From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Robert Haas <robertmhaas@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>,
        Justin Pryzby <pryzby@telsasoft.com>,
        Ashutosh Sharma <ashu.coek88@gmail.com>,
        Maciek Sakrejda <m.sakrejda@gmail.com>,
        Bruce Momjian <bruce@momjian.us>,
        Alvaro Herrera <alvherre@alvh.no-ip.org>,
        Andrew Dunstan <andrew@dunslane.net>,
        Heikki Linnakangas <hlinnaka@iki.fi>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
        Thomas Munro <thomas.munro@gmail.com>
Subject: Re: [Proposal] Fully WAL logged CREATE DATABASE - No Checkpoints
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Thu, 04 Aug 2022 16:59:22 -0700"
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Date: Thu, 04 Aug 2022 20:21:26 -0400
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Andres Freund <andres@anarazel.de> writes:
> On August 4, 2022 4:11:13 PM PDT, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> [pile^2]  Also, what is the rationale for locking the target buffer
>> but not the source buffer?  That seems pretty hard to justify from
>> here, even granting the assumption that we don't expect any other
>> processes to be interested in these buffers (which I don't grant,
>> because checkpointer).

> I'm not arguing it's good or should stay that way, but it's probably oka=
yish that checkpointer / bgwriter have access, given that they will never =
modify buffers. They just take a lock to prevent concurrent modifications,=
 which RelationCopyStorageUsingBuffer hopefully doesn't do. =


I'm not arguing that it's actively broken today --- but AFAIR,
every other access to a shared buffer takes a buffer lock.
It does not seem to me to be very future-proof for this code to
decide it's exempt from that rule, without so much as a comment
justifying it.  Furthermore, what's the gain?  We aren't expecting
contention here, I think.  If we were, then it probably *would* be
actively broken.

			regards, tom lane