This question was also posted on my blog on Dev X. Here is the response I gave at that location:
A query is not allocated to a resource partition directly. It is mapped to a performance group. A query is under the control of an allocation group, based on which allocation group its performance group (PG) points to. The resource partition is a collection of performance groups. So whatever PG a query belongs to will dictate the resource partition it is associated with.
A performance period is simply an attribute of a performance group. It allows a query to map to a single performance group, but based on time or CPU usage to be moved under the control of a different allocation group than the default allocation group. The performance period has no independent existence outside a performance group. It is simply a bridge between a performance group and an allocation group. It is a carry-over from previous releases where it had a more important role. Today, you can basically ignore performance periods.
Most performance groups only have a single (default) performance period. For appliance platforms, the performance groups (most of them) have built-in performance periods in order to control the automatic demotions. The first PP is the default one where the query starts; the second PP is for the allocation group after the demotion. However, you don't have to be concerned about performance periods at all or how to define them. In fact, on the appliance, you will not be allowed to add or delete performance periods. If you are using full TASM, you don't need to worry about them either, as you have things like workload exceptions to provide similar functionality.
The TASM workload is a performance group under the covers (at least in the SLES 10 versions). TASM workloads do not know about or get involved in performance periods. So if you are a TASM user, you don't have to be concerned or ever think about performance periods. Just set up workload exceptions if you need them, using whatever exception criteria you like.
Thanks, -Carrie
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