6. DATA SOURCES › 6.7 z/Architecture Highlights › 6.7.2 Removal of Expanded Storage

6.7.2 Removal of Expanded Storage

z/OS in z/Architecture mode does not support expanded storage
anymore.  In order to retain support for applications that
use these services, hiperspace and VIO pages are now in real
storage.  SRM has to make a placement decision for these
pages between real and auxiliary, as opposed to between
expanded and auxiliary in ESA/390 mode.  Instead of basing
this decision on the expanded storage migration age, the
system high UIC is compared to the criteria table value of
the page type (Hiperspace or VIO).  If the high UIC is less
than this value, the page is put in real storage.  Otherwise
it is sent to auxiliary storage.

There is a performance concern related to frame queues
growing much larger than in ESA/390 environment.  Changes to
UIC Steal and Update have been required so that the CPU cost
of these functions does not impact other functions in the
system.  Previously, the UIC interval was 1 second, meaning
that when a frame's UIC value was increased by 1, it
represented 1 second when the frame was not referenced.  In
z/Architecture mode, the UIC increment has been increased to
10 seconds to reduce the overhead of the UIC Update.

Also, z/Architecture introduces a new storage "line":  the
2-gigabyte line.  As systems grow much larger than 2 GB, the
area between 16 MB and 2 GB becomes a scarce resource, and
there will probably always be applications with a dependency
on real in this range.  This is why these regions are managed
as a separate resource when SRM detects pageable frame
shortages.  The logic is driven by new SRM counts of frames
between 16 MB and 2 GB.