202 IMS Performance and Tuning Guide
11.1 Hardware and microcode
The Coupling Facilities can have a significant impact on IMS performance. Depending on
your particular hardware configuration, you might need to use certain functions to maintain
high availability, which are less than ideal for performance.These facilities include
system-managed duplexing and the use of staging data sets for the MVS system logger.
These things are discussed more in this chapter.
11.1.1 Coupling Facility configuration
Coupling Facilities can be part of the same physical box as the z/OS LPAR or they can be
stand-alone boxes. If they are stand-alone boxes, you might generally be able to achieve high
availability without having to use system-managed duplexing or staging data sets. If the CF
LPARS are on the same physical box as the z/OS images running IMS, then it might be
necessary to use these functions to prevent a loss of integrity. A few other things to be aware
of in a production environment are:
򐂰 Dynamic dispatching for the CF LPAR might cause poor or erratic CF response times.
򐂰 Multiple CF engines, even when utilization is relatively low, can help keep response times
more stable.
򐂰 Multiple CF links not only help availability but also benefit performance.
11.1.2 Coupling Facility microcode
New levels of CF microcode generally introduce new capabilities. Some of these new
functions can take up additional control storage so that there might be less storage available
for user data in the same size structure. The good news is that except for preloaded VSO, the
microcode changes rarely cause any failures and IMS continues to run. The bad news is that
your IMS performance could suffer because of something you might not even know about. For
this reason, you should make sure your systems people let you know any time there is a
change to the CF microcode or the configuration. Make sure you know how to obtain and read
the RMF CF Structure Activity reports and keep a repository of these reports for future
reference in the event of problems.
11.2 Structure sizing
Individual structure sizing considerations are discussed later in this chapter, but for all IMS
structures as well as other system and subsystem structures, you might reference the
following IBM Web site:
If you click IMS, you can select any or all of the IMS-related structures to estimate the size.
This Web site normally calculates the size based on the most current CFLEVEL available,
which is displayed in the results page. Structure sizes are always a multiple of 256K so if you
specify a size, which is not a multiple of 256K, the system rounds it up to the next 256K
11.3 IRLM considerations
A couple of IRLM startup parameters need to be considered for IRLM performance:

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