1. A system, comprising: physical resources for data processing, for data storage and for network communications to couple the physical resources for data processing and data storage, the physical resources including one or more servers, each server from the one or more servers configured to run virtual machines and a storage manager arranged to handle access requests from the virtual machines at that server to shared virtual storage and, depending on a mapping of the shared virtual storage, to pass the access request to a storage manager at another server from the one or more servers; two 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 of that virtual overlay infrastructure, the overlay infrastructures being isolated from each other; and an inter-infrastructure interface for providing a controlled interaction between different ones of the isolated virtual overlay infrastructures.