Patent ID: 8352435

Claim:
A method of remote mirroring of data between a first data storage system and a second data storage system, the second data storage system being remote from the first data storage system and linked to the first data storage system for transfer of remote copy data from the first data storage system to the second data storage system, the first data storage system including a first data processor and a first non-transitory computer-readable storage medium, the second data storage system including a second data processor and a second non-transitory computer readable storage medium, said method comprising the first data processor executing a first set of computer instructions stored in the first non-transitory computer-readable storage medium, and the second data processor executing a second set of computer instructions stored in the second non-transitory computer-readable storage medium, to perform the steps of: (a) maintaining secondary storage in the second data storage system as a remote mirror of primary storage in the first data storage system as a host computer writes data to the primary storage in the first data storage system, the secondary storage being maintained in a synchronous mode in which the first data storage system returns an acknowledgement of completion of a write operation to the host computer when the second data storage system has returned an acknowledgement of receipt of remote copy data of the write operation; (b) the first data storage system performing data reduction upon the remote copy data prior to transmitting the remote copy data in a reduced form from the first data storage system to the second data storage system; and (c) the second data storage system returning the acknowledgement of receipt of the remote copy data to the first data storage system upon receipt of the remote copy data in the reduced form, and later reversing the data reduction upon the remote copy data in the reduced form to produce data written to the secondary storage to maintain the secondary storage as the remote mirror of the primary storage; wherein step (b) includes performing data reduction upon the remote copy data by finding that the remote copy data includes a copy of a block of data that was previously transmitted from the first data storage system to the second data storage system, and replacing the copy of the block of data with a pointer to the block of data that was previously transmitted from the first data storage system to the second data storage system; and wherein step (b) further includes searching a block index to find that the remote copy data includes said copy of a block of data that was previously transmitted from the first data storage system to the second data storage system.