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Re: Why clearing the VM doesn't require registering vm buffer in wal record

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Robert Haas <robertmhaas@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Noah Misch <noah@leadboat.com>
Date: 2026-03-05T20:16:28Z
Lists: pgsql-hackers 
Hi,

On 2026-03-05 14:56:13 -0500, Melanie Plageman wrote:
> Today Andres and I noticed that heap_{update,insert,delete}() don't
> register the VM buffer when they are clearing the VM. I was under the
> impression that any buffers modified needed to be registered in the
> WAL record. Without which, you'll never do an FPI. It seems like this
> could cause checksum failures. What are we missing?

Besides checksum errors, I was also worried that this would break
pg_rewind. But it seems we're saved by pg_rewind only parsing the WAL for
changes to the main fork.

But it does seem like it could be a problem for incremental backup /
walsummarizer?

Greetings,

Andres Freund

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Revamp the WAL record format.

  2. Allow I/O reliability checks using 16-bit checksums

  3. Make the visibility map crash-safe.