A storage virtualization computer system, comprising: a host unit for sending an I/O request; an external storage virtualization controller coupled to the host unit for executing an I/O operation in response to the I/O request; and at least one physical storage device coupled to the storage virtualization controller via a serial attached SCSI interconnect for providing data storage space to the storage virtualization computer system via the storage virtualization controller; wherein the external storage virtualization controller is configured to execute a snapshot function for storing the data state of at least one Source-Volume consisted of the physical storage devices at a particular moment, said snapshot function is performed via a section allocation system for managing operations of at least one Snapshot-Volume, and said section allocation system comprises: a section allocation table for recording information on said section allocation system; and a plurality of block association sets, each one of the block association sets corresponding to one of said at least one Source-Volume as the basis for performing a point-in-time backup (PIT backup) procedure, wherein said section allocation table has a field containing information pointing to said block association sets (BASs); at least one block association table in each of said plurality of block association sets (BASs), for storing information about data originally stored in said at least one Source-Volume, each said BAT corresponding to one Snapshot-Image in a one-to-one manner; wherein when receiving a first write request that writes first data into a first location in said at least one Source-Volume, a copy-on-write procedure is performed to copy data originally stored in data blocks and its LBA, to one of at least one Snapshot-Image, and said first data are written into said at least one Source-Volume, and when receiving a second write request that is later than said first write request, and that writes second data into said first location in said at least one Source-Volume, said second data are directly written into said first location in said at least one Source-Volume without performing said copy-on-write procedure on said one of at least one Snapshot-Image; and whereby the section allocation system manages operation of the at least one Snapshot-Image so as to store a status of the Source-Volume at a particular moment. a host unit for sending an I/O request; an external storage virtualization controller coupled to the host unit for executing an I/O operation in response to the I/O request; and at least one physical storage device coupled to the storage virtualization controller via a serial attached SCSI interconnect for providing data storage space to the storage virtualization computer system via the storage virtualization controller; a section allocation table for recording information on said section allocation system; and a plurality of block association sets, each one of the block association sets corresponding to one of said at least one Source-Volume as the basis for performing a point-in-time backup (PIT backup) procedure, wherein said section allocation table has a field containing information pointing to said block association sets (BASs); at least one block association table in each of said plurality of block association sets (BASs), for storing information about data originally stored in said at least one Source-Volume, each said BAT corresponding to one Snapshot-Image in a one-to-one manner;