RE: Synchronizing slots from primary to standby

On Friday, February 16, 2024 3:43 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Feb 16, 2024 at 11:43 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)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