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.