IBM SC34-7012-01 - Manual (Page 123)

However, using model log streams defined with the CICS default name are always
assigned to the same structure within an MVS image. This may not give you the
best allocation in terms of recovery considerations if you are using structures
defined across two or more coupling facilities.
For example, consider a two-way sysplex that uses two coupling facilities, each
with one log structure defined for use by CICS system logs, structures
LOG_DFHLOG_001 and LOG_DFHLOG_002. In this situation, it is better from a
recovery point of view for the CICS regions in each MVS to balance log stream
allocations across both log structures. This scenario is illustrated in Figure 8, which
shows a 2-way sysplex comprising MVSA and MVSB, with four CICS regions in
each MVS. CICSA1 through CICSA4 reside in MVSA, and CICSB1 through CICSB4
reside in MVSB. Each MVS is using two coupling facilities, CF1 and CF2, each of
which has one log structure referenced by model log streams. The first and second
CICS regions in each MVS use log streams defined in the first log structure, and
the third and fourth CICS regions in each MVS use log streams defined in the
second log structure. In this scenario, each MVS image has a partner to recover its
log streams in the event of an MVS failure.
Another example, shown in Figure 9 on page 112, is based on a 4-way sysplex
comprising MVSA, MVSB, MVSC, and MVSD. In this case, ensure that the CICS
regions that normally run on MVSA and MVSB use structure LOG_DFHLOG_001,
and that the regions that run on MVSC and MVSD use structure
LOG_DFHLOG_002. Thus each MVS image has a partner to recover its log streams
in the event of an MVS failure. If a structure fails, the two MVS images using the
other structure can take over the workload.
2-Way Sysplex
MVSA CF1 MVSB
CICSA1 CICSB1
LOG_DFHLOG_001
CICSA2 CICSB2
CF2
CICSA3 CICSB3
LOG_DFHLOG_002
CICSA4 CICSB4
Figure 8. Sharing system logger structures between 2 MVS images
Chapter 11. Defining system and general log streams 111