2. PERFORMANCE REPORTING ANALYSIS › 2.3 Auxiliary Storage Management Analysis › 2.3.4 MVS Concepts › 2.3.4.2 Paging › 2.3.4.2.1 Data Set Selection

2.3.4.2.1 Data Set Selection

For any page-in request, Auxiliary Storage Manager (ASM) must
naturally select the page data set to which the virtual page
was paged-out.  To improve page fault response time, page-in
requests are given priority over page-outs.

With the exception of a create LPA (CLPA) operation at IPL,
pageable link pack area virtual pages are read-only, and are
therefore paged-in from the PLPA data set, but never paged-
out.  Changed Common System Area (CSA) virtual pages are
always paged-out to the common data set.  The common and PLPA
data sets may spill to one another, though common should not
be allowed to spill to PLPA for performance reasons.  For
private area page-out processing, however, ASM must choose
between the local data sets.

In MVS/XA and MVS/ESA, considerations are made for the
projected response times of the individual page data sets.
Page data sets with faster projected response times tend to
be selected over those with slower projected response times,
balancing the paging load to provide a more consistent level
of paging response across page-out requests.

Naturally there also must be enough space on the selected
page data set to service the request.  For a group (or
related) request, this means enough contiguous slots.  For a
demand paging request, there must be enough slots to hold the
pages to be paged-out.