Subject: Re: Bump default wal_level to logical
To: Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>
References: <20200608171619.GA11063@alvherre.pgsql>
 <2021525.1591637270@sss.pgh.pa.us>
 <20200608213215.mgk3cctlzvfuaqm6@alap3.anarazel.de>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
 Tomas Vondra <tomas.vondra@2ndquadrant.com>,
 Magnus Hagander <magnus@hagander.net>, Michael Paquier
 <michael@paquier.xyz>, Amit Kapila <amit.kapila16@gmail.com>,
 David Fetter <david@fetter.org>,
 PostgreSQL Development <pgsql-hackers@postgresql.org>
From: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Organization: 2ndQuadrant
Message-ID: <977b6922-9e5c-eb6e-778d-9b19c0145b54@2ndquadrant.com>
Date: Tue, 9 Jun 2020 08:52:24 +0200
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On 2020-06-08 23:32, Andres Freund wrote:
> On 2020-06-08 13:27:50 -0400, Tom Lane wrote:
>> If we can allow wal_level to be changed on the fly, I agree that would
>> help reduce the pressure to make the default setting more expensive.
>> I don't recall why it's PGC_POSTMASTER right now, but I suppose there
>> was a reason for that ...
> 
> There's reasons, but IIRC they're all solvable with reasonable effort. I
> think most of it boils down to only being able to rely on the new
> wal_level after a while. For minimal->recovery we basically need a
> checkpoint started after the change in configuration, and for
> recovery->logical we need to wait until all sessions have a) read the
> new config setting b) finished the transaction that used the old
> setting.

The best behavior from a user's perspective would be if the WAL level 
automatically switched to logical if logical replication slots are 
present.  You might not even need 'logical' as an actual value of 
wal_level anymore, you just need to keep a flag in shared memory that 
records whether at least one logical slot exists.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services