A distributed lock manager (DLM) provides distributed applications with a means to synchronize their accesses to shared resources.
Each machine in a VMScluster runs a separate instance of the OpenVMS operating system, but sees a unified view of all the disks attached to the cluster, with cluster-wide file locking. This is accomplished by using a DLM to not only manage the locks but also coordinate all disk access.
A DLM is also a key component of more ambitious single system image projects such as OpenSSI.