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223 lines
6.9 KiB
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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
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wait ``mds_beacon_grace`` seconds (default 15 seconds) before marking
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the daemon as *laggy*.
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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 daemons
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---------------------------
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There are four configuration settings that control how a daemon
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will behave while in standby:
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::
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mds_standby_replay
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mds_standby_for_name
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mds_standby_for_rank
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mds_standby_for_fscid
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These may be set in the ceph.conf on the host where the MDS daemon
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runs (as opposed to on the monitor). The daemon loads these settings
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when it starts, and sends them to the monitor.
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By default, if none of these settings are used, all MDS daemons
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which do not hold a rank will be used as standbys for any rank.
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The settings which associate a standby daemon with a particular
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name or rank do not guarantee that the daemon will *only* be used
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for that rank. They mean that when several standbys are available,
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the associated standby daemon will be used. If a rank is failed,
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and a standby is available, it will be used even if it is associated
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with a different rank or named daemon.
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mds_standby_replay
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~~~~~~~~~~~~~~~~~~
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If this is set to true, then the standby daemon will continuously read
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the metadata journal of an up rank. This will give it
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a warm metadata cache, and speed up the process of failing over
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if the daemon serving the rank fails.
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An up rank may only have one standby replay daemon assigned to it,
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if two daemons are both set to be standby replay then one of them
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will arbitrarily win, and the other will become a normal non-replay
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standby.
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Once a daemon has entered the standby replay state, it will only be
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used as a standby for the rank that it is following. If another rank
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fails, this standby replay daemon will not be used as a replacement,
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even if no other standbys are available.
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*Historical note:* In Ceph prior to v10.2.1, this setting (when ``false``) is
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always true when ``mds_standby_for_*`` is also set.
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mds_standby_for_name
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~~~~~~~~~~~~~~~~~~~~
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Set this to make the standby daemon only take over a failed rank
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if the last daemon to hold it matches this name.
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mds_standby_for_rank
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~~~~~~~~~~~~~~~~~~~~
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Set this to make the standby daemon only take over the specified
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rank. If another rank fails, this daemon will not be used to
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replace it.
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Use in conjunction with ``mds_standby_for_fscid`` to be specific
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about which filesystem's rank you are targeting, if you have
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multiple filesystems.
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mds_standby_for_fscid
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~~~~~~~~~~~~~~~~~~~~~
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If ``mds_standby_for_rank`` is set, this is simply a qualifier to
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say which filesystem's rank is referred to.
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If ``mds_standby_for_rank`` is not set, then setting FSCID will
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cause this daemon to target any rank in the specified FSCID. Use
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this if you have a daemon that you want to use for any rank, but
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only within a particular filesystem.
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mon_force_standby_active
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~~~~~~~~~~~~~~~~~~~~~~~~
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This setting is used on monitor hosts. It defaults to true.
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If it is false, then daemons configured with mds_standby_replay=true
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will **only** become active if the rank/name that they have
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been configured to follow fails. On the other hand, if this
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setting is true, then a daemon configured with mds_standby_replay=true
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may be assigned some other rank.
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Examples
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--------
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These are example ceph.conf snippets. In practice you can either
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copy a ceph.conf with all daemons' configuration to all your servers,
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or you can have a different file on each server that contains just
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that server's daemons' configuration.
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Simple pair
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~~~~~~~~~~~
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Two MDS daemons 'a' and 'b' acting as a pair, where whichever one is not
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currently assigned a rank will be the standby replay follower
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of the other.
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::
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[mds.a]
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mds standby replay = true
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mds standby for rank = 0
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[mds.b]
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mds standby replay = true
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mds standby for rank = 0
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Floating standby
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~~~~~~~~~~~~~~~~
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Three MDS daemons 'a', 'b' and 'c', in a filesystem that has
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``max_mds`` set to 2.
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::
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# No explicit configuration required: whichever daemon is
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# not assigned a rank will go into 'standby' and take over
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# for whichever other daemon fails.
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Two MDS clusters
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~~~~~~~~~~~~~~~~
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With two filesystems, I have four MDS daemons, and I want two
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to act as a pair for one filesystem and two to act as a pair
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for the other filesystem.
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::
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[mds.a]
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mds standby for fscid = 1
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[mds.b]
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mds standby for fscid = 1
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[mds.c]
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mds standby for fscid = 2
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[mds.d]
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mds standby for fscid = 2
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