Image Number 6 for United States Patent #8131963.
Even when a host does not give a write time to write data, consistency can be kept among data stored in secondary storage systems. The present system has plural primary storage systems each having a source volume and plural secondary storage systems each having a target volume. Once data is received from a host, each of the plural storage systems creates write-data management information having sequential numbers and reference information and sends, to one of the primary storage systems, the data, sequential number and reference information. Each of the secondary storage systems records reference information corresponding to the largest sequential number among serial sequential numbers and stores, in a target volume in an order of sequential numbers, data corresponding to reference information having a value smaller than the reference information based on the smallest value reference information among reference information recorded in each of the plural secondary storage systems.