doc: update mds_autoscaler documentation to reflect recent changes

Notably, Sage changed how this worked in [1].

[1] 6d8c0e4722eb5f3014dc76f6733afea08bcc8e72

Fixes: https://tracker.ceph.com/issues/51187
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>

diff --git a/doc/mgr/mds_autoscaler.rst b/doc/mgr/mds_autoscaler.rst
index 39dadd8829298ac25884c29374efd879c2459883..46fc4415522483d8970f6894f9a9ffcee59b1992 100644 (file)
--- a/doc/mgr/mds_autoscaler.rst
+++ b/doc/mgr/mds_autoscaler.rst
@@ -1,31 +1,23 @@
 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:
+The MDS Autoscaler Module monitors file systems to ensure sufficient MDS
+daemons are available. It works by adjusting the placement specification for
+the orchestrator backend of the MDS service. To enable, use:
 
-- ``max_mds`` config value
-- ``standby_count_wanted`` config value
-- standby promotions to active MDS state in case of active MDS rank death
+.. sh:
 
-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.
+   ceph mgr module enable mds_autoscaler
 
-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.
+The module will monitor the following file system settings to inform
+placement count adjustments:
 
-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.
+- ``max_mds`` file system setting
+- ``standby_count_wanted`` file system setting
 
-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.
+The Ceph monitor daemons are still responsible for promoting or stopping MDS
+according to these settings. The ``mds_autoscaler`` simply adjusts the
+number of MDS which are spawned by the orchestrator.
 
-NOTE: There is no CLI associated with the MDS Autoscaler Module.
+.. note: There is no CLI or module configurations as of now. Enable or disable
+   the module to turn on or off.