Patent ID: 7461224

Claim:
A remote copy system comprising: a primary site having a first storage system coupled to a host computer; and a secondary site having a second storage system, wherein said first storage system is coupled to said second storage system via a communication line, wherein said first storage system receives a write request from said host computer, asynchronously transfers write data transferred according to said write request to said second storage system via said communication line, and writes said write data into a first storage area owned by said first storage system, wherein said second storage system writes the write data transferred from said first storage system into a second storage area owned by said second storage system, wherein said first storage system recognizes a size of differential data which is data written into said first storage area but not written into said second storage area, decides whether to start a process to reduce said size based on whether said size is larger than a threshold, and stops said process based on said size having been reduced, after said process is started, and wherein said process includes limiting input of write requests from said host computer to said first storage system.