Patent ID: 8015351

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 only has an access authority to said external volume during normal operation, wherein if said first storage system receives write data to said first logical unit, said first storage system controls to store data in said second logical unit by accessing, by using cache through mode, wherein said second storage system deletes data stored in said second logical unit after said first storage system destages dirty data from said first logical unit to said external volume, wherein said host computer switches access to data stored in said external volume by using said second logical unit instead of said first logical unit, when a failure occurs in said first storage system, and wherein said second storage system retrieves an access authority to said external volume, destages dirty data stored in said second logical unit to said external volume.