Recommendations made by DFHLSCU

DFHLSCU is a migration utility that helps you to define your CICS® Transaction Server for z/OS®, Version 3 Release 1 log streams, based on your CICS/ESA 4.1 or CICS/ESA 3.3 journaling activity. It examines one or more CICS/ESA 4.1 or CICS/ESA 3.3 journal data sets, and produces a report directed to SYSPRINT. The report contains a summary for every interval (as specified on the INTERVAL SYSIN statement), showing the largest calculated AVGBUFSIZE. The end of the report provides conclusions and recommendations. The conclusion of the report is divided into two sections, dealing with the CF log stream and the DASD-only log stream.

Note:
If you plan to define all your log streams as DASD-only, but there is a possibility that, at a later date, you might want to convert some of them to CF log streams, it is a good idea to save the output from DFHLSCU for later reference.

The values provided by DFHLSCU are estimates and may not match your actual experience. In particular, they may be affected by changes in the pattern of logging such as:

While the recommended values provide a starting point for structure sizing, you should monitor actual usage and adjust it as required.

Recommendations for a CF log stream

The first section of the report’s conclusion assumes that you want to define a log stream that uses a coupling facility structure. For a CF log stream, the report provides the following information (see below for descriptions of the attributes involved):

You must edit the sample definitions and provide appropriate values for the structure name and preflist name. See Sample report from DFHLSCU for an example of the output from DFHLSCU.

The attributes for which the report provides recommended values are explained as follows:

AVGBUFSIZE
The average buffer size, in bytes, of a log stream structure in the coupling facility. It is important, particularly in MVS™ releases before OS/390® Release 3, that the value you specify for AVGBUFSIZE reflects as accurately as possible the real size of most log blocks written to the structure. This leads to efficient use of the space in the coupling facility and minimum DASD offloading frequency. This is less important in OS/390 Release 3 and later, because OS/390 performs some dynamic tuning.
INITSIZE
The initial amount of space, in kilobytes, to be allocated for the log stream structure in the coupling facility. You define this attribute in your CFRM policy.
HIGHOFFLOAD
The point in primary storage (that is, in either the coupling facility structure or the staging data set), as a percentage of space consumed, at which the MVS system logger starts its offload process--which can cause data from the log stream to be offloaded to log stream DASD data sets. (The offload process is described in the CICS Transaction Server for z/OS Installation Guide.) You define this attribute in your DEFINE LOGSTREAM job.
LOWOFFLOAD
The point in primary storage, as a percentage of space consumed, at which the MVS system logger stops offloading data from the log stream to log stream DASD data sets. You define this attribute in your DEFINE LOGSTREAM job.
MAXBUFSIZE
The size, in bytes, of the largest block of data that can be written to the log stream.
SIZE
The maximum size, in kilobytes, of the log stream structure in the coupling facility. You define this attribute in your CFRM policy.
STG_SIZE
Optionally, a staging data set can be used with a CF log stream. For guidance about when to use a staging data set, see the CICS Transaction Server for z/OS Installation Guide. STG_SIZE is the size, as a number of 4K blocks, of the staging data set, if one is required. You define this attribute in your DEFINE LOGSTREAM job.

Recommendations for a DASD-only log stream

The second section of the report’s conclusion assumes that you want to define a log stream that uses DASD-only logging. It provides a sample log stream definition which contains recommended values for the following attributes (all of which are defined in your DEFINE LOGSTREAM job):

See Sample report from DFHLSCU for an example of the output from DFHLSCU.

The attributes for which the report provides recommended values are explained as follows:

HIGHOFFLOAD
The point in primary storage (that is, in the staging data set), as a percentage of space consumed, at which the MVS system logger starts its offload process.
LOWOFFLOAD
The point in primary storage as a percentage of space consumed, at which the MVS system logger stops offloading data from the log stream to log stream DASD data sets.
MAXBUFSIZE
The size, in bytes, of the largest block of data that can be written to the log stream.
STG_SIZE
The size, as a number of 4K blocks, of the staging data set for the log stream.

A DASD-only log stream always uses a staging data set.

Related reference
Log stream sizing migration utility (DFHLSCU)
Job control statements to run the DFHLSCU program
SYSIN control statements for the DFHLSCU utility
Considerations when using DFHLSCU
DFHLSCU return codes
Sample report from DFHLSCU
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