From: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Magnus Hagander <magnus@hagander.net>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Amit Kapila <amit.kapila16@gmail.com>
Cc: David Fetter <david@fetter.org>
Cc: PostgreSQL Development <pgsql-hackers@postgresql.org>
Subject: Re: Bump default wal_level to logical
Date: Tue, 9 Jun 2020 08:52:24 +0200
Message-ID: <977b6922-9e5c-eb6e-778d-9b19c0145b54@2ndquadrant.com> (raw)
In-Reply-To: <20200608213215.mgk3cctlzvfuaqm6@alap3.anarazel.de>
References: <20200608171619.GA11063@alvherre.pgsql>
<2021525.1591637270@sss.pgh.pa.us>
<20200608213215.mgk3cctlzvfuaqm6@alap3.anarazel.de>
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
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To: pgsql-hackers@postgresql.org
Cc: peter.eisentraut@2ndquadrant.com, andres@anarazel.de, tgl@sss.pgh.pa.us, alvherre@2ndquadrant.com, tomas.vondra@2ndquadrant.com, magnus@hagander.net, michael@paquier.xyz, amit.kapila16@gmail.com, david@fetter.org
Subject: Re: Bump default wal_level to logical
In-Reply-To: <977b6922-9e5c-eb6e-778d-9b19c0145b54@2ndquadrant.com>
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