Patent ID: 7610511

Claim:
A computer system, comprising: a first storage subsystem; and a second storage subsystem coupled to the first storage subsystem via a network, wherein the first storage subsystem has a first storage unit, which stores data requested by a host computer to be written, and a first control unit, which controls input/output of data to/from the first storage unit, wherein the second storage subsystem has a second storage unit, which stores data requested by the host computer to be written, and a second control unit, which controls input/output of data to/from the second storage unit, and wherein the first control unit is configured to: record journals related to data stored in the first storage unit; identify, in the case where one of a part and all of pieces of data stored in the first storage unit is to be migrated to the second storage unit, which ones of the recorded journals are related to the data that is to be migrated; choose the identified journals one at a time in reverse chronological order; judge whether or not the chosen journal corresponds to a data update made at some point between a current time and a time that precedes the current time by a length of a journal retention term set for the second storage subsystem; send, upon judging that the chosen journal corresponds to the update, the chosen journal to the second storage subsystem via the network; and stop, upon judging that the chosen journal does not correspond to the update, choosing the journals.