doc: document the mds_autoscaler mgr plugin

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>

diff --git a/doc/mgr/index.rst b/doc/mgr/index.rst
index 0dad31e929fb531039c6282c9be525c58a236686..0b03acfb9385a156e37bd9f415dab0332cf91f8a 100644 (file)
--- a/doc/mgr/index.rst
+++ b/doc/mgr/index.rst
@@ -45,3 +45,4 @@ sensible.
     Insights module <insights>
     Orchestrator module <orchestrator>
     Rook module <rook>
+    MDS Autoscaler module <mds_autoscaler>

diff --git a/doc/mgr/mds_autoscaler.rst b/doc/mgr/mds_autoscaler.rst
new file mode 100644 (file)
index 0000000..39dadd8
--- /dev/null
+++ b/doc/mgr/mds_autoscaler.rst
@@ -0,0 +1,31 @@
+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.