A primary copy of data is held at a primary storage facility, while data that is redundant of the primary copy is held at a secondary storage facility. To inhibit propagation of errors from the primary copy to the redundant data, write transactions to the redundant data are queued at the second storage...http://www.google.de/patents/US7380081?utm_source=gb-gplus-sharePatent US7380081 - Asynchronous data redundancy technique
