Abstract: The present invention relates to an efficient snapshot technique based on a mapping for a large logical volume shared in multiple hosts. According to the present invention, problems of time delays in a conventional snapshot technique is solved by employing a FAB and an SSB, which are bits representing whether a COW operation is carried out to a mapping entry. In other words, the present invention solves the problems of delaying a write operation of corresponding volume, which is simultaneously executed when a snapshot is created, until the snapshot creation is completed. Further, in the write operation carried out after the snapshot creation, an operation of determining whether the COW operation is carried out is achieved by reading only an original mapping block by using the FAB and the SSB, without reading out the snapshot mapping block.