Flash Express Allocation In Z/Os Lpars - IBM z13s Technical Manual

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.
Storage Class
Memory (SCM)
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
Pageable link pack area (PLPA)
Virtual input/output (VIO)
IBM HyperSwap Critical Address
Space data
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IBM z13s Technical Guide
z/OS z/OS
SCM
SPACE
Main
Memory Memory
Partition
Maximum
LP1 LP2
Data transfer between
Main Memory and
Storage Class Memory is
via EADMF (4KB or
1MB blocks)
Partition
Initial
Value
Storage Class
Memory (SCM)
SCM
SPACE
Partition
Main
Maximum
Partition
Initial
Value
Data page placement
At IPL/NIP time, PLPA pages are placed both on flash and
disk.
VIO data is always placed on disk (first to VIO accepting data
sets, with any spillover flowing to non-VIO data sets).
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.