[pve-devel] [PATCH docs] local-zfs: mention `zfs_arc_max` clamping by the installer

[pve-devel] [PATCH docs] local-zfs: mention `zfs_arc_max` clamping by the installer

[Christoph Heiss]

This was forgotten to be updated when it changed it the installer and
now reported in the forum [0] that the docs are a bit outdated in this
regard.

[0] https://forum.proxmox.com/threads/hat-proxmox-8-neue-zfs_arc_max-settings.142754/

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>
---
 local-zfs.adoc | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/local-zfs.adoc b/local-zfs.adoc
index a1a14e4..c977558 100644
--- a/local-zfs.adoc
+++ b/local-zfs.adoc
@@ -568,11 +568,16 @@ Limit ZFS Memory Usage
 ~~~~~~~~~~~~~~~~~~~~~~
 
 ZFS uses '50 %' of the host memory for the **A**daptive **R**eplacement
-**C**ache (ARC) by default. 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 GiB/TiB-Storage+. For example, if you have a pool with
-+8 TiB+ of available storage space then you should use +10 GiB+ of memory for
-the ARC.
+**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`.
+
+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
+GiB/TiB-Storage+. For example, if you have a pool with +8 TiB+ of available
+storage space then you should use +10 GiB+ of memory for the ARC.
+
+ZFS also enforces a minimum value of +64 MiB+.
 
 You can change the ARC usage limit for the current boot (a reboot resets this
 change again) by writing to the +zfs_arc_max+ module parameter directly:
@@ -581,8 +586,8 @@ change again) by writing to the +zfs_arc_max+ module parameter directly:
  echo "$[10 * 1024*1024*1024]" >/sys/module/zfs/parameters/zfs_arc_max
 ----
 
-To *permanently change* the ARC limits, add the following line to
-`/etc/modprobe.d/zfs.conf`:
+To *permanently change* the ARC limits, add (or change if already present) the
+following line to `/etc/modprobe.d/zfs.conf`:
 
 --------
 options zfs zfs_arc_max=8589934592
-- 
2.43.1

[pve-devel] [PATCH docs 2/2] installation: document `ARC max size` option for ZFS-on-root installations

[Christoph Heiss]

This has been part of the installer since the 8.1 release.

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>
---
 pve-installation.adoc | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/pve-installation.adoc b/pve-installation.adoc
index 6b44fc0..3e12727 100644
--- a/pve-installation.adoc
+++ b/pve-installation.adoc
@@ -277,6 +277,13 @@ Defines which checksumming algorithm should be used for `rpool`.
 Defines the `copies` parameter for `rpool`. Check the `zfs(8)` manpage for the
 semantics, and why this does not replace redundancy on disk-level.

+`ARC max size`::
+
+Defines the maximum size the ARC can grow to and thus limits the amount of
+memory ZFS will use. See also the section on
+xref:sysadmin_zfs_limit_memory_usage[how to limit ZFS memory usage] for more
+details.
+
 `hdsize`::

 Defines the total hard disk size to be used. This is useful to save free space
--
2.43.1

[pve-devel] applied-series: [PATCH docs] local-zfs: mention `zfs_arc_max` clamping by the installer

[Thomas Lamprecht]

Am 05/03/2024 um 13:13 schrieb Christoph Heiss:
> This was forgotten to be updated when it changed it the installer and
> now reported in the forum [0] that the docs are a bit outdated in this
> regard.
> 
> [0] https://forum.proxmox.com/threads/hat-proxmox-8-neue-zfs_arc_max-settings.142754/
> 
> Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>
> ---
>  local-zfs.adoc | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
>

applied both patches, thanks!