Defining VSAM buffer allocations for NSR (INDEXBUFFERS and DATABUFFERS)

For files using nonshared resources (NSR), the INDEXBUFFERS and DATABUFFERS parameters define VSAM index buffers and data buffers respectively.

Effects

INDEXBUFFERS and DATABUFFERS specify the number of index and data buffers for an NSR file.

The number of buffers can have a significant effect on performance. The use of many buffers can permit multiple concurrent operations (if there are the corresponding number of VSAM strings) and efficient sequential operations and CA splits. Providing extra buffers for high-level index records can reduce physical I/O operations.

Buffer allocations above the 16MB line represent a significant part of the virtual storage requirement of most CICS® systems.

INDEXBUFFERS and DATABUFFERS have no effect if they are specified for files using LSR.

Where useful

The INDEXBUFFERS and DATABUFFERS parameters should be used in CICS systems that use VSAM NSR files in CICS file control.

Limitations

These parameters can be overridden by VSAM if they are insufficient for the strings specified for the VSAM data set. The maximum specification is 255. A specification greater than this will automatically be reduced to 255. Overriding of VSAM strings and buffers should never be done by specifying the AMP= attribute on the DD statement.

Recommendations

See VSAM tuning: general objectives.

How implemented

The INDEXBUFFERS and DATABUFFERS parameters are defined in the file definition on the CSD. They correspond exactly to VSAM ACB parameters: INDEXBUFFERS is the number of index buffers, DATABUFFERS is the number of data buffers.

For LSR files, they are ignored.

How monitored

The effects of these parameters can be monitored through transaction response times and data set and paging I/O rates. The CICS file statistics show data set activity to VSAM data sets. The VSAM catalog and RMF™ can show data set activity, I/O contention, space usage, and CI size.

Related tasks
VSAM and file control: improving performance
VSAM tuning: general objectives
Defining VSAM buffer allocations for LSR
Defining VSAM string settings for NSR (STRINGS)
Defining VSAM string settings for LSR (STRINGS)
Specifying maximum keylength for LSR (KEYLENGTH and MAXKEYLENGTH)
Specifying resource percentile for LSR (SHARELIMIT)
Using VSAM local shared resources (LSR)
Using Hiperspace buffers
Permitting VSAM subtasking (SUBTSKS=1)
Using data tables to improve performance
Using coupling facility data tables to gain performance benefits
Performance aspects of VSAM record-level sharing (RLS)
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