A control unit for a data storage system is shown, which provides at least two buffers for updating mapping information through a host memory buffer HMB. A first buffer is provided for dynamic management of a physical-to-logical mapping table F2H that records a mapping relationship which maps a physical address within a target block to a logical address of a sector of user data stored at the physical address. The control unit performs reverse conversion on the mapping relationship to get reversed mapping information for the logical address and, accordingly, selects a target logical-to-physical mapping sub-table. A second buffer is provided to buffer the target logical-to-physical mapping sub-table when the target logical-to-physical mapping sub-table is read from the host memory buffer HMB. The control unit updates the target logical-to-physical mapping sub-table on the second buffer based on the reversed mapping information about the logical address.