An apparatus for controlling a storage system having a data replication function, comprises: a storage array component being operable to send notification to a replication engine that a write of data to a primary storage location by a host is subject to data replication; the replication engine being operable to receive the notification and in response to instruct the storage array to copy the data to a secondary storage location; wherein the data is copied to the secondary storage location unmediated by the replication engine.