VSAM specifications for NSR

Defining VSAM string settings for NSR and defining VSAM buffer allocations for NSR.

Defining VSAM buffer allocations for NSR

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

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.
  • Effects

    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 control area (CA) splits. Providing extra buffers for high-level index records can reduce physical I/O operations.

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

  • 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 value is automatically reduced to 255. Never override VSAM strings and buffers by specifying the AMP attribute on the DD statement.

  • Limitations

    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 control interval (CI) size.

Defining VSAM string settings for NSR

The STRINGS parameter is used to determine the number of concurrent operations possible against the file, and against the VSAM base cluster to which the file relates.

Use the STRINGS parameter in CICS systems that use VSAM NSR files in CICS file control.

The number of strings is defined by the STRINGS parameter in the CICS file definition on the CSD. It corresponds to the VSAM parameter in the ACB, except where a base file is opened as the first for a VSAM data set. In this case, the CICS -accumulated BSTRNO value is used as the STRNO value for the ACB.
  • Effects
    The STRINGS parameter for files using NSR has the following effects:
    • It specifies the number of concurrent asynchronous requests that can be made against that specific file.
    • It is used as the STRINGS value in the VSAM ACB.
    • It is used, with the BASE parameter, to calculate the VSAM BSTRNO value.
    • A number greater than 1 can adversely affect performance for ESDS files used exclusively in write mode. With a string number greater than 1, the cost of invalidating the buffers for each of the strings is greater than the cost of waiting for the string, and there can be a significant increase in the number of VSAM EXCP requests.

    Strings represent a significant part of the virtual storage requirement of most CICS systems. With CICS, this storage is above the 16 MB line.

  • Limitations

    A maximum of 255 strings can be used as the STRNO or BSTRNO values in the ACB.

  • Monitoring

    The effects of the STRINGS parameter can be seen in changes to response times. The CICS performance data field 427, FCVSWTT, in the DFHFILE group, shows the elapsed time in which each user task waited for a VSAM string. The CICS LSR pool statistics give information about the number of strings, the number of requests that waited for strings, and the maximum number of strings that were active at one time. RMF can show I/O contention in the DASD subsystem.