PM99575: CHANGE THE DISCARDDATA LOGIC FOR REAL FRAMES BASED ON THE CUSTOMER SETTINGS FOR REALSTORAGE_MANAGEMENT.

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• Existing logic, though working as designed, results in different
  behaviors based on customer workload and thread control
  specifications.  Desire is to move to more uniform and
  predictable function.
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All DB2 V10 and V11 users.                   *
  ****************************************************************
  * PROBLEM DESCRIPTION: Functional change to the way 64-bit     *
  *                      REAL storage frames under DB2 storage   *
  *                      management control are handled when     *
  *                      FREE.  Eligible FREE frames will now be *
  *                      returned to z/OS RSM by using           *
  *                      KEEPREAL(YES) on the DISCARDDATA        *
  *                      under normal execution.                 *
  *                      If the user has specified zparm         *
  *                      RealStorage_MAX and DB2 reaches that    *
  *                      limit, or system experiences Auxiliary  *
  *                      storage shortage, KEEPREAL(NO) will     *
  *                      be specified until the condition is     *
  *                      relieved.                               *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  Currently, when DB2 has FREE frames eligible for discard back to
  z/OS RSM, the use of KEEPREAL(NO) tells RSM to remove ownership
  immediately.  This results in a new first reference Page Fault
  and the associated CPU cost when those pages are again used by
  DB2.  This does however, allow DB2 to provide accurate
  statistics on how much REAL storage is being used by each DB2
  for their 64-bit shared and common storage.  The current logic
  does the DISCARDDATA under the control set by the user zparm
  RealStorage_Managment as either ON, OFF, AUTO and certain
  additional system critical events such as Auxiliary storage
  shortage.  The default setting is AUTO which tells DB2 to
  perform the DISCARDDATA only if the system is PAGING.  This
  occurs only when thread pool storage is contracted as part of
  commit processing or freed when the thread is de-allocated.
  If during contraction or de-allocation, the LPAR is not paging
  when the storage is returned to DB2's large storage object, it
  is done without the REAL frames being discarded or unbacked.
  Those frames will remain backed even if paging occurs,
  until those frames are again allocated to an active thread.
  This sequestering of backed frames, can result in unnecessary
  system paging and prevent real storage reconfiguration on the
  LPAR.
  

Problem conclusion

• DB2 is changing the logic for DISCARDDATA back to KEEPREAL(YES)
  and always performing the discard unless the user has elected to
  retain all backed frames by specifying RealStorage_Management =
  OFF.  If RealStorage_Management = OFF, frames will only be
  discarded if a critical storage event is detected by DB2 storage
  monitor and DB2 goes into discard mode. This allows z/OS to
  steal those frames should that become necessary.  If the user
  has specified RealStorage_MAX and DB2 hits that level or if
  the LPAR experiences critical auxiliary storage shortage, DB2
  will change DISCARDDATA to KEEPREAL(NO).  This remains in effect
  as long as DB2 is in discard mode for those events.
  Since z/OS RSM tracks 64-bit Shared and Common only at the LPAR
  level, DB2 Real Frame statistics will only be able to provide
  numbers which are worst case. This worst case value may result
  in DB2 termination with RC00E20033 for RealStorage_MAX even
  though many of the Shared frames have been discarded with
  Keepreal(YES). For LPARs which do not page, the
  REAL frame usage values before and after PM99575, will remain
  about the same since RSM maintains the charge against DB2 for
  discarded KEEPREAL(YES) frames which have not been stolen.
  Summary of results:
  After PM88804                   After PM99575
  * Auxiliary shortage means      * Auxiliary shortage means
      DISCARDDATA KEEPREAL(NO)        DISCARDDATA KEEPREAL(NO)
  * REALSTORAGE_MANAGEMENT=OFF    * REALSTORAGE_MANAGEMENT=OFF
      No DISCARDDATA                  No DISCARDDATA
  * REALSTORAGE_MANAGEMENT=AUTO   * REALSTORAGE_MANAGEMENT=AUTO
    with no paging means            with no paging means
      No DISCARDDATA at Thread        DISCARDDATA KEEPREAL(YES)
      De-allocation or 120 Commits    at Thread De-allocation or
                                      120 commits
  * REALSTORAGE_MANAGEMENT=AUTO   * REALSTORAGE_MANAGEMENT=AUTO
    or ON with paging means         or ON with paging means
      DISCARDDATA KEEPREAL(NO) at     DISCARDDATA KEEPREAL(YES) at
      De-allocation or 30 commits     De-allocation or 30 commits
      and STACK is also discarded     and STACK is also discarded
  * REALSTORAGE_MAX specified     * REALSTORAGE_MAX specified
    means                           means
      DISCARDDATA KEEPREAL(NO) at     DISCARDDATA KEEPREAL(NO) at
      80%                             100%
  .
  If you have XCF CRITICAL PAGING ENABLED, you also need to apply
  z/OS RSM APAR OA44913. Without this apar, the KEEPREAL(YES)
  Shared frames are not stolen to replenish frame queues when
  paging occurs.
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

  UI17080 UI17081

Modules/Macros

• DSNSCON2 DSNSCTL  DSNSVSFM DSNSVSPC DSNSVSVP DSNVMON
  

Fix information

• Fixed component name

  DB2 OS/390 & Z/

• Fixed component ID

  5740XYR00

Applicable component levels

• RA10 PSY UI17080

     UP14/05/01 P F404

• RB10 PSY UI17081

     UP14/05/01 P F404

Fix is available

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