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doc: document the mds_autoscaler mgr plugin

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The MDS Autoscaler provides automation to maintain a required count of
MDSs in the system.
The MDS count in the system can be afected by:
* max_mds config option
* standby_count_wanted config option
* death of an active MDS Rank

Fixes: https://tracker.ceph.com/issues/46884
Signed-off-by: Milind Changire <mchangir@redhat.com>
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Milind Changire 2020-10-20 09:19:48 +05:30
parent 3ec03efc70
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doc/mgr/index.rst
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Insights module <insights>
Orchestrator module <orchestrator>
Rook module <rook>
MDS Autoscaler module <mds_autoscaler>

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MDS Autoscaler Module
=====================
The MDS Autoscaler Module monitors ``fsmap`` update notifications from the mgr
daemon and takes action to spawn or kill MDS daemons for a file-system as per
changes to the:
- ``max_mds`` config value
- ``standby_count_wanted`` config value
- standby promotions to active MDS state in case of active MDS rank death
Bumping up the ``max_mds`` config option value causes a standby mds to be promoted
to hold an active rank. This leads to a drop in standby mds count. The MDS
Autoscaler module detects this deficit and the orchestrator module is notified
about the required MDS count. The orchestrator back-end then takes necessary
measures to spawn standby MDSs.
Dropping the ``max_mds`` config option causes the orchestrator back-end to kill
standby mds to achieve the new reduced count. Preferably standby mds are chosen
to be killed when the ``max_mds`` count is dropped.
An increment and decrement of the ``standby_count_wanted`` config option value
has a similar effect on the total MDS count. The orchestrator is notified about
the change and necessary action to spawn or kill standby MDSs is taken.
A death of an active MDS rank also causes promotion of a standby mds to occupy
the required active rank. The MDS Autoscaler notices the change in the standby
mds count and a message is passed to the orchestrator to maintain the necessary
MDS count.
NOTE: There is no CLI associated with the MDS Autoscaler Module.