6. DATA SOURCES › 6.4 Expanded Storage Concepts › 6.4.7 Expanded Storage Management

6.4.7 Expanded Storage Management

Expanded storage can be used to reduce the response time of
online systems, to increase the capacity of the processor
complex, or a combination of the two.  The best way of using
expanded storage depends on the business and technical
environment in which it is used.

Unfortunately, the mechanisms used to control expanded
storage allocation do not provide for workload assignments,
such as performance group or domain.  It is therefore
necessary to have a good understanding of the storage
utilization characteristics of the individual workloads and
the system as a whole before attempting to modify the
criteria ages that affect expanded storage control.

There are some general guidelines that apply to all systems,
however.  For example, if you wish to improve the response
times of online systems running as non-swappable address
spaces, you could set low criteria ages for type 0 and high
ones for types 1 and 2.  This will favor the onlines over
other work, but only if there is other work in the same
system.  In a system used for application development and
testing with ISPF/PDF users submitting batch compiles and
executions, you might want to favor type 2 over types 0 and
1.  If you have a processor complex that is performing a wide
variety of tasks and with changing workload characteristics
over time, then it is quite possible that you would achieve
both somewhat better response time and more throughput by
using the default values for all criteria ages.

When making criteria age changes, keep in mind the following:

o  Assigning a low criteria age to a particular page type and
   status is saying, in effect, that you want the RSM to move
   such pages to expanded storage before other page
   type/status classifications even if the contention for
   expanded storage is high.  You would do this if (a) you
   regard the workload owning the page as important and
   deserving of a performance improvement, or (b) you expect
   the page to be referenced quickly and want to lower the
   load on the ASM by making it unnecessary to page-out and
   then page-in the same page in a brief time period.

o  Assigning a high criteria age to pages has the opposite
   effect:  such pages are sent to expanded storage by the
   RSM after page types with lower criteria ages.  In periods
   of contention, such pages tend to go to auxiliary storage
   instead.  You would do this if you either (a) want to use
   your expanded storage resources for more important work,
   or (b) expect that such a page will not be referenced for
   a relatively long time and would be migrated to auxiliary
   storage anyway.

Although IBM has established reasonable default values, it is
ultimately the responsibility of individual installations to
set these values for best use of expanded storage.  The
performance analyst must monitor page movement to ensure that
the specified values are optimizing the use of expanded
storage for the installation's requirements.  If criteria
ages are set too low, expanded storage will tend to be over-
utilized.  This will result in a low migration age, large
values for both central-to-expanded page movement, and page
migration.  If the difference between these two measurements
is small, it could indicate that too many pages are being
migrated.