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From: Marc Millas <marc.millas@mokadb.com>
Date: Tue, 18 Nov 2025 21:23:18 +0100
Message-ID: <CADX_1aah__ktrnw-yT5ADp6ppxVpz=r6_ZHLQ0cmM=ZgfOMzZg@mail.gmail.com>
Subject: Re: postgres in swap space
To: Laurenz Albe <laurenz.albe@cybertec.at>
Cc: "pgsql-general@lists.postgresql.org" <pgsql-general@lists.postgresql.org>
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Clear !
thanks Laurentz

Marc MILLAS
Senior Architect
+33607850334
www.mokadb.com


On Mon, Nov 17, 2025 at 9:50=E2=80=AFPM Laurenz Albe <laurenz.albe@cybertec=
.at>
wrote:

> On Mon, 2025-11-17 at 18:25 +0100, Marc Millas wrote:
> > Can someone point me to any doc describing why and how much space
> postgres uses on the swap of a debian machine ?
> > it's an old postgres 10, because it is used by a product for which only
> this version is certified.
> > (no comment on that, please)
>
> I'm biting down a comment.
>
> PostgreSQL itself doesn't use any swap space.
>
> The kernel can decide to swap out memory used by PostgreSQL.  How much
> that is depends
> on how you configured the Linux kernel and how much memory PostgreSQL
> uses.  The latter
> is mostly determined by "shared_buffers", "work_mem",
> "maintenance_work_mem" and
> "max_connections".
>
> On the kernel side, it is mostly the "vm.swappiness" parameter that
> determines how
> eager the kernel is to swap out memory, even if there is no pressure.  Fo=
r
> best
> performance, that should happen as little as possible, and the database
> should
> determine what to keep in memory and what to store on disk.
>
> One tool you have to prevent shared buffers from being swapped out is to
> define enough
> Linux hugepages, so that PostgreSQL can allocate shared buffers there.
> Linux does not
> swap out hugepages.
>
> You can find the PostgreSQL parameters described in the PostgreSQL
> documentation and the
> kernel parameters in the kernel documentation.
>
> Yours,
> Laurenz Albe
>

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<div dir=3D"ltr"><div>Clear !</div><div>thanks Laurentz</div><div><br></div=
><div><div dir=3D"ltr" class=3D"gmail_signature" data-smartmail=3D"gmail_si=
gnature"><div dir=3D"ltr"><div><div dir=3D"ltr">Marc MILLAS<div>Senior Arch=
itect</div><div>+33607850334</div><div><a href=3D"http://www.mokadb.com" ta=
rget=3D"_blank">www.mokadb.com</a></div><div><br></div></div></div></div></=
div></div><br></div><br><div class=3D"gmail_quote gmail_quote_container"><d=
iv dir=3D"ltr" class=3D"gmail_attr">On Mon, Nov 17, 2025 at 9:50=E2=80=AFPM=
 Laurenz Albe &lt;<a href=3D"mailto:laurenz.albe@cybertec.at">laurenz.albe@=
cybertec.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 Mon, 2025-11-17 at 18:25 +0100, Marc Millas wrote:<br>
&gt; Can someone point me to any doc describing why and how much=C2=A0space=
 postgres uses on the swap of a debian machine ?<br>
&gt; it&#39;s an old postgres 10, because it is used by a product for which=
 only this version is certified.<br>
&gt; (no comment on that, please)<br>
<br>
I&#39;m biting down a comment.<br>
<br>
PostgreSQL itself doesn&#39;t use any swap space.<br>
<br>
The kernel can decide to swap out memory used by PostgreSQL.=C2=A0 How much=
 that is depends<br>
on how you configured the Linux kernel and how much memory PostgreSQL uses.=
=C2=A0 The latter<br>
is mostly determined by &quot;shared_buffers&quot;, &quot;work_mem&quot;, &=
quot;maintenance_work_mem&quot; and<br>
&quot;max_connections&quot;.<br>
<br>
On the kernel side, it is mostly the &quot;vm.swappiness&quot; parameter th=
at determines how<br>
eager the kernel is to swap out memory, even if there is no pressure.=C2=A0=
 For best<br>
performance, that should happen as little as possible, and the database sho=
uld<br>
determine what to keep in memory and what to store on disk.<br>
<br>
One tool you have to prevent shared buffers from being swapped out is to de=
fine enough<br>
Linux hugepages, so that PostgreSQL can allocate shared buffers there.=C2=
=A0 Linux does not<br>
swap out hugepages.<br>
<br>
You can find the PostgreSQL parameters described in the PostgreSQL document=
ation and the<br>
kernel parameters in the kernel documentation.<br>
<br>
Yours,<br>
Laurenz Albe<br>
</blockquote></div>

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