A first storage system stores information relating to the updating of data
stored in that system as a journal. More specifically, the journal is
composed of a copy of data that was used for updating and update
information such as a write command used during updating. Furthermore,
the second storage system acquires the journal via a communication line
between the first storage system and the second storage system. The
second storage system holds a duplicate of the data held by the first
storage system and updates the data corresponding to the data of the
first storage system in the data update order of the first storage system
by using the journal.