1. A system having physical resources for data processing, for data storage resources and for network communications to couple the data processing and data storage resources, the system also having one or more virtual overlay infrastructures mapped onto the physical resources, each virtual overlay infrastructure comprising one or more virtual entities for data processing, one or more virtual data storage entities and one or more virtual network entities arranged to couple others of the virtual entities, the system being arranged to passivate any of the virtual overlay infrastructures so as to allow later reactivation.