Message-ID: <ef9349445ef9bd14e1e9285c4eb3e6d48f47ec9b.camel@cybertec.at>
Subject: Re: postgres in swap space
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Marc Millas <marc.millas@mokadb.com>, 
 "pgsql-general@lists.postgresql.org"
	 <pgsql-general@lists.postgresql.org>
Date: Mon, 17 Nov 2025 21:49:59 +0100
In-Reply-To: <CADX_1ab6U-T_SZP7L1uiOMNdBm9vV07u22i-KgQvu1E=BHguWA@mail.gmail.com>
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On Mon, 2025-11-17 at 18:25 +0100, Marc Millas wrote:
> Can someone point me to any doc describing why and how much=C2=A0space po=
stgres uses on the swap of a debian machine ?
> it's an old postgres 10, because it is used by a product for which only t=
his 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 determi=
nes how
eager the kernel is to swap out memory, even if there is no pressure.  For =
best
performance, that should happen as little as possible, and the database sho=
uld
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 de=
fine enough
Linux hugepages, so that PostgreSQL can allocate shared buffers there.  Lin=
ux does not
swap out hugepages.

You can find the PostgreSQL parameters described in the PostgreSQL document=
ation and the
kernel parameters in the kernel documentation.

Yours,
Laurenz Albe