Re: [PATCH docs v2 1/2] local-zfs: rephrase ARC limit

[Friedrich Weber <f.weber@proxmox.com>]

Hi, thanks for looking into this!

On 26/02/2026 09:47, Maximiliano Sandoval wrote:
> We rephrase the current section putting more emphasis on the current
> state of things given than Proxmox VE 8.1 was released over two years
> ago.
> 
> We also document ZFS' default size. See [zfs-6a629f3] for more details.
> 
> This also frames the problem for pre-existing setups in its own
> paragraph.
> 
> [zfs-6a629f3] https://github.com/openzfs/zfs/commit/6a629f32344468ae81b264055916641480cb438d
> 
> Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
> ---
>  local-zfs.adoc | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/local-zfs.adoc b/local-zfs.adoc
> index 85fa092..b3a462d 100644
> --- a/local-zfs.adoc
> +++ b/local-zfs.adoc
> @@ -586,10 +586,15 @@ configured for the root user.
>  Limit ZFS Memory Usage
>  ~~~~~~~~~~~~~~~~~~~~~~
>  
> -ZFS uses '50 %' of the host memory for the **A**daptive **R**eplacement
> -**C**ache (ARC) by default. For new installations starting with {pve} 8.1, the
> -ARC usage limit will be set to '10 %' of the installed physical memory, clamped
> -to a maximum of +16 GiB+. This value is written to `/etc/modprobe.d/zfs.conf`.
> +ZFS uses '10 %' of the host memory, clamped to a maximum of +16 GiB+, for the
> +**A**daptive **R**eplacement **C**ache (ARC) by default. This value is written
> +to `/etc/modprobe.d/zfs.conf` during installation.
> +
> +Before {pve} 8.1, the ARC usage limit was not set during installation and
> +matched ZFS' default value which was either 62.5% starting on ZFS 2.3.0 or 50%

I'm not sure there is a case where a user could have PVE < 8.1 but ZFS
>= 2.3? But it's probably OK to still mention the 62.5%.

There is one more complication: IIRC PVE 8.1 installers only set the ARC
size limit when installing on ZFS, so e.g. installations on ext4+LVM
which later got a zpool would still have ZFS defaults. This was only
changed with the PVE 8.4 installer I think [1], now all installations
limit the ARC size.

> +for older versions. For existing installations that predate {pve} 8.1, manual
> +steps would have to be performed in order to lower the usage limit as described
> +below.
>  
>  Allocating enough memory for the ARC is crucial for IO performance, so reduce it
>  with caution. As a general rule of thumb, allocate at least +2 GiB Base + 1

[1] https://bugzilla.proxmox.com/show_bug.cgi?id=6285