Specifying SMS data set characteristics for DASD data sets

When you define or update a log stream definition in the LOGR policy, you can assign the SMS storage class, data class, and management class for both the DASD log data sets and staging data sets. For example, you can specify parameters STG_DATACLAS and LS_DATACLAS in the LOGR policy to assign a data class with the correct VSAM share options. These parameters intersect with other parameters and the system environment in ways that might produce unpredictable results if you do not plan well. So, think carefully before specifying the SMS parameters in the LOGR policy. For instance:

SMS parameters and ACS routines:
If you use installation-written automatic class selection (ACS) routines to assign classes for these DASD data sets, the ACS class might override SMS classes you explicitly specify in the LOGR policy using the IXCMIAPU utility. Make sure you coordinate your ACS routines and the SMS characteristics you specify in the LOGR policy to avoid surprises.
Using SMS parameters and the LIKE parameter together:
If you are using the LIKE parameter for a log stream, note that any explicit definitions for the log stream you are defining override the definitions of the log stream referenced on the LIKE parameter. System logger will not use an SMS class from a log stream referenced on a LIKE parameter if there is any value specified for that class on an SMS parameter.
Optimizing Control Interval Size:
Increasing the size of the control interval for DASD log data sets can provide performance improvements in the areas of record processing speed and storage requirements. You can modify the control interval size attribute of DFSMS Data Classes defined for DASD log data sets to gain performance improvements in I/O operations associated with the offloading of log stream data as well as during browse processing when retrieving log stream data from DASD log data sets.

A VSAM linear data set requires a control interval size of from 4096 to 32768 bytes, and can be expressed in increments of 4096 bytes. The VSAM default is 4096 bytes. System logger is designed to obtain optimal I/O performance when DASD log data sets reside on devices with 3390 formats. IBM® suggests a control interval size of 24576 bytes to realize optimal I/O performance if system logger DASD log data sets reside on devices with 3390 formats. Specify a DFSMS data class that is defined with a control interval size of 24576 on the LS_DATACLAS parameter of a log stream definition to have DASD log data sets allocated with control interval sizes of 24576. Using a control interval size other than 4096 in conjunction with the LS_SIZE parameter can result in a slightly different DASD space allocation than expected. Confirm that the actual data set size is suitable for your log stream.

Note: Do not change the Control Interval Size attributes for staging data sets to be anything other than 4096. System logger requires that the Control Interval Size for staging data sets be set to 4096. If staging data sets are defined with characteristics that result in a Control Interval Size other than 4096, system logger will not use staging data sets to keep a duplicate copy of log data. Operations involving staging data sets defined with a Control Interval Size other than 4096 will fail to complete successfully.

If you intend to use control interval sizes other than 4096, you might need to modify the STG_DATACLAS specifications or change your DFSMS ACS routine. This action may be required regardless of whether or not you specify STG_DUPLEX (YES) for your log streams. If you explicitly provide a STG_DATACLAS specification for the log stream, ensure the DFSMS data class uses a control interval size of 4096. If you use an ACS routine to have attributes assigned to any log stream data sets, you must ensure the staging data sets do not use a data class that has a control interval size other than 4096. For example, you might need to change the ACS routine to filter based on the data set name up to and including the last qualifier in order to recognize staging data sets.

If your IXCMIAPU request does not complete successfully, you might get return and reason codes returned in the output. These return and reason codes are documented along with IXGINVNT in z/OS MVS Programming: Assembler Services Reference IAR-XCT.

Before using the IXCMIAPU utility, you will need to define the SAF authorizations for system logger applications. See Authorization for system logger applications.