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102 lines
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102 lines
3.7 KiB
ReStructuredText
.. _mds-standby:
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Terminology
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-----------
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A Ceph cluster may have zero or more CephFS *filesystems*. CephFS
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filesystems have a human readable name (set in ``fs new``)
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and an integer ID. The ID is called the filesystem cluster ID,
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or *FSCID*.
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Each CephFS filesystem has a number of *ranks*, one by default,
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which start at zero. A rank may be thought of as a metadata shard.
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Controlling the number of ranks in a filesystem is described
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in :doc:`/cephfs/multimds`
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Each CephFS ceph-mds process (a *daemon*) initially starts up
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without a rank. It may be assigned one by the monitor cluster.
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A daemon may only hold one rank at a time. Daemons only give up
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a rank when the ceph-mds process stops.
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If a rank is not associated with a daemon, the rank is
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considered *failed*. Once a rank is assigned to a daemon,
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the rank is considered *up*.
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A daemon has a *name* that is set statically by the administrator
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when the daemon is first configured. Typical configurations
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use the hostname where the daemon runs as the daemon name.
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Each time a daemon starts up, it is also assigned a *GID*, which
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is unique to this particular process lifetime of the daemon. The
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GID is an integer.
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Referring to MDS daemons
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------------------------
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Most of the administrative commands that refer to an MDS daemon
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accept a flexible argument format that may contain a rank, a GID
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or a name.
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Where a rank is used, this may optionally be qualified with
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a leading filesystem name or ID. If a daemon is a standby (i.e.
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it is not currently assigned a rank), then it may only be
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referred to by GID or name.
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For example, if we had an MDS daemon which was called 'myhost',
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had GID 5446, and was assigned rank 0 in the filesystem 'myfs'
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which had FSCID 3, then any of the following would be suitable
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forms of the 'fail' command:
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::
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ceph mds fail 5446 # GID
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ceph mds fail myhost # Daemon name
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ceph mds fail 0 # Unqualified rank
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ceph mds fail 3:0 # FSCID and rank
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ceph mds fail myfs:0 # Filesystem name and rank
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Managing failover
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-----------------
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If an MDS daemon stops communicating with the monitor, the monitor will wait
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``mds_beacon_grace`` seconds (default 15 seconds) before marking the daemon as
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*laggy*. If a standby is available, the monitor will immediately replace the
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laggy daemon.
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Each file system may specify a number of standby daemons to be considered
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healthy. This number includes daemons in standby-replay waiting for a rank to
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fail (remember that a standby-replay daemon will not be assigned to take over a
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failure for another rank or a failure in a another CephFS file system). The
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pool of standby daemons not in replay count towards any file system count.
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Each file system may set the number of standby daemons wanted using:
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::
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ceph fs set <fs name> standby_count_wanted <count>
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Setting ``count`` to 0 will disable the health check.
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Configuring standby-replay
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--------------------------
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Each CephFS file system may be configured to add standby-replay daemons. These
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standby daemons follow the active MDS's metadata journal to reduce failover
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time in the event the active MDS becomes unavailable. Each active MDS may have
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only one standby-replay daemon following it.
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Configuring standby-replay on a file system is done using:
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::
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ceph fs set <fs name> allow_standby_replay <bool>
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Once set, the monitors will assign available standby daemons to follow the
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active MDSs in that file system.
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Once an MDS has entered the standby-replay state, it will only be used as a
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standby for the rank that it is following. If another rank fails, this
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standby-replay daemon will not be used as a replacement, even if no other
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standbys are available. For this reason, it is advised that if standby-replay
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is used then every active MDS should have a standby-replay daemon.
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