Technical Information › Fixed and Modified LPA › Finding the Linklist Libraries

Finding the Linklist Libraries

Although you specify data set name and module names in IEALPAxx or IEAFIXxx, in systems where the library name can refer to a concatenation (as explained above), more than one module can appear to qualify. In these systems, careful review of duplicate modules in linklist is important for both integrity and security.

You can find the system linklist libraries with the Linklist Library Display (2.4.2). If multiple copies of the modules exist in the linklist or LPA libraries, z/OS searches the libraries in the order they are concatenated in, stopping at the first module it finds. z/OS loads that module into FLPA or MLPA and ignores all other modules.

When a module is requested without using JOBLIB or STEPLIB, z/OS searches the FLPA first, then the MLPA, the LPA, and finally the system linklist. One key test determines if the same module is located in several places. See the System Review Checklist for specific tests to perform. It is important to remember that z/OS considers FLPA and MLPA resident modules to be APF‑authorized, just like those in PLPA.

Normally, FLPA is used for performance tuning of z/OS. For example, IBM recommends that some modules be placed in FLPA, such as the timer SVCs (IGC0004F and IGC0004G). They work faster because they do not have to be paged in. IBM advises that the best candidates for FLPA are those modules that are used infrequently but are needed for fast response, such as those supporting interactive transaction processing.

Systems programmers use MLPA primarily for testing new versions of LPA modules. However, you can also find that they also use dynamic LPA extension programs, such as MODREP, to do testing. These programs load test copies of modules into common storage, and then modify the LPA directory to “activate” the new module.

Because most audit and security experts recommend reviewing LPA, MLPA, and FLPA as a group, be sure to read the LPA section of this guide carefully and perform the tests specified in the System Review Checklist for a complete review.