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From: Laurenz Albe <[email protected]>
To: Marc Millas <[email protected]>
To: [email protected] <[email protected]>
Subject: Re: postgres in swap space
Date: Mon, 17 Nov 2025 21:49:59 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <CADX_1ab6U-T_SZP7L1uiOMNdBm9vV07u22i-KgQvu1E=BHguWA@mail.gmail.com>
References: <CADX_1ab6U-T_SZP7L1uiOMNdBm9vV07u22i-KgQvu1E=BHguWA@mail.gmail.com>

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.  For 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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