US Patent 7734591 Coherent device to device data replication

An apparatus provides coherent data copying operations where data replication is controlled by a source storage controller directly to a destination controller and managed by a remote application. The apparatus generates a snapshot version for each source data block changed by one or more write operations to the source data block during the course of a copy operation; and copies each block of the source data to a corresponding block in the destination data in the absence of the snapshot version of the block and otherwise copying the snapshot version of the source data block to the corresponding block in the destination data.