Determining I/O Buffer Space for Nonshared Resource

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.

To increase performance, there are parameters to override the VSAM default values. There are five places where these parameters can be specified:

BUFFERSPACE, specified in the access method services DEFINE command. This is the least amount of storage ever provided for I/O buffers.
BUFSP, BUFNI, and BUFND, specified in the VSAM ACB macro. This is the maximum amount of storage to be used for a data set's I/O buffers. If the value specified in the ACB macro is greater than the value specified in DEFINE, the ACB value overrides the DEFINE value.
BUFSP, BUFNI, and BUFND, specified in the JCL DD AMP parameter. This is the maximum amount of storage to be used for a data set's I/O buffers. A value specified in JCL overrides DEFINE and ACB values if it is greater than the value specified in DEFINE.

ACCBIAS specified in the JCL DD AMP parameter. Record access bias has six specifications:

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.

Record Access Bias specified in the ISMF data class panels. The data class keyword Record Access Bias has the same six mutually exclusive specifications as the JCL AMP ACCBIAS shown above.

VSAM must always have sufficient space available to process the data set as directed by the specified processing options.