Creating a pair causes the storage system to start the initial copy. During the initial copy, the P-VOL remains available for read and write operations from the host. After the initial copy, the storage system periodically copies the differential data in the P-VOL to the S-VOL. Subsequent write operations to the P-VOL are regularly duplicated to the S-VOL. The data in the P-VOL is copied to the S-VOL.