(VSAM) An IBM disk file storage scheme used in OS360. VSAM uses inverted indexes and clever data organisation to improve access time and reduce the need for reorganisation of certain types of randomly accessed disk files.
Search algorithms that rely on the records of a database being sorted are significantly more efficient than reading the records sequentially until the required record is located. Therefore, each time a new record is inserted in the correct sort order, the records following it must be shifted to accommodate the new record. In order to avoid this "shuffle" for each new record, empty space between records may be allocated. However, once a large number of new records have been added, the empty space will fill up, requiring that the records be reorganised to create additional space.
Rather than "shuffle" and reorganize large records in a large dataset, one can create an inverted index of record locations and sort that rather than the actual records. But eventually the empty space provided in the index for new keys fills up, requiring a reorganisation. VSAM applies statistical algorithms in an attempt to optimise this process.
(08 Feb 1995)