6. DATA SOURCES › 6.4 Expanded Storage Concepts › 6.4.3 Swapping and Migration to Auxiliary Storage

6.4.3 Swapping and Migration to Auxiliary Storage

Swapping to expanded storage differs from swapping to
auxiliary storage.  When an address space is swapped out to
expanded storage, the RSM and the System Resources Manager
(SRM) divide the working set pages into primary and secondary
working set groups.  The primary group, which is managed by
the RSM as a single entity, contains all LSQA and fixed
pages, plus one page from each virtual storage segment
included in the whole working set.  The secondary group
comprises all other working set pages not included in the
primary group.

Once the address space has been swapped out to expanded
storage, the RSM recognizes the difference between the
primary and secondary working sets when it makes its
decisions about migration.  Secondary pages are migrated
first, and only after all secondary pages have been migrated
will the primary working set be migrated.

If the address space is swapped back in before it has been
migrated, the primary working set is swapped in first, at
which time the address space becomes dispatchable.  Secondary
working set pages are then treated the same as all other
pages in expanded storage.  This means that if they are
migrated, they will be sent to local page data sets rather
than swap data sets.  If you do a substantial amount of
swapping to expanded storage, this could be an argument in
favor of an auxiliary storage subsystem configuration
consisting only of local page data sets with no swap data
sets.