Horton

RE: Synchronizing slots from primary to standby

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Bruce Momjian <bruce@momjian.us>, Ashutosh Sharma <ashu.coek88@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-02-16T08:22:20Z
Lists: pgsql-hackers

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On Friday, February 16, 2024 3:43 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> On Fri, Feb 16, 2024 at 11:43 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > Thanks for noticing this. I have pushed all your debug patches. Let's
> > hope if there is a BF failure next time, we can gather enough
> > information to know the reason of the same.
> >
> 
> There is a new BF failure [1] after adding these LOGs and I think I know what is
> going wrong. First, let's look at standby LOGs:
> 
> 2024-02-16 06:18:18.442 UTC [241414][client backend][2/14:0] DEBUG:
> segno: 4 of purposed restart_lsn for the synced slot, oldest_segno: 4 available
> 2024-02-16 06:18:18.443 UTC [241414][client backend][2/14:0] DEBUG:
> xmin required by slots: data 0, catalog 741
> 2024-02-16 06:18:18.443 UTC [241414][client backend][2/14:0] LOG:mote  could
> not sync slot information as reslot precedes local slot: remote slot "lsub1_slot":
> LSN (0/4000168), catalog xmin (739) local slot: LSN (0/4000168), catalog xmin
> (741)
> 
> So, from the above LOG, it is clear that the remote slot's catalog xmin (739)
> precedes the local catalog xmin (741) which makes the sync on standby to not
> complete.
> 
> Next, let's look at the LOG from the primary during the nearby time:
> 2024-02-16 06:18:11.354 UTC [238037][autovacuum worker][5/17:0] DEBUG:
>  analyzing "pg_catalog.pg_depend"
> 2024-02-16 06:18:11.360 UTC [238037][autovacuum worker][5/17:0] DEBUG:
>  "pg_depend": scanned 13 of 13 pages, containing 1709 live rows and 0 dead
> rows; 1709 rows in sample, 1709 estimated total rows ...
> 2024-02-16 06:18:11.372 UTC [238037][autovacuum worker][5/0:0] DEBUG:
> Autovacuum VacuumUpdateCosts(db=1, rel=14050, dobalance=yes,
> cost_limit=200, cost_delay=2 active=yes failsafe=no)
> 2024-02-16 06:18:11.372 UTC [238037][autovacuum worker][5/19:0] DEBUG:
>  analyzing "information_schema.sql_features"
> 2024-02-16 06:18:11.377 UTC [238037][autovacuum worker][5/19:0] DEBUG:
>  "sql_features": scanned 8 of 8 pages, containing 756 live rows and 0 dead rows;
> 756 rows in sample, 756 estimated total rows
> 
> It shows us that autovacuum worker has analyzed catalog table and for updating
> its statistics in pg_statistic table, it would have acquired a new transaction id. Now,
> after the slot creation, a new transaction id that has updated the catalog is
> generated on primary and would have been replication to standby. Due to this
> catalog_xmin of primary's slot would precede standby's catalog_xmin and we see
> this failure.
> 
> As per this theory, we should disable autovacuum on primary to avoid updates to
> catalog_xmin values.
> 

Agreed. Here is the patch to disable autovacuum in the test.

Best Regards,
Hou zj

Commits

  1. Doc: Add the new section "Logical Replication Failover".

  2. Fix the review comments and a bug in the slot sync code.

  3. Fix a test in failover slots regression test.

  4. Fix the intermittent buildfarm failures in 040_standby_failover_slots_sync.

  5. Split XLogCtl->LogwrtResult into separate struct members

  6. Ensure that the sync slots reach a consistent state after promotion without losing data.

  7. Introduce a new GUC 'standby_slot_names'.

  8. Fix BF failure introduced by commit b3f6b14cf4.

  9. Fixups for commit 93db6cbda0.

  10. Fix BF failure in commit 93db6cbda0.

  11. Add a new slot sync worker to synchronize logical slots.

  12. Improve ERROR/LOG messages added by commits ddd5f4f54a and 7a424ece48.

  13. Disable autovacuum on primary in 040_standby_failover_slots_sync test.

  14. Fix the incorrect format specifier used in commit 7a424ece48.

  15. Change the LOG level in 040_standby_failover_slots_sync.pl to DEBUG2.

  16. Add more LOG and DEBUG messages for slot synchronization.

  17. Another try to fix BF failure introduced in commit ddd5f4f54a.

  18. Fix BF introduced in commit ddd5f4f54a.

  19. Add a slot synchronization function.

  20. Clean-ups for 776621a5e4 and 7329240437.

  21. Enhance libpqrcv APIs to support slot synchronization.

  22. Add a failover option to subscriptions.

  23. Allow setting failover property in the replication command.

  24. Allow to enable failover property for replication slots via SQL API.

  25. Add macros for looping through a List without a ListCell.

  26. Track conflict_reason in pg_replication_slots.

  27. Allow upgrades to preserve the full subscription's state.

  28. Add support for incremental backup.

  29. Simplify some logic in CreateReplicationSlot()

  30. Fix uninitialized access to InitialRunningXacts during decoding after ERROR.

  31. Flexible options for CREATE_REPLICATION_SLOT.

  32. Empty search_path in logical replication apply worker and walsender.

  33. Fix oldest xmin and LSN computation across repslots after advancing

  34. Require C99 (and thus MSCV 2013 upwards).