Patent ID: 8046551

Claim:
A method executed in a data storage system in connection with processing an I/O request, the method including: receiving the I/O request; determining whether the I/O request is a write request; and if the I/O request is a write request, performing write request processing including: copying write request data of the write request to cache; destaging the write request data from the cache to a primary storage device; and copying, in accordance with a heuristic, the write request data from the primary storage device to an asynchronous mirror device including an asynchronous copy of data from the primary storage device, wherein said asynchronous mirror device is write disabled with respect to received write requests requesting to write data thereto, said asynchronous mirror device used for servicing data requested in connection with read requests upon the occurrence of a cache miss, wherein said heuristic specifies a threshold amount of a block having a current copy which is stored on the primary storage device and not on the asynchronous mirror device and said copying copies a first block of the write request data from the primary storage device to the asynchronous mirror device when said threshold amount of the first block has a current copy stored on the primary storage device and not on the asynchronous mirror device.