Horton

Re: Synchronizing slots from primary to standby

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: shveta malik <shveta.malik@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@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: 2024-02-09T04:34:42Z
Lists: pgsql-hackers 
On Fri, Feb 9, 2024 at 10:00 AM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> Here is the V82 patch set which includes the following changes:
>

+reserve_wal_for_local_slot(XLogRecPtr restart_lsn)
{
...
+ /*
+ * Find the oldest existing WAL segment file.
+ *
+ * Normally, we can determine it by using the last removed segment
+ * number. However, if no WAL segment files have been removed by a
+ * checkpoint since startup, we need to search for the oldest segment
+ * file currently existing in XLOGDIR.
+ */
+ oldest_segno = XLogGetLastRemovedSegno() + 1;
+
+ if (oldest_segno == 1)
+ {
+ TimeLineID cur_timeline;
+
+ GetWalRcvFlushRecPtr(NULL, &cur_timeline);
+ oldest_segno = XLogGetOldestSegno(cur_timeline);
...
...

This means that if the restart_lsn of the slot is from the prior
timeline then the standby needs to wait for longer times to sync the
slot. Ideally, it should be okay because I don't think even if
restart_lsn of the slot may be from some prior timeline than the
current flush timeline on standby, how often that case can happen?
OTOH, in the prior version patch(v80_2-0001*), we search for the
oldest segment in all possible timelines via code like:
+reserve_wal_for_local_slot(XLogRecPtr restart_lsn)
{
...
+ */
+ oldest_segno = XLogGetLastRemovedSegno() + 1;
+
+ if (oldest_segno == 1)
+ oldest_segno = XLogGetOldestSegno(0);

I don't see a problem either way as in both scenarios this is a very
rare case and doesn't seem to cause any problem but would like to know
the opinion of others.

-- 
With Regards,
Amit Kapila.

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).