Patent ID: 7111189

Claim:
A method for logging data written by a host computer to a local data storage system at a first site including a local array of data storage devices, comprising: providing a first array controller and a second array controller coupled to each other and also coupled between the host computer and the array; storing, on a log unit in primary cache memory in the first array controller, the data and associated command for every write transaction that occurs between the host computer and the local array; mirroring the primary cache in backup cache memory in the second array controller at the first site; sending the data to a remote storage system located at a second site and linked by at least one link to the local data storage system to create a remote copy; and in the situation wherein the remote copy has not successfully completed and the first array controller has failed, re-playing the data on the log unit by performing: for each entry in the log unit mirrored in the backup cache in the second array controller: reading the data from the backup cache for each said transaction in the log unit; and writing the data to the remote storage system in transaction order.