Patent ID: 8566543

Claim:
A computer system comprising: a file server for providing a primary volume, which is a data read/write access destination, and a snapshot of the primary volume to a host, wherein the file server includes a difference volume for storing pre-update data of the primary volume; and a storage system for setting a virtual volume to the primary volume, allocating a storage block from a pool to the virtual volume in response to write access from the host to the primary volume, and writing data to the storage block; wherein if it is determined that there is a specified area of the primary volume to which a file system does not refer and the snapshot refers, the file server stores the pre-update data of the specified area in the difference volume by writing temporary data to a specified storage block corresponding to the specified area among a plurality of storage blocks after obtaining the snapshot, and thereby releases the reference from the snapshot to the specified area; and the storage system releases the specified storage block from allocation to the virtual volume.