Re: Doc about how to set max_wal_senders when setting minimal wal_level - Mailing list pgsql-hackers

From Tom Lane
Subject Re: Doc about how to set max_wal_senders when setting minimal wal_level
Date
Msg-id 2040586.1657065753@sss.pgh.pa.us
Whole thread Raw
In response to Re: Doc about how to set max_wal_senders when setting minimal wal_level  (Japin Li <japinli@hotmail.com>)
Responses Re: Doc about how to set max_wal_senders when setting minimal wal_level
Re: Doc about how to set max_wal_senders when setting minimal wal_level
List pgsql-hackers
Japin Li <japinli@hotmail.com> writes:
> [ v4-wal-level-documentation.patch ]

Hm, I don't care for the wording here:

+        A precondition for using minimal WAL is to disable WAL archiving and
+        streaming replication by setting <varname>archive_mode</varname> to
+        <literal>off</literal>, and <xref linkend="guc-max-wal-senders"/> to
+        <literal>0</literal>.

"Precondition" is an overly fancy word that makes things less clear
not more so.  Does it mean that setting wal_level = minimal will fail
if you don't do these other things, or does it just mean that you
won't be getting the absolute minimum WAL volume?  If the former,
I think it'd be better to say something like "To set wal_level to minimal,
you must also set [these variables], which has the effect of disabling
both WAL archiving and streaming replication."

+        servers.  If setting <varname>max_wal_senders</varname> to
+        <literal>0</literal> consider also reducing the amount of WAL produced
+        by changing <varname>wal_level</varname> to <literal>minimal</literal>.

I don't think this is great advice.  It will encourage people to use
wal_level = minimal even if they have other requirements that weigh
against it.  If they feel that their system is producing too much
WAL, I doubt they'll have a hard time finding the wal_level knob.

TBH, I think the real problem with the docs in this area is that
the first para about wal_level is disjointed and unclear; we ought
to nuke and rewrite that.  In particular it fails to provide adequate
context about what "logical decoding" means.  I think possibly what
it needs to say is that replica mode supports *physical* replication
but if you want to use *logical* replication you need logical mode;
if you need neither, and are not doing WAL archiving either, you
can get away with minimal mode.

            regards, tom lane



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