1. A method for backing up a primary computer storage system, comprising: executing a plurality of virtual servers on the primary storage system, each virtual server having an external configuration information, the external configuration information used to describe one or more instances of resources available on the primary storage system, and having an internal configuration information; mirroring a data set served by the primary storage system onto a backup storage system by copying all data blocks and external configuration information and internal configuration information to a backup server of the backup storage system at a first time; at a selected time after the first time, performing a backup by copying to the backup server the external configuration information of each virtual server which has changed since the first time, and by copying to the backup server the internal configuration information of each virtual server which has changed since the first time, and by performing a snapshot of the primary storage system by copying data blocks which have changed since the first time from the primary storage system to the backup server; determining unavailability of the primary storage system; and serving the mirrored data set from the backup storage system in response to unavailability of the primary storage system by using the external configuration information and the internal configuration information and the snapshot of the primary storage system stored on the backup server to assume an identity of at least one of the virtual servers of the plurality of virtual servers of the primary storage system.