This section applies only to log streams that use coupling facility structures.
Coupling facility space is divided into structures by the CFRM policy, the maximum permitted being 255 structures. Multiple log streams can use the same structure. Generally, the more log streams per structure, the more difficult it is to tune the various parameters that affect the efficiency and performance of the CICS® log manager.
As far as performance considerations go, you should try to ensure that log streams used by applications that write similar sized data records share the same structure. The reasons for this relate to the values defined in the AVGBUFSIZE and MAXBUFSIZE parameters on the structure definition.
When a coupling facility structure is defined, it is divided into two areas:
List elements are units of logged data and are either 256 bytes or 512 bytes long. List entries are index pointers to the list elements. There is one list entry per log record. There is at least one element per log record.
If you define MAXBUFSIZE with a value greater than 65276, data is written in 512-byte elements. If you define MAXBUFSIZE with a value less than, or equal to, 65276, data is written in 256-byte elements. The maximum value permitted for this parameter is 65532.
The proportion of the areas occupied by the list entries and the list elements is determined by a ratio calculated as follows:
AVGBUFSIZE / element size
The resulting ratio represents the ratio, nn : 1, where nn represents element storage, and '1' represents entry storage. This is subject to a minimum of 1:1.
This ratio has performance significance because it may be inappropriate for a combination of many different applications with different logging requirements and behavior.
AVGBUFSIZE is set at the structure level and dictates the ratio for the whole structure. As a general rule, the greater the number of log streams per structure, the greater the chance that the element/entry ratio is inappropriate for certain applications using the log streams. If many applications write to their log streams significantly differing amounts of data, and at significantly differing intervals, some of those applications can experience unexpected DASD offloading, with the extra processing overhead that this incurs. The DASD offloading is unexpected because the log stream may not yet have reached its HIGHOFFLOAD threshold.
Each log record places an entry in the list entry area of the structure, and the data is loaded as one or more elements in the list element area. If the list entry area exceeds 90% of its capacity, all log streams are offloaded to DASD. DASD offloading commences at this point, regardless of the current utilization of the log stream, and continues until an amount of data equal to the difference between the HIGHOFFLOAD threshold and the LOWOFFLOAD threshold has been offloaded.
For example, the list entry area may exceed 90% of its capacity while log stream A is only 50% utilized. Its HIGHOFFLOAD threshold is 80% and its LOWOFFLOAD threshold is 60%. Even though log stream A has not reached its HIGHOFFLOAD threshold, or even its LOWOFFLOAD threshold, data is offloaded until 20% of the log stream has been offloaded. This is the difference between 80% and 60%. After the offloading operation has completed, log stream A is at 30% utilization (50% minus 20%).
Thus, the log stream used by an application issuing very few journal write requests may be offloaded to DASD because of frequent journal write requests by other applications using other log streams in the same structure.
However, if multiple log streams share the same structure, a situation where list entry storage reaches 90% utilization should only occur where all the log streams have a similar amount of logging activity.
However, there is the likelihood of wasted list entry storage if you underestimate the value for AVGBUFSIZE in order to avoid frequent DASD offloading.
Whenever a log stream connects to or disconnects from a coupling facility structure, the structure undergoes dynamic repartitioning. This means that the space in the structure is partitioned between all the log streams connected to the structure. As more log streams connect, the less space each log stream is apportioned. This can lead to a higher frequency of DASD offloading as reduced log stream space means that the log stream HIGHOFFLOAD threshold percentages are reached more often.
A value of 64000 for MAXBUFSIZE should be appropriate for most environments and should be suitable for most purposes.
If MAXBUFSIZE is set to greater than 65276, the element size is 512 bytes. With an 512-byte element, there is more likelihood of space being unused, and, therefore, wasted because of padding to the end of the last element for the log record. This likelihood lessens where records are larger and where systems are busier.
AVGBUFSIZE and MAXBUFSIZE are parameters for use in the IXCMIAPU program which you would run to define coupling facility structures. For more information, see the System/390® MVS™ Setting up a Sysplex manual.
The following facilities are available to monitor the data traffic to log streams on structures, and from log streams to DASD: