Minor Description
In large clusters with many clients, there may be some fraction of clients that are only working in a subdirectory tree that is located on a single MDT, or only accessing the filesystem infrequently. This is the same as the existing OST idle disconnect feature in LU-7236.
In such cases, it would be desirable for the client to disconnect from MDTs that it is no longer using, in order to reduce the saved state and recovery load on the MDT if it should fail. This would also reduce log noise on both the client and server if an unreliable node is removed from scheduling but the filesystem is not unmounted.
We may (optionally) want to prevent idle disconnection from MDT0000 to avoid repeated connect/disconnect cycles from clients that are periodically running "df/statfs" from various libraries and monitoring tools. In that case, lmv_select_statfs_mdt() should select an MDT connection that is FULL (any will work) rather than using an IDLE connection.
