Analysis of VSAM's free-space behavior

Abstract

The main difference between IBM's VSAM (Virtual Storage Access Method) and ISAM (Indexed Sequential Access Method) is that with the former a certain amount of free space has been reserved at loading time for later insertion of records while with the latter none has been reserved. Therefore, VSAM maintains a balanced tree structure, while no balancing of the structure is done with ISAM. The amount of free space which should be pre-reserved for a VSAM file needs careful study. A VSAM file with an underestimated amount of free-space will require more frequent, time-consuming file-reorganization because of the likelihood of using up reserved free-space sooner. On the other hand, a file with an overestimated amount of free-space will waste useful and highly demanded storage space. In this paper, the appropriate free space is represented as a function of two variables, namely: (1) the insertion rate and (2) the number of records in the file at loading time. Using these two variables as known parameters, the appropriate amount of free space can be reasonably determined and reserved in every data block (called CINV by IBM) as follows.