Datensicherung in einer Datenverarbeitungsanlage

A method and means for asynchronous remote data duplexing at a distant location from copies based at a primary site storage subsystem, which copying is non-disruptive to executing applications, and further in which any data loss occasioned by losses in flight or updates never received at the time of any interruption between the primary and remote sites are accounted for at the remote site. The method assigns a token and unique sequence number responsive to each write operation at the primary site, and sends the tokens + numbers and data updates to the remote site. The method relies upon the sequence number to establish a sequence and define gaps therein to ascertain missing updates.