A command is issued from a processing unit to an on-line storage unit and transferred from the on-line storage unit to a back-up storage unit. The received commands are processed in the respective storage units. An overhead of the on-line storage unit is reduced in a hot stand-by system. Storage...http://www.google.de/patents/US5845328?utm_source=gb-gplus-sharePatent US5845328 - Method for ensuring data coherency for redundat storage by transferring copies of access requests from on-line storage to back-up storage 
