it fills its own identifier into the id_resp field of the message of sector transfer request if the id_resp fields of all nodes in the message of sector transfer request are null it is indicated that no sector data will be transferred therefore a transfer rejection message is directly returned to the originating party of the sector transfer request otherwise the processed message is sent to the server the server determines if the received message of the sector transfer request is correct and each of the fields are reasonable and non null then local disk operations are performed according to the content of the op_type field and copying of sector data is performed between the disk images corresponding to the originating party and receiving party of the sector transfer request after the successful completion of the above mentioned steps the server returns to the receiving party of the sector transfer request a message indicating the successful completion of the operations including the information on read or written sectors and the receiving party updates the locally maintained mapping table based on the message then the receiving party of the sector transfer request returns to the originating party of sector transfer request a message indicating the completion.