Patent ID: 7512757

Claim:
A computer system comprising: a first storage system connected to a first host computer for establishing communications therewith; a second storage system connected to a second host computer and said first storage system for establishing communications therewith; and a third storage system connected to said first and second storage systems for establishing communications therewith, wherein: data written from said first host computer into said first storage system is transferred to said second storage system by synchronous remote copy, and to said third storage system by asynchronous remote copy; said second and third storage systems include reservation means for registering beforehand transfer setting of asynchronous remote copy between said second and third storage systems, before data written in said second storage system starts being transferred to said third storage system by asynchronous remote copy when a failure occurs in one of said first host computer and said first storage system, and for maintaining a state not starting transfer; said second storage system includes transfer start designating means for receiving a command of starting transfer based on said transfer setting; and said first storage system is implemented with a program for realizing delta-resync which uses a pair of a replication subject and replication data, by transferring a difference between journals of said second storage system and said third storage system, when the failure occurs in said first storage system or said first host computer.