Primary storage subsystems synchronously send write data to an intermediate storage system. Upon receiving the write data, the intermediate storage system 100c generates control data, which contains information to identifying the write order. In one embodiment, a portion of the control data, e.g., a sequence number, is generated and attached to the control data of the write data. The intermediate storage system sends the write data along with its control data to the secondary storage systems asynchronously. The secondary storage systems store the write data to the secondary volumes based on the control data. The write order is maintained using the sequence number generated by the intermediate subsystem.