Figure 1 shows the potential resource contention that exists in the SDSP environment:

Figure 1. The SDSP Data Set Contention Environment

It is important to plan the number of SDSP data sets in relation to the number of concurrent migration tasks and the amount of processing done by functions with a higher usage priority for the SDSP data sets.

Because of their higher usage priority, any of these activities can gain control of your SDSP data sets and leave you with fewer than the expected number of SDSP data sets for migration.

When an activity with a higher usage priority for SDSP data sets has or requests an SDSP data set, that SDSP data set is no longer a candidate for migration. The small data set that is in need of migration must find a different, available SDSP data set or it is skipped and left unprocessed until your next migration window.

Additionally, if all SDSP data sets should become full (as a result of migrations to them), the filled SDSP data sets are not candidates for further migration. Full SDSP data sets are not seen by migration processing, and, as a result, any small user data sets are migrated as data sets to level-1 migration volumes.

The following three-part example illustrates how SDSP data sets become unavailable for use as level-0 to level-1 migration targets. Figure 2 shows three concurrent migration tasks that move three small user data sets from level-0 user volumes to level-1 migration volumes (with SDSP data sets that are defined on the level-1 migration volumes).

Figure 2. Part 1—Ideal Multitasking Migration. Three migration tasks migrate three small user data sets to three level-1 migration volumes on which SDSP data sets are defined.

Figure 3 shows how a recall of a small user data set from an SDSP data set during level-0 to level-1 migration has effectively eliminated one concurrent migration task. The small user data set, whose migration was preempted by a recall, sees that all SDSP volumes are not full and defers this migration for your next migration window.

Figure 3. Part 2—Recall Processing Has a Higher Priority than Migration. One migration task does not process the small data sets because recall processing has a higher usage priority for the SDSP than migration processing.

Figure 4 shows that all SDSP data sets have become full. They are no longer seen as candidates for level-0 to level-1 migration destinations and the small-user data sets migrate as data sets.

Figure 4. Part 3—All SDSP Data Sets Are Full. The small user data sets migrate as large data sets to the level-1 migration volumes.

Because other activity can effect your migrations, you must plan and monitor those activities that can cause your small user data set migrations to be skipped. You must define ample SDSP data sets to manage your worst-case scenario.

Related reading: For more information about the SDSP parameter of the SETSYS command and for a table of SDSP migration contention priorities, see z/OS DFSMShsm Storage Administration.