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From: Andrey Borodin <[email protected]>
To: Melanie Plageman <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Andres Freund <[email protected]>
Subject: Re: Missing FSM Update when Updating VM On-access
Date: Sat, 4 Jul 2026 23:41:07 +0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAAKRu_b2StZrEC=HmW8LePuQbczyFRnfs8qTAJwn_=W76-y24w@mail.gmail.com>
References: <CAAKRu_b2StZrEC=HmW8LePuQbczyFRnfs8qTAJwn_=W76-y24w@mail.gmail.com>



> On 30 Jun 2026, at 03:32, Melanie Plageman <[email protected]> wrote:
> 
> <v1-0001-Update-FSM-after-updating-VM-on-access.patch>


Hi Melanie,

I applied the patch ("Update FSM after updating VM on-access") and it does
what it says. On a single node, on-access pruning now sets the VM and
refreshes the FSM to the post-prune free space, matching what a plain VACUUM
records (in my run summed avail 3200000 -> 7360000, VM 10000|0). On a
primary + streaming standby the FSM comes out identical on both sides after
replay (in this simple test) - i.e. the primary now records what the standby
already writes in heap_xlog_prune_freeze.

A dedicated test would be nice but is difficult here: on-access VM-setting is
nondeterministic (see the enum/plancache flakiness earlier), and even a TAP
test with autovacuum off + forced horizon advancement would likely be
unstable on the buildfarm - so it might even be net negative.

One comment nit: the reserve behavior is preserved (we skip the FSM update
unless the page became newly_all_visible), but the outer comment explaining
*why* we don't update the FSM in the common case is gone; the rationale now
survives only as a parenthetical inside the `if (presult.newly_all_visible)`
block. A one-line note at the point where we *skip* the update would spare a
future reader from wondering why it isn't unconditional.

Thanks for working on this!


Best regards, Andrey Borodin.

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