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From: Colin 't Hart <[email protected]>
To: Laurenz Albe <[email protected]>
Cc: Andrew <[email protected]>
Cc: Greg Sabino Mullane <[email protected]>
Cc: PostgreSQL General <[email protected]>
Subject: Re: wal segment size
Date: Fri, 19 Dec 2025 14:25:58 +0100
Message-ID: <CAMon-aTR63KpFutZfgO5Jz1h9XngA4odHUvrqVRX2h9TPX=jYQ@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>

What's the behaviour when pg_resetwal is used to change the WAL segment
size?

This note is worrying to me:
--
While pg_resetwal will set the WAL starting address beyond the latest
existing WAL segment file, some segment size changes can cause previous WAL
file names to be reused. It is recommended to use -l together with this
option to manually set the WAL starting address if WAL file name overlap
will cause problems with your archiving strategy.
--
Why can a segment size change cause previous WAL file names to be reused?

Do we need to take a new backup immediately after changing the WAL segment
size?

Thanks,

Colin

On Fri, 19 Dec 2025 at 14:09, Laurenz Albe <[email protected]> wrote:

> On Fri, 2025-12-19 at 09:26 +0100, Andrew wrote:
> > As an oracle dba new to Postgres, I’m used to the concept of context
> switches and latch issues
> > with regards to transaction log switches. Does Postgres have a similar
> mechanism with latching
> > etc when it switches to a new wal segment that is alleviated when
> increasing the size of the
> > wal segments?
>
> Not really.  PostgreSQL doesn't reuse WAL segments in a circular fashion
> like Oracle does.
> At the end of a checkpoint, it creates new, empty WAL segments for future
> use, so if there
> is a need to switch to a new segment, there is no need to wait for
> anything.
>
> Yours,
> Laurenz Albe
>

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