Re: recovery_command has precedence over phisical slots?

At Fri, 19 Aug 2022 18:37:53 +0200, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote in
> On Fri, 2022-08-19 at 16:54 +0200, Giovanni Biscontini wrote:
> > Hello everyone, 
> > I'm experiencing a behaviour I don't really understand if is a misconfiguration or a wanted behaviour:
> > 1) I set up a primary server (a.k.a. db1) with and archive_command to a storage
> > 2) I set up a replica (a.k.a. db2) that created a slot named as slot_2 and that has the recovery_command set to read archived wal on the storage.
> > If I shutdown replica db2 during a pgbench I see the safe_wal_size queried from pg_replication_slots on the primary decrease to a certain amount but still in the max_slot_wal_kepp_size window: even
> > if I restart the replica db2 before the slot_state changes to unreserved or lost I see that the replica gets needed wals from the storage using recovery_command but doesn't use slot on primary.
> > Only if I comment the recovery command on the .conf of the replica then it uses slot.
> > If this is a wanted behaviour I can't understand the need of slots on primary.
>
> This is normal behavior and is no problem.
>
> After the standby has caught up using "restore_command", it will connection to
> the primary as defined in "primary_conninfo" and stream WAL from there.

The reason that db2 ran recovery beyond the slot LSN is the db2's
restore_command (I guess) points to db1's archive. If db2 had its own
archive directory or no archive (that is, restore_command is empty),
archive recovery stops at (approximately) the slot LSN and replication
will start from there (from the beginning of the segment, to be
exact).

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center