Patent ID: 8170996

Claim:
A computer system comprising: a first computer; a first storage system, which is coupled to the first computer, and which receives a write request from the first computer; and a second computer which is coupled to the first computer; and a second storage system which is coupled to the second computer and the first storage system, wherein the first storage system further comprises: a first control unit which processes the write request from the first computer; and a first storage which stores data related to the write request targeted to the first storage system from the first computer; wherein the second storage system further comprises: a second control unit which processes data from at least one of the second computer and the first storage system; a second storage which stores data related to the transferred data targeted to the second storage system from the first storage system; and a third storage, coupled to the second control unit, which records data from the second computer with one of time information and sequence number, and a plurality of checkpoints in time for recovery, as log data of the third storage, wherein the checkpoints identify a system state when certain types of transactions are detected, wherein the second computer monitors heartbeat with the first computer, and wherein, if there are no responses to the heartbeat a predetermined number of times, the second computer uses a communication channel of an alternative system and further monitors the heartbeat with the first computer.