IT34081: DDT TABLE GROWING AGAIN AFTER MASS EVICTION EVEN WITH FIX FOR IT32392

APAR status

• Closed as program error.

Error description

• With APAR IT32392, the mass eviction process was updated to
  ensure the parameter used to control the vSnap Deduplication
  Table (DDT) size was kept active after reboot even if the DDT
  monitored internal threshold value was slightly above the hard
  coded value.
  It has been seen that the tolerance currently
  coded is too small and that, on some occasions, the DDT size
  control parameter again was disabled unexpectedly after vSnap
  reboot.
  
  IBM Spectrum Protect Plus Versions Affected:
  IBM
  Spectrum Protect Plus 10.1.5 e-fix4 and above
  IBM Spectrum
  Protect Plus 10.1.6
  
  | MDVPARTL 10.1.5 e-fix4 TIV_5737SPLUS |
  IT32392
  | MDVPARTL 10.1.6 TIV_5737SPLUS | IT32392
  
  Initial
  Impact: Medium
  
  Additional Keywords: SPP, SPPLUS, TS003935044,
  mass eviction, DDT, dedup
  

Local fix

• Until the fix is available, contact IBM Support to proceed with
  the mass eviction process.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * IBM Spectrum Protect Plus level 10.1.5 and 10.1.6.           *
  ****************************************************************
  * 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     *
  * 10.1.7. Note that this is subject to change at the           *
  * discretion of IBM.                                           *
  ****************************************************************
  

Problem conclusion

• Starting with the fixes introduced in IT31918, vSnap uses a
  kernel module parameter to control the amount of unique entries
  in the DDT. The parameter is set dynamically on startup and it
  sets a threshold on the amount of unique entries. In the normal
  course of operations as new writes occur, the number of entries
  can grow slightly over this threshold. A background task is
  responsible for monitoring the growth and pruning entries to
  bring the number back to the threshold value.
  
  Under abnormal conditions (e.g. when the vSnap server is
  experiencing I/O hangs due to issues described in IT31841 or
  IT34064), the background task is not able to prune unique
  entries, so the number grows beyond the threshold. The next time
  the vSnap is rebooted, it checks the current number of unique
  entries. If the current number is greater than [threshold +
  additional headroom], vSnap deliberately disables the threshold.
  This is by design, because if the headroom is too large, then a
  large amount of entries would need to be pruned immediately
  which would result in further slowdowns. Instead, the solution
  in this case is to manually perform a mass eviction of unique
  entries at a later convenient time.
  
  If the headroom is too small, the threshold gets disabled even
  when the number of unique entries is only slightly above the
  threshold.
  
  To resolve this issue, the default value of the headroom has
  been increased. The value of the headroom is also configurable
  via a vSnap preference named ddtUniqueMaxHeadroom.
  

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