6. DATA SOURCES › 6.4 Expanded Storage Concepts › 6.4.6 Measurement and Analysis of Expanded Storage Activity

6.4.6 Measurement and Analysis of Expanded Storage Activity

RMF measures and reports many data elements concerning
paging, swapping, and memory use.  Numerous metrics are
provided that differentiate between paging or swapping to
expanded versus auxiliary storage, and between the use of
central versus expanded storage for CSA, LPA, SQA, and other
pages.  Obviously, expanded storage has added to the
complexity of understanding and managing the storage
hierarchy.

Even though well-tuned systems or systems with low storage
contention are likely to experience expanded storage-related
delays so small as to be relatively unimportant when compared
to other sources of delay, installations with expanded
storage should now monitor its utilization and activity as
well as auxiliary storage activity.  Expanded storage delay
can become substantial to the point of significantly reducing
the benefit from your investment in it.

In a system with expanded storage, the degradation due to
page movement delay from expanded storage to central storage
is not directly measurable .  While the system control
program maintains (and RMF records) the average time that it
takes to read a page from auxiliary to central storage, no
such estimate is made of the time it takes to transfer a page
from expanded to central storage.

Page migration should be a primary concern because, if
excessive, it represents ineffective page placement and
ineffective use of expanded storage, just as we've already
seen in swapping situations.  If there is contention for
expanded storage, it makes no sense to move a page from
central to expanded storage if that page will eventually be
moved back into central and then paged out to auxiliary.  It
would be much better to page it out directly to auxiliary
storage.

Minimizing such ineffective page placement depends on how
ccurately an address space's future activity and storage
 references can be predicted.  The RSM makes these predictions
based on the criteria age values specified in the IEAOPTxx
parameters for the various categories of expanded storage
use.