Defining the number of log streams: Performance considerations

Coupling facility space is divided into structures by the coupling facility resource management (CFRM) policy; the maximum is 255 structures. Multiple log streams can use the same structure. 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.

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.

When a coupling facility structure is defined, it is divided into two areas: one holds list entries, and the other holds list elements.

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 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 can be inappropriate for a combination of many different applications with different logging requirements and behavior.