OA46475: ADDITIONAL STORAGE-CLASS MEMORY AND PAGESCM GUIDELINES

APAR status

• Closed as documentation error.

Error description

• Additional guidelines and considerations on usage of
  Storage-Class Memory (SCM) for auxiliary storage including the
  PAGESCM parameter should be added to the z/OS MVS Initialization
  and Tuning Guide
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: Users of JBB778H and up.                     *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  The MVS Initialization and Tuning Guide should provide some
  guidelines on use of the PAGESCM IEASYSxx parameter.
  

Problem conclusion

• The MVS Initialization and Tuning Guide will be updated
  with considerations for the PAGESCM IEASYSxx parmlib
  parameter.
  
   Documentation Changes
  =======================
  
  The information in the z/OS MVS Initialization and Tuning Guide
  will be updated as indicated below. Updates will only be made
  to the publication in a future release.
  
  Publication Title - z/OS MVS Initialization and Tuning Guide
  Location - Chapter 2 - Auxiliary Storage Management
  Initialization
  
  New/changed text is noted below with a change bar '|'
  
   *-----------------------------------------------------*
  
  Update 1 - At the beginning of the section titled
  'Paging operations and algorithms for storage-class memory
  (SCM)', add a sentence to clarify the hardware environments
  to which SCM usage apply:
  
   Paging operations and algorithms for storage-class memory
   (SCM)
  
   | On IBM zEnterprise  EC12 (zEC12), BC12 (zBC12) and later
   | processors, storage-class memory (SCM) implemented through
   | the optional Flash Express feature may be used for
   | auxiliary storage in conjunction with page data sets.
  
     Once page data sets on DASD are selected by ASM as the
     preferred storage medium, all of the factors in
     "Paging operations and algorithms" on page 59 remain
     applicable. However, if ASM selects SCM as the preferred
     storage medium, then additional operations and algorithms
     apply.
  
   *-----------------------------------------------------*
  
  Update 2 - At the end of the section titled
  'Paging operations and algorithms for storage-class memory
  (SCM)', add the following PAGESCM considerations:
  
   | When using or planning to use SCM, the PAGESCM IEASYSxx
   | parameter specifies the amount of SCM that is to be reserved
   | for auxiliary storage.
   |
   | The following are some items to consider when setting the
   | PAGESCM value.
   |
   | PAGESCM=ALL (default) or some value other than NONE:
   |
   |  - Should be specified if SCM is available and will be used
   |    for auxiliary storage.  If SCM is not currently available
   |    but is planned to be used at some time during this IPL,
   |    specifying a PAGESCM value allows SCM to be subsequently
   |    brought online and used for auxiliary storage without the
   |    need for an IPL.
   |
   |  - The pageable large page area (PLAREA) is defined
   |    regardless of whether SCM is installed. The PLAREA is a
   |    system-defined area of real storage from which 1M pageable
   |    large pages are allocated. Pageable large pages are
   |    requested via the IARV64, DSPSERV, CPOOL and
   |    STORAGE OBTAIN services. The PLAREA is also used to
   |    fulfill 4K page requests in cases where there's a
   |    shortage of 4K real storage frames.
   |
   |    If SCM is not in use and the need arises to page-out
   |    a pageable large page, the page will be paged-out as
   |    256 4K pages thus losing some of benefits provided by
   |    the large page.
   |
   |  - When specifying a value other than NONE, it is recommended
   |    to specify ALL (or take the default of ALL) since
   |    auxiliary storage is currently the only exploiter of SCM
   |    in z/OS. This value will need to be reexamined if other
   |    exploiters are introduced.
   |
   | PAGESCM=NONE:
   |
   |  - Indicates that SCM will not be used during this IPL.
   |    Subsequent use of SCM will require an IPL with a PAGESCM
   |    value other than none.
   |
   |  - The pageable large page area is not defined. Requests for
   |    pageable large pages will be fulfilled with 4K pages.
  

Temporary fix

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