A VMScluster is a computer cluster involving a group of computers running the Open VMS operating system. Whereas tightly-coupled multiprocessor systems run a single copy of the operating system, a VMScluster is loosely-coupled: each machine runs its own copy of OpenVMS, but the disk storage, lock manager, and security domain are all cluster-wide. Machines can join or leave a VMScluster without affecting the rest of the cluster.