Re: pg_rewind: Doc update for PostgreSQL 18

On Mon, Jun 09, 2025 at 01:38:09PM -0400, Jesper Pedersen wrote:
> <application>pg_rewind</application> requires that the target server either has
> the <xref linkend="guc-wal-log-hints"/> option enabled
> in <filename>postgresql.conf</filename> or data checksums enabled when
> - the cluster was initialized with <application>initdb</application>. Neither of these
> - are currently on by default. <xref linkend="guc-full-page-writes"/>
> + the cluster was initialized with <application>initdb</application>. Data checksums
> + are enabled by default starting from PostgreSQL 18. <xref linkend="guc-full-page-writes"/>
> must also be set to <literal>on</literal>, but is enabled by default.

+1 for fixing this, but I'd like to bikeshed on the wording a bit. I think
the above proposal loses information since it doesn't mention that
wal_log_hints is off by default. But it might be tricky to work in all
those details in a readable fashion. How about we change it to something
like this:

pg_rewind requires that the target server has full_page_writes enabled
and at least one of either wal_log_hints or data checksums (see Section
28.2) enabled. full_page_writes and data checksums are enabled by
default, but wal_log_hints is not.

I think we ordinarily try to avoid too many version notes in the docs, and
I see that initdb makes no mention of the fact that the default for data
checksums changed in v18, so my first instinct is to leave that out.

--
nathan