Horton

Re: Synchronizing slots from primary to standby

Ajin Cherian <itsajin@gmail.com>

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From: Ajin Cherian <itsajin@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, shveta malik <shveta.malik@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Peter Smith <smithpb2250@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: 2024-03-01T09:09:23Z
Lists: pgsql-hackers

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On Thu, Feb 29, 2024 at 12:34 PM Zhijie Hou (Fujitsu) <
houzj.fnst@fujitsu.com> wrote:

> On Monday, February 26, 2024 7:52 PM Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> >
> > On Mon, Feb 26, 2024 at 7:49 AM Zhijie Hou (Fujitsu) <
> houzj.fnst@fujitsu.com>
> > wrote:
> > >
> > > Attach the V98 patch set which addressed above comments.
> > >
> >
> > Few comments:
> > =============
> > 1.
> >  WalSndWaitForWal(XLogRecPtr loc)
> >  {
> >   int wakeEvents;
> > + bool wait_for_standby = false;
> > + uint32 wait_event;
> > + List    *standby_slots = NIL;
> >   static XLogRecPtr RecentFlushPtr = InvalidXLogRecPtr;
> >
> > + if (MyReplicationSlot->data.failover && replication_active)
> > + standby_slots = GetStandbySlotList(true);
> > +
> >   /*
> > - * Fast path to avoid acquiring the spinlock in case we already know we
> > - * have enough WAL available. This is particularly interesting if we're
> > - * far behind.
> > + * Check if all the standby servers have confirmed receipt of WAL up to
> > + * RecentFlushPtr even when we already know we have enough WAL
> available.
> > + *
> > + * Note that we cannot directly return without checking the status of
> > + * standby servers because the standby_slot_names may have changed,
> > + which
> > + * means there could be new standby slots in the list that have not yet
> > + * caught up to the RecentFlushPtr.
> >   */
> > - if (RecentFlushPtr != InvalidXLogRecPtr &&
> > - loc <= RecentFlushPtr)
> > - return RecentFlushPtr;
> > + if (!XLogRecPtrIsInvalid(RecentFlushPtr) && loc <= RecentFlushPtr) {
> > + FilterStandbySlots(RecentFlushPtr, &standby_slots);
> >
> > I think even if the slot list is not changed, we will always process
> each slot
> > mentioned in standby_slot_names once. Can't we cache the previous list of
> > slots for we have already waited for? In that case, we won't even need
> to copy
> > the list via GetStandbySlotList() unless we need to wait.
> >
> > 2.
> >  WalSndWaitForWal(XLogRecPtr loc)
> >  {
> > + /*
> > + * Update the standby slots that have not yet caught up to the flushed
> > + * position. It is good to wait up to RecentFlushPtr and then let it
> > + * send the changes to logical subscribers one by one which are
> > + * already covered in RecentFlushPtr without needing to wait on every
> > + * change for standby confirmation.
> > + */
> > + if (wait_for_standby)
> > + FilterStandbySlots(RecentFlushPtr, &standby_slots);
> > +
> >   /* Update our idea of the currently flushed position. */
> > - if (!RecoveryInProgress())
> > + else if (!RecoveryInProgress())
> >   RecentFlushPtr = GetFlushRecPtr(NULL);
> >   else
> >   RecentFlushPtr = GetXLogReplayRecPtr(NULL); ...
> > /*
> > * If postmaster asked us to stop, don't wait anymore.
> > *
> > * It's important to do this check after the recomputation of
> > * RecentFlushPtr, so we can send all remaining data before shutting
> > * down.
> > */
> > if (got_STOPPING)
> > break;
> >
> > I think because 'wait_for_standby' may not be set in the first or
> consecutive
> > cycles we may send the WAL to the logical subscriber before sending it
> to the
> > physical subscriber during shutdown.
>
> Here is the v101 patch set which addressed above comments.
>
> This version will cache the oldest standby slot's LSN each time we waited
> for
> them to catch up. The cached LSN is invalidated when we reload the GUC
> config.
> In the WalSndWaitForWal function, instead of traversing the entire standby
> list
> each time, we can check the cached LSN to quickly determine if the standbys
> have caught up. When a shutdown signal is received, we continue to wait
> for the
> standby slots to catch up. When waiting for the standbys to catch up after
> receiving the shutdown signal, an ERROR is reported if any slots are
> dropped,
> invalidated, or inactive. This measure is taken to prevent the walsender
> from
> waiting indefinitely.
>
>
Thanks for the patch.
I did some performance test run on PATCH v101 with synchronous_commit
turned on to check how much logical replication changes affects transaction
speed on primary compared to HEAD code. In all configurations, there is a
primary, a logical subscriber and a physical standby and the logical
subscriber is listed in the synchronous_standby_names. This means all
transactions on primary will not be committed until the logical subscriber
has confirmed receipt of this transaction.


Machine details:
Intel(R) Xeon(R) CPU E7-4890 v2 @ 2.80GHz, 800GB RAM

My addition configuration on each instance is:
shared_buffers = 40GB
max_worker_processes = 32
max_parallel_maintenance_workers = 24
max_parallel_workers = 32
synchronous_commit = off
checkpoint_timeout = 1d
max_wal_size = 24GB
min_wal_size = 15GB
autovacuum = off

All tests are done using pgbench running for 15 minutes:
Creating tables: pgbench -p 6972 postgres -qis 2
Running benchmark: pgbench postgres -p 6972 -c 10 -j 3 -T 900 -P 5

HEAD code-
 Primary with Synchronous_commit=on, physical standby with
hot_standby_feedback=off
RUN1 (TPS) RUN2 (TPS) AVERAGE (TPS)
8.226658 8.17815 8.202404

HEAD code-
Primary with Synchronous_commit=on, physical standby with
hot_standby_feedback=on
RUN1 (TPS) RUN2 (TPS) AVERAGE (TPS)
8.134901 8.229066 8.1819835 -- degradation from first config -0.25%

PATCHED code - (v101-0001)
Primary with synchronous_commit=on, physical standby with
hot_standby_feedback=on, standby_slot_names not configured, logical
subscriber not failover enabled, physical standby not configured for sync
RUN1 (TPS) RUN2 (TPS) AVERAGE (TPS)
8.18839 8.18839 8.18839-- degradation from first config *-0.17%*

PATCHED code - (v98-0001)
Synchronous_commit=on, hot_standby_feedback=on, standby_slot_names
configured to physical standby, logical subscriber failover enabled,
physical standby configured for sync
RUN1 (TPS) RUN2 (TPS) AVERAGE (TPS)
8.173062 8.068536 8.120799-- degradation from first config* -0.99%*

Overall, I do not see any significant performance degradation with the
patch and sync-slot enabled with one logical subscriber and one physical
standby.
Attaching script for my final test configuration for reference.

regards,
Ajin Cherian
Fujitsu Australia

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).