[pve-devel] [PATCH pve-docs v8] fix #5644: Moved the section on swap partitions

[Christoph Heiss]

Please take earlier review comments into account. To quote Fiona from
v6:

> Please use present tense in the commit title/subject like is customary
> in Proxmox VE development.

Commit messages are always present tense, as the commit/patch itself
_changes_ something, but it _hasn't _changed before_ the commit.

Also, this does more than just moving the section, as it inserts a new
paragraph w.r.t. swap on zfs. I'd separate the local-zfs.adoc changes in
a separate patch, as it doesn't have much to do with the section move
itself.

On Tue Jun 3, 2025 at 3:12 PM CEST, Alexander Abraham wrote:
> The section on running swap on ZFS was split into two parts.
> The part describing setting the swappiness level was moved to a
> separate file and the part about creating swap partitions on a ZFS
> volume was replaced with a warning about doing so.
>
> Signed-off-by: Alexander Abraham <a.abraham at proxmox.com>
>
> ---
>
> Changes in v8:
> * Moved the changelog below the three dashes.
>
> Changes in v7:
> * Keep ZFS warning in its original file.
> * Corrected some formatting errors.

It's customary to always include the full changelog, e.g. since v1.
Makes it easier for reviewer.

>
>  local-zfs.adoc | 49 ++++++++++---------------------------------------
>  pve-swap.adoc  | 35 +++++++++++++++++++++++++++++++++++
>  sysadmin.adoc  |  2 ++
>  3 files changed, 47 insertions(+), 39 deletions(-)
>  create mode 100644 pve-swap.adoc
>
> diff --git a/local-zfs.adoc b/local-zfs.adoc
> index c64fb27..bfd2295 100644
> --- a/local-zfs.adoc
> +++ b/local-zfs.adoc
> @@ -1,9 +1,10 @@
>  [[chapter_zfs]]
>  ZFS on Linux
>  ------------
> -ifdef::wiki[]
> -:pve-toplevel:
> -endif::wiki[]
> +
> +-ifdef::wiki[]
> +-:pve-toplevel:
> +-endif::wiki[]

Erroneous hunk maybe from a bad rebase or such?

This was also already noted by Fiona in v6.

>
>  ZFS is a combined file system and logical volume manager designed by
>  Sun Microsystems. Starting with {pve} 3.4, the native Linux
> @@ -620,48 +621,18 @@ time this value changes:
>  ----
>
>  You *must reboot* to activate these changes.
> -====
>
> +====
>
>  [[zfs_swap]]
>  SWAP on ZFS
>  ~~~~~~~~~~~
>
> -Swap-space created on a zvol may generate some troubles, like blocking the
> -server or generating a high IO load, often seen when starting a Backup
> -to an external Storage.
> -
> -We strongly recommend to use enough memory, so that you normally do not
> -run into low memory situations. Should you need or want to add swap, it is
> -preferred to create a partition on a physical disk and use it as a swap device.
> -You can leave some space free for this purpose in the advanced options of the
> -installer. Additionally, you can lower the
> -``swappiness'' value. A good value for servers is 10:
> -
> -----
> -# sysctl -w vm.swappiness=10
> -----
> -
> -To make the swappiness persistent, open `/etc/sysctl.conf` with
> -an editor of your choice and add the following line:
> -
> ---------
> -vm.swappiness = 10
> ---------
> -
> -.Linux kernel `swappiness` parameter values
> -[width="100%",cols="<m,2d",options="header"]
> -|===========================================================
> -| Value               | Strategy
> -| vm.swappiness = 0   | The kernel will swap only to avoid
> -an 'out of memory' condition
> -| vm.swappiness = 1   | Minimum amount of swapping without
> -disabling it entirely.
> -| vm.swappiness = 10  | This value is sometimes recommended to
> -improve performance when sufficient memory exists in a system.
> -| vm.swappiness = 60  | The default value.
> -| vm.swappiness = 100 | The kernel will swap aggressively.
> -|===========================================================
> +Do not use a ZFS volume for creating a swap partition because this could lead

nit: [..] creating a swap partition as it could lead

sounds a bit better.

> +to deadlocks. These deadlocks could cause the  affected system to freeze. The

Whitespace errors, I'd also rephrase it a bit to

[..] to deadlocks, which can cause systems to freeze.

> +OpenZFS documentation
> +footnote:[https://openzfs.github.io/openzfs-docs/Project%20and%20Community/FAQ.html#using-a-zvol-for-a-swap-device-on-linux]

Any reason for a footnote and not just a direct link?

> +warns about using a ZFS volume for creating a swap partition.
>
>  [[zfs_encryption]]
>  Encrypted ZFS Datasets
> diff --git a/pve-swap.adoc b/pve-swap.adoc
> new file mode 100644
> index 0000000..6bc62b0
> --- /dev/null
> +++ b/pve-swap.adoc
> @@ -0,0 +1,35 @@
> +[[creating_swap_partitions]]
> +Creating SWAP Partitions
> +------------------------
> +
> +We strongly recommend to use enough memory, so that you normally do not
> +run into low memory situations. Should you need or want to add swap, it is
> +preferred to create a partition on a physical disk and use it as a swap device.
> +You can leave some space free for this purpose in the advanced options of the
> +installer. Additionally, you can lower the
> +``swappiness'' value. A good value for servers is 10:
> +
> +----
> +# sysctl -w vm.swappiness=10
> +----
> +
> +To make the swappiness persistent, open `/etc/sysctl.conf` with
> +an editor of your choice and add the following line:
> +
> +--------
> +vm.swappiness = 10
> +--------
> +
> +.Linux kernel `swappiness` parameter values
> +[width="100%",cols="<m,2d",options="header"]
> +|===========================================================
> +| Value               | Strategy
> +| vm.swappiness = 0   | The kernel will swap only to avoid
> +an 'out of memory' condition
> +| vm.swappiness = 1   | Minimum amount of swapping without
> +disabling it entirely.
> +| vm.swappiness = 10  | This value is sometimes recommended to
> +improve performance when sufficient memory exists in a system.
> +| vm.swappiness = 60  | The default value.
> +| vm.swappiness = 100 | The kernel will swap aggressively.
> +|===========================================================
> diff --git a/sysadmin.adoc b/sysadmin.adoc
> index dd43f73..5b12634 100644
> --- a/sysadmin.adoc
> +++ b/sysadmin.adoc
> @@ -71,6 +71,8 @@ include::pve-external-metric-server.adoc[]
>
>  include::pve-disk-health-monitoring.adoc[]
>
> +include::pve-swap.adoc[]
> +
>  include::local-lvm.adoc[]
>
>  include::local-zfs.adoc[]