A system has a virtual overlay infrastructure mapped onto physical resources for processing, storage and network communications, the virtual infrastructure having virtual entities for processing, storage and network communications. Each virtual infrastructure can be passivated by suspending applications, stopping operating systems, and storing state, to enable later reactivation. This is simpler for a complete virtual infrastructure than for groups of virtual entities and physical entities. It enables cloned virtual infrastructure to be created for testing, upgrading or sharing without risk to the parent. On failure, reversion to a previous working clone is feasible.