Virtual Memory System(VMS) is the proprietary operating system originally produced by Digital Equipment Corp (DEC, now part of HP) for its VAX minicomputer.
VMS V1 was released in August 1978. VMS was renamed "OpenVMS" around version 5.5. OpenVMS is a multi-user, multiprocessing virtual memory-based operating system (OS) designed for use in time sharing, batch processing, real time (process priorities can be set higher than OS kernel jobs) and transaction processing.
It offers high system availability through clustering, or the ability to distribute the system over multiple physical machines.
This allows the system to be "disaster-tolerant" against natural disasters that may disable individual data-processing facilities.