2.4 OpenVMS Galaxy Features An evolution in OpenVMS functionality, OpenVMS Galaxy leverages proven OpenVMS Cluster, symmetric multipro- cessing, and performance capabilities to offer greater levels of performance, scalability, and availability with extremely flexible operational capabilities. Clustering Fifteen years of proven OpenVMS Cluster technology facil- itates communication among clustered instances within an OpenVMS Galaxy. An OpenVMS Cluster is a software concept. It is a set of co- ordinated OpenVMS operating systems, one per computer , communicating over various communications media to com- bine the processing power and storage capacity of multiple computers into a single, shared-everything environment. An OpenVMS Galaxy is also a software concept. However, it is a set of coordinated OpenVMS operating systems, in a sin- gle computer , communicating through shared memory. An instance of the operating system in an OpenVMS Galaxy can be clustered with other instances within the Galaxy or with instances in other systems. An OpenVMS Galaxy is a complete system in and of itself. Although an OpenVMS Galaxy can be added to an existing cluster as multiple cluster nodes can be added today, the sin- gle system is the OpenVMS Galaxy architecture focus. An application running totally within an OpenVMS Galaxy can take advantage of performance opportunities not present in multisystem clusters. SMP Any instance in an OpenVMS Galaxy can be an SMP con- figuration. The number of CPUs is part of the definition of an instance. Because an instance in the OpenVMS Galaxy is a complete OpenVMS operating system, all applications be- have the same as they would on a traditional, single-instance computer. CPU reassignment A CPU can be dynamically reassigned from one instance to another while all applications on both instances continue to run. Reassignment is realized by three separate functions: stopping, reassigning, and starting the CPU in question. As resource needs of applications change, the CPUs can be reas- signed to the appropriate instances. There are some restric- tions; for example, the primary CPU in an instance cannot be reassigned, and a CPU cannot specifically be designated to handle certain interrupts. Dynamic Reconfiguration Multiple instances of the OpenVMS operating system al- low system managers to reassign processing power to the instances whose applications most need it. As that need varies over time, so can the configuration. OpenVMS al- lows dynamic reconfiguration while all instances and their applications continue to run.