534 IBM z13s Technical Guide
At IPL, z/OS detects whether flash is assigned to the partition. z/OS automatically uses Flash
Express for paging unless otherwise specified by using the new z/OS PAGESCM=NONE
parameter. All paging data can be on Flash Express memory. The function is easy to use, and
there is no need for capacity planning or placement of data on Flash Express cards.
Figure H-5 gives an example of Flash Express allocation between two z/OS LPARs.
Figure H-5 Flash Express allocation in z/OS LPARs
Flash Express memory is a faster paging device than HDD. It replaces disks, not memory. It is
suitable for workloads that can tolerate paging. It does not benefit workloads that cannot
afford to page. The z/OS design for Flash Express memory does not completely remove the
virtual constraints that are created by a paging spike in the system. The z/OS paging
subsystem works with a mix of internal Flash Express and external disks.
Currently, 1 MB large pages are not pageable. With the introduction of Flash Express, 1 MB
large pages can be on Flash Express and pageable.
Table H-2 introduces, for a few z/OS data types that are supported by Flash Express, the
choice criteria for data placement on Flash Express or on disk.
Table H-2 Flash Express z/OS supported data types
Data type Data page placement
Pageable link pack area (PLPA) At IPL/NIP time, PLPA pages are placed both on flash and
disk.
Virtual input/output (VIO) VIO data is always placed on disk (first to VIO accepting data
sets, with any spillover flowing to non-VIO data sets).
IBM HyperSwap Critical Address
Space data
If flash space is available, all virtual pages that belong to a
HyperSwap Critical Address Space are placed in flash
memory.
If flash space is not available, these pages are kept in memory
and only paged to disk when the system is real storage
constrained, and no other alternatives exist.
z/OS z/OS
Main
Memory
SCM
SPACE
Data transfer between
Main Memory and
Storage Class Memory i s
via EADMF (4KB or
1MB blocks)
Main
Memory
Storag e Class
Memory (SCM)
Partition
Maximum
Partition
Initial
Value
Partition
Initial
Va lu e
Partition
Maximum
LP2LP1
Storag e Class
Memory (SCM)
SCM
SPACE
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