Daemons only give up +a rank when the ceph-mds process stops. + +If a rank is not associated with a daemon, the rank is +considered *failed*. Once a rank is assigned to a daemon, +the rank is considered *up*. + +A daemon has a *name* that is set statically by the administrator +when the daemon is first configured. Typical configurations +use the hostname where the daemon runs as the daemon name. + +A ceph-mds daemons can be assigned to a particular file system by +setting the `mds_join_fs` configuration option to the file system +name. + +Each time a daemon starts up, it is also assigned a *GID*, which +is unique to this particular process lifetime of the daemon. The +GID is an integer. + +Referring to MDS daemons +------------------------ + +Most of the administrative commands that refer to an MDS daemon +accept a flexible argument format that may contain a rank, a GID +or a name. + +Where a rank is used, this may optionally be qualified with +a leading file system name or ID. If a daemon is a standby (i.e. +it is not currently assigned a rank), then it may only be +referred to by GID or name. + +For example, if we had an MDS daemon which was called 'myhost', +had GID 5446, and was assigned rank 0 in the file system 'myfs' +which had FSCID 3, then any of the following would be suitable +forms of the 'fail' command: + +:: + + ceph mds fail 5446 # GID + ceph mds fail myhost # Daemon name + ceph mds fail 0 # Unqualified rank + ceph mds fail 3:0 # FSCID and rank + ceph mds fail myfs:0 # File System name and rank + +Managing failover +----------------- + +If an MDS daemon stops communicating with the monitor, the monitor will wait +``mds_beacon_grace`` seconds (default 15 seconds) before marking the daemon as +*laggy*. If a standby is available, the monitor will immediately replace the +laggy daemon. + +Each file system may specify a number of standby daemons to be considered +healthy. This number includes daemons in standby-replay waiting for a rank to +fail (remember that a standby-replay daemon will not be assigned to take over a +failure for another rank or a failure in a another CephFS file system). The +pool of standby daemons not in replay count towards any file system count. +Each file system may set the number of standby daemons wanted using: + +:: + + ceph fs set <fs name> standby_count_wanted <count> + +Setting ``count`` to 0 will disable the health check. + + +.. _mds-standby-replay: + +Configuring standby-replay +-------------------------- + +Each CephFS file system may be configured to add standby-replay daemons. These +standby daemons follow the active MDS's metadata journal to reduce failover +time in the event the active MDS becomes unavailable. Each active MDS may have +only one standby-replay daemon following it. + +Configuring standby-replay on a file system is done using: + +:: + + ceph fs set <fs name> allow_standby_replay <bool> + +Once set, the monitors will assign available standby daemons to follow the +active MDSs in that file system. + +Once an MDS has entered the standby-replay state, it will only be used as a +standby for the rank that it is following. If another rank fails, this +standby-replay daemon will not be used as a replacement, even if no other +standbys are available. For this reason, it is advised that if standby-replay +is used then every active MDS should have a standby-replay daemon. + +.. _mds-join-fs: + +Configuring MDS file system affinity +------------------------------------ + +You may want to have an MDS used for a particular file system. Or, perhaps you +have larger MDSs on better hardware that should be preferred over a last-resort +standby on lesser or over-provisioned hardware. To express this preference, +CephFS provides a configuration option for MDS called ``mds_join_fs`` which +enforces this `affinity`. + +As part of any failover, the Ceph monitors will prefer standby daemons with +``mds_join_fs`` equal to the file system name with the failed rank. If no +standby exists with ``mds_join_fs`` equal to the file system name, it will +choose a `vanilla` standby (no setting for ``mds_join_fs``) for the replacement +or any other available standby as a last resort. Note, this does not change the +behavior that ``standby-replay`` daemons are always selected before looking at +other standbys. + +Even further, the monitors will regularly examine the CephFS file systems when +stable to check if a standby with stronger affinity is available to replace an +MDS with lower affinity. This process is also done for standby-replay daemons: +if a regular standby has stronger affinity than the standby-replay MDS, it will +replace the standby-replay MDS. + +For example, given this stable and healthy file system: + +:: + + $ ceph fs dump + dumped fsmap epoch 399 + ... + Filesystem 'cephfs' (27) + ... + e399 + max_mds 1 + in 0 + up {0=20384} + failed + damaged + stopped + ... + [mds.a{0:20384} state up:active seq 239 addr [v2:127.0.0.1:6854/966242805,v1:127.0.0.1:6855/966242805]] + + Standby daemons: + + [mds.b{-1:10420} state up:standby seq 2 addr [v2:127.0.0.1:6856/2745199145,v1:127.0.0.1:6857/2745199145]] + + +You may set ``mds_join_fs`` on the standby to enforce your preference: :: + + $ ceph config set mds.b mds_join_fs cephfs + +after automatic failover: :: + + $ ceph fs dump + dumped fsmap epoch 405 + e405 + ... + Filesystem 'cephfs' (27) + ... + max_mds 1 + in 0 + up {0=10420} + failed + damaged + stopped + ... + [mds.b{0:10420} state up:active seq 274 join_fscid=27 addr [v2:127.0.0.1:6856/2745199145,v1:127.0.0.1:6857/2745199145]] + + Standby daemons: + + [mds.a{-1:10720} state up:standby seq 2 addr [v2:127.0.0.1:6854/1340357658,v1:127.0.0.1:6855/1340357658]] + +Note in the above example that ``mds.b`` now has ``join_fscid=27``. In this +output, the file system name from ``mds_join_fs`` is changed to the file system +identifier (27). If the file system is recreated with the same name, the +standby will follow the new file system as expected. + +Finally, if the file system is degraded or undersized, no failover will occur +to enforce ``mds_join_fs``. |