Re: Synchronizing slots from primary to standby

Hi,

On Fri, Feb 16, 2024 at 01:12:31PM +0530, Amit Kapila wrote:
> On Fri, Feb 16, 2024 at 11:43 AM Amit Kapila <amit.kapila16@gmail.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.

Yeah, catalog_xmin was the other suspect (with restart_lsn) and agree it is the
culprit here.

> 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.

Makes sense to me.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com