6. DATA SOURCES › 6.4 Expanded Storage Concepts › 6.4.4 Effectiveness of Logical and Expanded Storage Swaps

6.4.4 Effectiveness of Logical and Expanded Storage Swaps

The concept of an effective swap placement decision refers to
the ability of the RSM and SRM to predict the address space's
future activity, allowing the swap pages to reside in the
most effective storage medium.  An address space can be
logically swapped and remain primarily in central storage, or
physically swapped either to expanded or auxiliary storage.

Logical swapping (also referred to as SRM demand swapping) is
used under certain conditions in an attempt to save the
processor cycles and expanded storage or I/O overhead of
physical swapping.  A TSO address space in a terminal wait
condition or any class of address space in a long or detected
wait condition is eligible for logical swapping.  When this
happens, the LSQA, fixed, and most recently referenced pages
are kept in central storage.

A logical swap is effective only if the address space is
swapped back in before being converted to a physical swap.
An expanded storage physical swap is effective only if the
address space is swapped back in before being migrated
through central to auxiliary storage.