A global namespace (GNS) is constructed across heterogeneous network-attached storage (NAS) devices by organizing metadata servers into a cluster layer having cluster nodes and a consistent hashing overlay layer having overlay nodes. The global namespace is distributed based on workload at the cluster and overlay nodes, and a portion of the GNS namespace may be migrated from one layer to the other. Cluster nodes may be added to the cluster layer, overlay nodes may be added to the consistent hashing overlay layer. Further, a cluster node may become an overlay node or an overlay node may become a cluster node. To access the global namespace, a request for namespace metadata managed by a cluster node is serviced by the cluster node, but if the requested namespace metadata have been migrated to the consistent hashing overlay layer, the cluster node will forward the request to the responsible overlay node for servicing.