PH61574: IMS LOG READER OLDS LOG SWITCH DETECTION UPDATES AND ENHANCED LSN VALIDATION DURING STREAM ACTIVATION

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• IMS log reader OLDS log switch detection updates and enhanced
  LSN validation during stream activation
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: Classic CDC for z/OS for IMS                 *
  *                 IIDR for IMS                                 *
  ****************************************************************
  * PROBLEM DESCRIPTION: IMS log reader OLDS log switch          *
  *                      detection updates and enhanced LSN      *
  *                      validation during stream activation     *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  This APAR includes updates to address two different IMS log
  reader issues.
  The first is to add additional initial log selection validation
  during stream activation processing that confirms the contents
  of the IMS log selected match what is recorded in DBRC. This is
  done by verifying that the LSN of the first record in the log
  matches the FIRST LSN information recorded for that log in DBRC
  .
  This additional validation prevents unexpected IMS log reader
  behavior for the following conditions:
    An OLDS is selected, the OLDS contents have been
  re-initialized, and the IMS log reader needs to wait for IMS to
  write data to the log. The IMS log reader does not support this
  wait logic during stream activation processing.
   An active OLDS is selected after IMS has performed a log
  switch, but before a new log block has been written to the log.
  Today IMS log reader, reads the prior OLDS contents, discards
  all the log records, and then processes the next log based on
  the last LSN read, until it reaches the current OLDS.
    A SLDS has been selected, there is a problem with archiving
  and the SLDS is recorded in the RECON multiple times, and the
  selected logs contents do not match the selected DBRC entry.
  The IMS log reader currently determines what to do based on the
  first block in the log which may cause unexpected behavior.
  Additional changes have been made to do a better job of
  handling when the PABLKCNT value wraps which can occur in high
  volume IMS control regions that are not reset by doing a cold
  start. Evey block in an IMS log contains a padding record as
  the last record in each block. In a DB/DC or DB-CTL record the
  padding record contains a PABLKCNT value that is set to one
  when an IMS cold start is performed and is incremented by one
  for each subsequent block written to the active OLDS.
  The PABLKCNT is 32-bit integer and if the IMS writes enough
  blocks before another cold start is performed, the PABLKCNT
  resets to zero and then continues to increment by 1.
  The APAR contains updates to perform a better job of
  identifying the correct SLDS to process when the IMS log reader
  is accessing an OLDS where the PABLKCNT is about to wrap,
  log read operations are suspended for a period, and the OLDS is
  over-written with new data. This condition only occurs during
  heavy batch workloads where the capture cache fills for one or
  more subscriptions and the OLDS that is being processed is
  close to being over-written by IMS when the IMS log reader
  starts reading the OLDS.
  Currently, the IMS log reader issues a CECZ0309E (un-expected
  error condition) and reports the change stream in error which
  causes replication to stop for all subscriptions being serviced
  for the change stream.
  The IMS log reader can also run into problems when reading the
  active OLDS and a logical-end-of-log condition is detected and
  the PABLKCNT is wrapping. The IMS log reader can get stuck
  reading the wrong block of data in the OLDS and does not
  advance until the OLDS is over-written by IMS at which point
  processing switches to the appropriate SLDS.
  

Problem conclusion

• The IMS log reader stream activation logic has been modified to
  verify the first LSN in the selected log matches the
  information in DBRC. If it does not a new CECZ0317E message
  identifying the IMS subsystem, log dataset, and DBRC/log LSN
  information is issued.
  When the log LSN is zeros, this indicates that the log has
  recently been initialized and this issue can be resolved by
  doing log switches until all new logs contain at least one log
  block written by IMS.
  When the message reports a non-zero log LSN and the log is an
  OLDS then likely this is due to a log switch where IMS has not
  written new data to the selected OLDS. Issuing an IMS
  /CHECKPOINT command will resolve this situation.
  With the APAR applied, and the active OLDS is overwritten and
  the PABLKCNT resets, instead of issuing a CECZ0309E message.
  the IMS log reader will switch to processing the SLDS that
  contains the log data for the OLDS that was being processed
  when reading was halted.
  The reason the IMS log reader cannot detect new data has been
  written to the OLDS when the log is close to a PABLKCNT reset
  is still unknown, but with APAR PH61574 applied, the IMS log
  reader will detect this condition shortly after IMS switches to
  the next OLDS. It can take the IMS log reader up to thirty
  seconds to detect that IMS has switch OLDS at which point the
  OLDS that the IMS log reader was reading is closed, reopened,
  and processing resumes
  There are no new or special messages issued when one of the
  PABLKCNT wrap conditions are resolved. The diagnostic log does
  contain new trace entries indicating this code has been
  activated.
  While these changes are not expected to have unintended side
  effects, they can be disabled by updating the global TASKPARM
  to include  PH61574=OFF . This value is case sensitive. The new
  setting takes effect the next time a new change stream is
  activated or immediately when the IMS log reader is waiting for
  IMS to write more data to the active OLDS.
  

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:

  UI98689

Modules/Macros

•    CACCHICT CACENGCT CACJPNCT CACZHOCT CECCLRFU
  CECCLRIC CECCLRID CECCLRIE CECCLRIH CECCLRII CECCLRIM CECCLRIQ
  CECCLRIR CECCLRIS CECCLRIT CECCREP2
  

Fix information

• Fixed component name

  CLASSIC FED & E

• Fixed component ID

  5697I8200

Applicable component levels

• RB30 PSY UI98689

     UP24/10/09 P F410

Fix is available

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