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: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Masahiko Sawada <sawada.mshk@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: 2024-04-01T11:34:53Z
Lists: pgsql-hackers 
On Mon, Apr 1, 2024 at 2:51 PM Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> Hi,
>
> On Mon, Apr 01, 2024 at 06:05:34AM +0000, Zhijie Hou (Fujitsu) wrote:
> > On Monday, April 1, 2024 8:56 AM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com> wrote:
> > Attach the V4 patch which includes the optimization to skip the decoding if
> > the snapshot at the syncing restart_lsn is already serialized. It can avoid most
> > of the duplicate decoding in my test, and I am doing some more tests locally.
> >
>
> Thanks!
>
> 1 ===
>
> Same comment as in [1].
>
> In LogicalSlotAdvanceAndCheckReadynessForDecoding(), if we are synchronizing slots
> then I think that we can skip:
>
> +               /*
> +                * Wait for specified streaming replication standby servers (if any)
> +                * to confirm receipt of WAL up to moveto lsn.
> +                */
> +               WaitForStandbyConfirmation(moveto);
>
> Indeed if we are dealing with synced slot then we know we're in RecoveryInProgress().
>
> Then there is no need to call WaitForStandbyConfirmation() as it could go until
> the RecoveryInProgress() in StandbySlotsHaveCaughtup() for nothing (as we already
> know it).
>

Won't it will normally return from the first check in
WaitForStandbyConfirmation() because standby_slot_names_config is not
set on standby?

> 2 ===
>
> +       {
> +               if (SnapBuildSnapshotExists(remote_slot->restart_lsn))
> +               {
>
> That could call SnapBuildSnapshotExists() multiple times for the same
> "restart_lsn" (for example in case of multiple remote slots to sync).
>
> What if the sync worker records the last lsn it asks for serialization (and
> serialized ? Then we could check that value first before deciding to call (or not)
> SnapBuildSnapshotExists() on it?
>
> It's not ideal because it would record "only the last one" but that would be
> simple enough for now (currently there is only one sync worker so that scenario
> is likely to happen).
>

Yeah, we could do that but I am not sure how much it can help. I guess
we could do some tests to see if it helps.

-- 
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).