Defining VSAM buffer allocations for LSR

For files using local shared resources (LSR), the number of buffers to be used is not specified explicitly by file. The files share the buffers of the appropriate sizes in the LSR pool. The number of buffers in the pool may either be specified explicitly using the BUFFERS parameter in the file definition on the CSD, or be left to CICS® to calculate. For more information about the CSD, see the CICS Resource Definition Guide.

Effects

The BUFFERS parameter allows for exact definition of specific buffers for the LSR pool.

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). It can also increase the chance of successful buffer lookaside with the resulting reduction in physical I/O operations.

The number of buffers should achieve an optimum between increasing the I/O saving due to lookaside and increasing the real storage requirement. This optimum is different for buffers used for indexes and buffers used for data. Note that the optimum buffer allocation for LSR is likely to be significantly less than the buffer allocation for the same files using NSR.

Where useful

The BUFFERS parameter should be used in CICS systems that use VSAM LSR files in CICS file control.

Recommendations

See VSAM tuning: general objectives.

How implemented

The BUFFERS parameter is defined in the file definition on the CSD. For more information about the CSD, see the CICS Resource Definition Guide.

How monitored

The effects of these parameters can be monitored through transaction response times and data set and paging I/O rates. The effectiveness affects both file and lsrpool statistics. 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 NSR (INDEXBUFFERS and DATABUFFERS)
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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