Feedback-ID: id4a34324:Fastmail
Date: Tue, 10 Feb 2026 11:49:36 -0500
From: Andres Freund <andres@anarazel.de>
To: Antonin Houska <ah@cybertec.at>
Cc: Kirill Reshke <reshkekirill@gmail.com>, 
	Heikki Linnakangas <hlinnaka@iki.fi>, Melanie Plageman <melanieplageman@gmail.com>, 
	Matthias van de Meent <boekewurm+postgres@gmail.com>, pgsql-hackers@postgresql.org, 
	Thomas Munro <thomas.munro@gmail.com>, Noah Misch <noah@leadboat.com>, 
	Robert Haas <robertmhaas@gmail.com>, Michael Paquier <michael.paquier@gmail.com>
Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
Message-ID: <aYtg622xasbSju7_@alap3.anarazel.de>
References: <1108f18d-cf7c-4f17-b29c-a119fe42f7e5@iki.fi>
 <5dwlfu2jyzkyf3nrlzxxblxctb6xio5es73ptgsahjnmfu5miu@772rc764hfhi>
 <ossv2eistssmubfsir6xjll76tynvxv5lup4zkrfzjkud7fycw@rf5vii6l6cha>
 <4csodkvvfbfloxxjlkgsnl2lgfv2mtzdl7phqzd4jxjadxm4o5@usw7feyb5bzf>
 <CALdSSPgyc7VuMLUZ8J7v2G-PebBa_vEi+mp-cLkcOwNycB56Hw@mail.gmail.com>
 <kjdgvws34gpnz4y6xu5aaeul5mspgy3ahvyiw4lndb3ecsacdb@b2ccyowotpqh>
 <jtg5cu4n6h5lib3kzx66ju4yhh6kmviaud7oq6dtut6c4q4rdi@xwsfoagt3c2b>
 <61812.1770637345@localhost>
 <aYpbjeiN2lKEBBn1@alap3.anarazel.de>
 <19720.1770709587@localhost>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <19720.1770709587@localhost>
Archived-At: <https://www.postgresql.org/message-id/aYtg622xasbSju7_%40alap3.anarazel.de>
Precedence: bulk

Hi,

On 2026-02-10 08:46:27 +0100, Antonin Houska wrote:
> Andres Freund <andres@anarazel.de> wrote:
> > I think we ought to instead restrict the page-at-a-time scans to only happen
> > with "real" mvcc snapshots. I.e. this:
> > 
> > 	/*
> > 	 * Disable page-at-a-time mode if it's not a MVCC-safe snapshot.
> > 	 */
> > 	if (!(snapshot && IsMVCCSnapshot(snapshot)))
> > 		scan->rs_base.rs_flags &= ~SO_ALLOW_PAGEMODE;
> >
> > should trigger for historic snapshots as well.
> 
> I suppose you mean changing it to
> 
> 	if (!(snapshot && IsMVCCSnapshot(snapshot) &&
> 		  !IsHistoricMVCCSnapshot(snapshot)))
> 		scan->rs_base.rs_flags &= ~SO_ALLOW_PAGEMODE;

Yes.

For something committable, I think we should probably split IsMVCCSnapshot
into IsMVCCSnapshot(), just accepting SNAPSHOT_MVCC, and IsMVCCLikeSnapshot()
accepting both SNAPSHOT_MVCC and SNAPSHOT_HISTORIC_MVCC. And then go through
all the existing callers of IsMVCCSnapshot() - only about half should stay
as-is, I think.


> > Does that fix the issue for you?
> 
> Yes, with this change, I don't hit the problem anymore.

Great!

Greetings,

Andres Freund