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From: Fabrice Chapuis <[email protected]>
To: Christoph Moench-Tegeder <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: wal recycling problem
Date: Tue, 17 Oct 2023 12:37:05 +0200
Message-ID: <CAA5-nLAaVhGo3XWiUvOP9FANF8PaSiVZiUP9FyAuptuMBfvaNg@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <CAA5-nLCrbc90DHbX_9Yav72hEXUetkZ9iO8zmKvCvYNZkDHx9g@mail.gmail.com>
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	<[email protected]>

Thanks for your feedback
> How would you know which part of WAL is needed for any specific
replication slot?
change are captured for each published table and written twice,  once in
the current wal and once in the slot-specific wal
> How would you handle multiple replications
for the same table
added information about from which publication a table belongs is entered
in the wal slot
> be it logical or physical replication, retains WAL up to
max_slot_wal_keep_size
ok but if max_slot_wal_keep_size is exceeded the changes are lost and all
of the replicated tables must be resynchronized

Regards

Fabrice

On Sun, Oct 8, 2023 at 3:57 PM Christoph Moench-Tegeder <[email protected]>
wrote:

> ## Fabrice Chapuis ([email protected]):
>
> > From a conceptual point of view I think that specific wals per
> subscription
> > should be used and stored in the pg_replslot folder in order to avoid
> > working directly on the wals of the instance.
> > What do you think about this proposal?
>
> I think that would open a wholly new can of worms.
> The most obvious point here is: that WAL is primarily generated for
> the operation of the database itself - it's our kind of transaction
> log, or "Redo Log" in other systems' lingo. Replication (be it physical
> or logical) is a secondary purpose (an obvious and important one, but
> still secondary).
> How would you know which part of WAL is needed for any specific
> replication slot? You'd have to decode and filter it, and already
> you're back at square one. How would you handle multiple replications
> for the same table (in the same publication, or even over multiple
> (overlapping) publications) - do you multiply the WAL?
>
> For now, we have "any replication using replication slots, be it logical
> or physical replication, retains WAL up to max_slot_wal_keep_size
> (or "unlimited" if not set - and on PostgreSQL 12 and before); and you
> need to monitor the state of your replication slots", which is a
> totally usabe rule, I think.
>
> Regards,
> Christoph
>
> --
> Spare Space
>

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