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IT19471: EXCESSIVE SHARED MEMORY ALLOCATION FOR FMP MEMORY LEADS TO UNEXPECTED MEMORY ALLOCATION FAILURES, LARGE COREDUMPS

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You can track all active APARs for this component.

 

APAR status

  • Closed as program error.

Error description

  • While the excess memory will not be referenced and not backed by
    memory or paging space/swap during normal operations, the
    virtual allocation amount may still cause some problematic
    symptoms.
    
    1. unexpected memory allocation errors on Linux if the shmall
    limit is reached (maximum allowed virtual shared memory
    allocation on the system).  This is more likely on
    multi-instance systems where the INSTANCE_MEMORY setting is
    AUTOMATIC (each instance will create an FMP memory segment
    roughly 50% * RAM)
    2. excessive coredump file sizes
    3. on AIX, if there is a DB2 crash/trap, the large virtual
    allocation becomes committed as a coredump is written out.  This
    may lead to a much longer outage due to paging, and eventually a
    system-level hang or crash as paging space becomes exhausted.
    
    Note there is a distinct problem symptom documented in APAR
    IT19442 that is also caused by the large FMP shared memory
    allocation.  Restricting the allocation size as per the Local
    Fix will also address that problem symptom.
    

Local fix

  • Set the FMP memory segment size to the original 256MB amount
       db2set DB2NTMEMSIZE=APLD:268435456
    This requires recycling the instance (db2stop force; db2start)
    
    While the documentation indicates that DB2NTMEMSIZE is valid
    only for the Windows platform, it is also effective for UNIX
    platforms on the DB2 level vulnerable to this APAR (DB2 v11.1
    mod 1 fix pack 1).  Its use is not generally supported on UNIX
    platforms, and the setting should be removed upon upgrading to a
    level containing the APAR fix.
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED:                                              *
    * All users are affected                                       *
    ****************************************************************
    * PROBLEM DESCRIPTION:                                         *
    * See Error Description                                        *
    ****************************************************************
    * RECOMMENDATION:                                              *
    * use workaround in "Local Fix"                                *
    ****************************************************************
    

Problem conclusion

  • Projected: Problem first fixed in DB2 Version 11.1 mod 2 fix
    pack 1
    

Temporary fix

  • see Local Fix
    

Comments

APAR Information

  • APAR number

    IT19471

  • Reported component name

    DB2 FOR LUW

  • Reported component ID

    DB2FORLUW

  • Reported release

    B10

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2017-02-28

  • Closed date

    2017-06-23

  • Last modified date

    2017-06-23

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

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

Fix information

  • Fixed component name

    DB2 FOR LUW

  • Fixed component ID

    DB2FORLUW

Applicable component levels

[{"Business Unit":{"code":"BU053","label":"Cloud & Data Platform"},"Product":{"code":"SSEPGG","label":"Db2 for Linux, UNIX and Windows"},"Platform":[{"code":"PF025","label":"Platform Independent"}],"Version":"11.1","Line of Business":{"code":"LOB10","label":"Data and AI"}}]

Document Information

Modified date:
29 June 2020