The metadata service should be modified to use the RWStore. The RWStore will be allow the MDS to address petabytes of shards when deployed on a modest hardware configuration. Failover for the MDS will use the write replication mechanism for the RWStore. Clients can read from any MDS instance, except if they are requesting the unisolated view. All writes will go to the master MDS instance and be replicated to the failover nodes.
This issue has a dependency on  (integrate and test the RWStore).