IT32392: DDT TABLE GROWING AGAIN AFTER MASS EVICTION

APAR status

• Closed as program error.

Error description

• After completing mass eviction on a vSnap storage pool, steady
  state eviction is enabled automatically causing entries with
  refcnt=1 being evicted.
  This is done by setting parameter "zfs_unique_ddt_max" to
  1073741824 after mass eviction completed.
  
  If a vSnap is rebooted when the existing refcnt=1 entries were a
  few bytes above the threshold, the zfs_unique_ddt_max is reset
  to 0, disabling steady state eviction.
  This will cause the DDT table to grow again.
  
  Note: Mass eviction was introduced in Spectrum Protect Plus
  10.1.5 patch1
  
  | MDVPARTL 10.1.5.1-TIV_5737SPLUS |  IT31918
  
  IBM Spectrum Protect Plus Versions Affected:
  IBM Spectrum Protect Plus 10.1.5 patch1
  
  Initial Impact: Medium
  
  Additional Keywords: SPP, SPPlus, TS003444940
  

Local fix

• Perform mass eviction again to remove all entries with refcnt=1
  from DDT table and reset "zfs_unique_ddt_max" to 1073741824.
  Contact support for assistance.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * IBM Spectrum Protect Plus level 10.1.5 patch1                *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * See Error Description                                        *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply fixing level when available. This problem is currently *
  * projected to be fixed in IBM Spectrum Protect Plus level     *
  * 10.1.5.2204 and 10.1.6. Note that this is subject to change  *
  * at the discretion of IBM.                                    *
  ****************************************************************
  

Problem conclusion

• As part of the changes made to address APAR IT31918, the mass
  eviction process was introduced in SPP 10.1.5 patch1 as a way to
  perform a one-time optimization of the vSnap DDT to reduce its
  size. After a mass eviction has been performed, vSnap
  dynamically sets a parameter during startup to control the size
  of the DDT and prevent it from growing again beyond a
  pre-defined threshold. The parameter is supposed to be disabled
  if the existing DDT size is above the threshold (i.e. mass
  eviction has not yet been performed), and is supposed to be
  enabled once the DDT size is at or below the threshold (i.e.
  mass eviction has been performed).
  
  Due to a bug in the startup logic, if the vSnap was restarted at
  a moment when the size of the DDT is exactly at the threshold or
  just above the threshold by a factor of a few hundred bytes, the
  parameter was disabled during the next startup. This causes the
  DDT to start growing again.
  
  The problem has been resolved by fixing the bug in the startup
  logic to ensure the parameter is set correctly.
  

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:

Fix information

• Fixed component name

  SP PLUS

• Fixed component ID

  5737SPLUS

Applicable component levels