Metadata Server Cluster plays vital role in distributed storage systems for storage area network. In this paper, we present the specification and design of a light-weight group communication service (GCSLight), by which MDS Cluster may be built into a modular and scalable architecture. GCSLight is light-weight in that: i) Simple design. It provides only the necessary services for MDS Cluster, excluding those complex and extraneous features, such as multicast and virtual synchrony, which exist in many other group communication services aiming at replicated service. ii) Optimized performance in failure-free runs by adopting a lazy failure detection-based processor group membership protocol.