Lock management is a common cluster-infrastructure service that provides a mechanism for other cluster infrastructure components to synchronize their access to shared resources. In a Red Hat cluster, one of the following Red Hat Cluster Suite components operates as the lock manager: DLM (Distributed Lock Manager) or GULM (Grand Unified Lock Manager). The major difference between the two lock managers is that DLM is a distributed lock manager and GULM is a client-server lock manager. DLM runs in each cluster node; lock management is distributed across all nodes in the cluster (refer to Figure 1.2, “CMAN/DLM Overview”). DLM can be the lock manager only in a cluster configured with CMAN as its cluster manager. GULM runs in nodes designated as GULM server nodes; lock management is centralized in the nodes designated as GULM server nodes. GULM server nodes manage locks through GULM clients in the cluster nodes (refer to Figure 1.3, “GULM Overview”). With GULM, lock management operates in a limited number of nodes: either one, three, or five nodes configured as GULM servers. GFS and CLVM use locks from the lock manager. GFS uses locks from the lock manager to synchronize access to file system metadata (on shared storage). CLVM uses locks from the lock manager to synchronize updates to LVM volumes and volume groups (also on shared storage).