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From: "Colin 't Hart" <colinthart@gmail.com>
Date: Fri, 19 Dec 2025 14:25:58 +0100
Message-ID: <CAMon-aTR63KpFutZfgO5Jz1h9XngA4odHUvrqVRX2h9TPX=jYQ@mail.gmail.com>
Subject: Re: wal segment size
To: Laurenz Albe <laurenz.albe@cybertec.at>
Cc: Andrew <adhenry.9@gmail.com>, Greg Sabino Mullane <htamfids@gmail.com>, 
	PostgreSQL General <pgsql-general@lists.postgresql.org>
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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 <laurenz.albe@cybertec.at> wrote=
:

> On Fri, 2025-12-19 at 09:26 +0100, Andrew wrote:
> > As an oracle dba new to Postgres, I=E2=80=99m used to the concept of co=
ntext
> 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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<div dir=3D"ltr"><div>What&#39;s the behaviour when pg_resetwal is used to =
change the WAL segment size?</div><div><br></div><div>This note is worrying=
 to me:</div><div>--</div><div>While <code class=3D"gmail-command">pg_reset=
wal</code> will set the WAL starting
 address beyond the latest existing WAL segment file, some segment size=20
changes can cause previous WAL file names to be reused. It is=20
recommended to use <code class=3D"gmail-option">-l</code> together with thi=
s=20
option to manually set the WAL starting address if WAL file name overlap
 will cause problems with your archiving strategy.</div><div>--</div><div>W=
hy can a segment size change cause previous WAL file names to be reused?</d=
iv><div><br></div><div>Do we need to take a new backup immediately after ch=
anging the WAL segment size?</div><div><br></div><div>Thanks,</div><div><br=
></div><div>Colin</div></div><br><div class=3D"gmail_quote gmail_quote_cont=
ainer"><div dir=3D"ltr" class=3D"gmail_attr">On Fri, 19 Dec 2025 at 14:09, =
Laurenz Albe &lt;<a href=3D"mailto:laurenz.albe@cybertec.at">laurenz.albe@c=
ybertec.at</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote" style=
=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding=
-left:1ex">On Fri, 2025-12-19 at 09:26 +0100, Andrew wrote:<br>
&gt; As an oracle dba new to Postgres, I=E2=80=99m used to the concept of c=
ontext switches and latch issues<br>
&gt; with regards to transaction log switches. Does Postgres have a similar=
 mechanism with latching<br>
&gt; etc when it switches to a new wal segment that is alleviated when incr=
easing the size of the<br>
&gt; wal segments?<br>
<br>
Not really.=C2=A0 PostgreSQL doesn&#39;t reuse WAL segments in a circular f=
ashion like Oracle does.<br>
At the end of a checkpoint, it creates new, empty WAL segments for future u=
se, so if there<br>
is a need to switch to a new segment, there is no need to wait for anything=
.<br>
<br>
Yours,<br>
Laurenz Albe<br>
</blockquote></div>

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