Horton

Re: Synchronizing slots from primary to standby

shveta malik <shveta.malik@gmail.com>

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From: shveta malik <shveta.malik@gmail.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Amit Kapila <amit.kapila16@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>, shveta malik <shveta.malik@gmail.com>, Ajin Cherian <itsajin@gmail.com>
Date: 2023-08-14T09:52:24Z
Lists: pgsql-hackers 
On Tue, Aug 8, 2023 at 11:11 AM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> Hi,
>
> On 8/8/23 7:01 AM, shveta malik wrote:
> > On Mon, Aug 7, 2023 at 3:17 PM Drouvot, Bertrand
> > <bertranddrouvot.pg@gmail.com> wrote:
> >>
> >> Hi,
> >>
> >> On 8/4/23 1:32 PM, shveta malik wrote:
> >>> On Fri, Aug 4, 2023 at 2:44 PM Drouvot, Bertrand
> >>> <bertranddrouvot.pg@gmail.com> wrote:
> >>>> On 7/28/23 4:39 PM, Bharath Rupireddy wrote:
> >>
> >
> > Agreed. That is why in v10,v11 patches, we have different infra for
> > sync-slot worker i.e. it is not relying on "logical replication
> > background worker" anymore.
>
> yeah saw that, looks like the right way to go to me.
>
> >> Maybe we should start some tests/benchmark with only one sync worker to get numbers
> >> and start from there?
> >
> > Yes, we can do that performance testing to figure out the difference
> > between the two modes. I will try to get some statistics on this.
> >
>
> Great, thanks!
>

We (myself and Ajin) performed the tests to compute the lag in standby
slots as compared to primary slots with different number of slot-sync
workers configured.

3 DBs were created, each with 30 tables and each table having one
logical-pub/sub configured. So this made a total of 90 logical
replication slots to be synced. Then the workload was run for aprox 10
mins. During this workload, at regular intervals, primary and standby
slots' lsns were captured (from pg_replication_slots) and compared. At
each capture, the intent was to know how much is each standby's slot
lagging behind corresponding primary's slot by taking the distance
between confirmed_flush_lsn of primary and standby slot. Then we took
the average (integer value) of this distance over the span of 10 min
workload and this is what we got:

With max_slot_sync_workers=1, average-lag =  42290.3563
With max_slot_sync_workers=2, average-lag =  24585.1421
With max_slot_sync_workers=3, average-lag =  14964.9215

This shows that more workers have better chances to keep logical
replication slots in sync for this case.

Another statistics if it interests you is, we ran a frequency test as
well (this by changing code, unit test sort of) to figure out the
'total number of times synchronization done' with different number of
sync-slots workers configured. Same 3 DBs setup with each DB having 30
logical replication slots. With 'max_slot_sync_workers' set at 1, 2
and 3; total number of times synchronization done was 15874, 20205 and
23414 respectively. Note: this is not on the same machine where we
captured lsn-gap data, it is on  a little less efficient machine but
gives almost the same picture.

Next we are planning to capture this data for a lesser number of slots
like 10,30,50 etc. It may happen that the benefit of multi-workers
over single workers in such cases could be less, but let's have the
data to verify that.

Thanks Ajin for jointly working on this.

thanks
Shveta

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).