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doc: update documentation for standby-replay
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
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Patrick Donnelly
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@ -58,9 +58,10 @@ forms of the 'fail' command:
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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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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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@ -76,148 +77,25 @@ Each file system may set the number of standby daemons wanted using:
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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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Configuring standby-replay
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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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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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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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ceph fs set <fs name> allow_standby_replay <bool>
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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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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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@ -428,6 +428,12 @@ These changes occurred between the Mimic and Nautilus releases.
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``mds_recall_warning_decay_rate`` (default: 60s) sets the threshold
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for this warning.
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* The MDS mds_standby_for_*, mon_force_standby_active, and mds_standby_replay
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configuration options have been obsoleted. Instead, the operator may now set
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the new "allow_standby_replay" flag on the CephFS file system. This setting
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causes standbys to become standby-replay for any available rank in the file
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system.
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* The Telegraf module for the Manager allows for sending statistics to
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an Telegraf Agent over TCP, UDP or a UNIX Socket. Telegraf can then
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send the statistics to databases like InfluxDB, ElasticSearch, Graphite
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