Getting rid of wal_level=archive and default to hot_standby + wal_senders

Hi,

I think these days there's no reason for the split between the archive
and hot_standby wal levels. The split was made out of volume and
stability concerns. I think we can by now be confident about the
wal_level = hot_standby changes (note I'm not proposing hot_standby =
on).

So let's remove the split. It just gives users choice between two options
that don't have a meaningful difference.

Additionally I think we should change the default for wal_level to
hot_standby and max_wal_senders (maybe to 5). That way users can use
pg_basebackup and setup streaming standbys without having to restart the
primary. I think that'd be a important step in making setup easier.

Previously there have been arguments against changing the default of
wal_level because it'd mean the regression tests wouldn't exercise
minimal anymore. That might be true, but then right now we just don't
exercise the more complex levels. If we're really concerned we can just
force a different value during the tests, just as we do for prepared
xacts.

Comments?

Additionally, more complex and further into the future, I wonder if we
couldn't also get rid of wal_level = logical by automatically switching
to it whenever logical slots are active.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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