I/O buffers are used by VSAM to read and write control intervals from DASD to virtual storage. For a key-sequenced data set or variable-length RRDS, VSAM requires a minimum of three buffers, two for data control intervals and one for an index control interval. (One of the data buffers is used only for formatting control areas and splitting control intervals and control areas.) The VSAM default is enough space for these three buffers. Only data buffers are needed for entry-sequenced, fixed-length RRDSs or for linear data sets.
Parameter | Purpose |
---|---|
SYSTEM | Force system-managed buffering and let the system determine the buffering technique based on the ACB MACRF and storage-class specification. |
USER | Bypass system-managed buffering. |
SO | System-managed buffering with sequential optimization. |
SW | System-managed buffering weighted for sequential processing. |
DO | System-managed buffering with direct optimization. |
DW | System-managed buffering weighted for direct optimization. |
VSAM must always have sufficient space available to process the data set as directed by the specified processing options.