Each of a storage apparatus having a primary volume and a storage apparatus having a secondary volume, upon reception of a write command, changes a data attribute of a data area allocated to an address designated by the write command, to a first attribute if the data attribute is other than the first attribute. In an offloaded data copy processing sequence, each of the storage apparatus changes a data attribute of a data area allocated to a copy source address to a second attribute if the data attribute is other than the second attribute. When an allocation information copy process is to be performed to allocate the data area to a copy destination address according to an offloaded data copy command, each of the storage apparatus discontinues the offloaded data copy processing sequence if the data attribute of the data area is the first attribute.