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help / color / mirror / Atom feedFrom: Andres Freund <[email protected]>
To: Antonin Houska <[email protected]>
Cc: Kirill Reshke <[email protected]>
Cc: Heikki Linnakangas <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: Matthias van de Meent <[email protected]>
Cc: [email protected]
Cc: Thomas Munro <[email protected]>
Cc: Noah Misch <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Michael Paquier <[email protected]>
Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
Date: Tue, 10 Feb 2026 11:49:36 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <19720.1770709587@localhost>
References: <[email protected]>
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<[email protected]>
<19720.1770709587@localhost>
Hi,
On 2026-02-10 08:46:27 +0100, Antonin Houska wrote:
> Andres Freund <[email protected]> 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
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Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
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