Patent ID: 7861052

Claim:
A computer system, comprising: a host computer; an external storage system having an external volume storing data used by said host computer; a first storage system having a first expansion device configured by virtualizing said external volume via a first path between said external device and said first storage system, providing a first logical unit corresponding to said first expansion device to said host computer, and controlling access to said first logical unit from said host computer; and a second storage system having a second expansion device configured by virtualizing said external volume via a second path between said external device and said second storage system, providing a second logical unit corresponding to said second expansion device to said host computer, and controlling access to said second logical unit from said host computer, wherein said first storage system further includes: a third expansion device configured by virtualizing said second logical unit of said second storage system via a third path between said first and said second storage systems, and wherein if said first storage system receives write data to said first logical unit, said first storage system stores data in said external volume by accessing said external volume via said third expansion device, said third path, said second expansion device, and said second path, by using cache through mode.