The invention relates to retrieving data from a storage system. One embodiment of the invention comprises receiving a write operation, establishing a correspondence relationship between a logic block address and a physical block address of the write operation, and determining whether a valid data percentage in a mapping table is greater than a predetermined threshold after the correspondence relationship is added in stored metadata. In response to the valid data percentage being less than the predetermined threshold, the embodiment adds the correspondence relationship to a B-tree data structure of stored metadata.