DATA COMPRESSION METHOD OF DIRECT-ACCESS STORAGE DEVICE AND ITS DATA COMPRESSION STRUCTURE

摘要

PROBLEM TO BE SOLVED: To obtain consistency between a performance resembling to that of an uncompressed DASD subsystem and response time by providing a compressed data exception containing updated compressed data exceeding the original storage space corresponding to original compressed data. SOLUTION: A DASD 36 has a plurality of variable-length compressed data groups 38 and each group 38 keeps related host system data. Each group 38 has a compressed data area which stores original and updated compressed data, an exception managing area 42 which stores updated compressed data larger than the space originally assigned to the data, and a directory table or micro-table 44 which decides the location of data in the group 38. The exception managing area 42 is a space prespecified in each compressed data group 38 for such a compressed page that cannot be contained in a currently assigned space and directory data. Since the area 42 is provided in such a way, the updated data are not stored at distance far from their original storage space and adjacent file data.