doc/cephfs: add documentation for mds last-seen

Signed-off-by: Patrick Donnelly <[email protected]>

Author: batrick

doc/cephfs/administration.rst

@@ -280,6 +280,17 @@ Mark the file system rank as repaired. Unlike the name suggests, this command
 does not change a MDS; it manipulates the file system rank which has been
 marked damaged.
 
+::
+
+    ceph mds last-seen <name>
+
+Learn the when the MDS named ``name`` was last in the FSMap. The JSON output
+includes the epoch the MDS was last seen. Historically information is limited by
+the following ``mon`` configuration:
+
+
+.. confval:: mon_fsmap_prune_threshold
+
 
 Required Client Features
 ------------------------