[pve-devel] [PATCH pve-docs v3] fix #5644: Split the section on creating a swap partition on a ZVol.

[pve-devel] [PATCH pve-docs v3] fix #5644: Split the section on creating a swap partition on a ZVol.

[Alexander Abraham]

The section "SWAP on ZFS" was split into two parts. The first part
contains general instructions on how to create a swap partition and
how to set the level of "swappiness". The second part contains a
warning about creating a swap partition on a ZFS volume.The official
ZFS documentation, which has been linked in a footnote, explains
that creating a swap partition on a ZFS volume could lead to
dangerous and undefined behavior.

Signed-off-by: Alexander Abraham <a.abraham@proxmox.com>
---
 local-zfs.adoc | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/local-zfs.adoc b/local-zfs.adoc
index c64fb27..d411222 100644
--- a/local-zfs.adoc
+++ b/local-zfs.adoc
@@ -622,14 +622,9 @@ 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.
+[[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
@@ -637,18 +632,18 @@ 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"]
 |===========================================================
@@ -663,6 +658,15 @@ improve performance when sufficient memory exists in a system.
 | vm.swappiness = 100 | The kernel will swap aggressively.
 |===========================================================
 
+[[zfs_swap]]
+SWAP on ZFS
+~~~~~~~~~~~
+
+It is strongly recommended to not use a ZFS volume for creating a swap partition
+because this could lead to deadlocks. These deadlocks could cause the affected 
+system to freeze. The OpenZFS documentation footnote:[https://openzfs.github.io/openzfs-docs/Project%20and%20Community/FAQ.html#using-a-zvol-for-a-swap-device-on-linux]
+warns about using a ZFS volume for creating a swap partition.
+
 [[zfs_encryption]]
 Encrypted ZFS Datasets
 ~~~~~~~~~~~~~~~~~~~~~~
-- 
2.39.5



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Re: [pve-devel] [PATCH pve-docs v3] fix #5644: Split the section on creating a swap partition on a ZVol.

[Fiona Ebner]

Nit: we usually don't use a dot at the end of a commit title

Am 05.02.25 um 13:41 schrieb Alexander Abraham:
> The section "SWAP on ZFS" was split into two parts. The first part
> contains general instructions on how to create a swap partition and
> how to set the level of "swappiness". The second part contains a
> warning about creating a swap partition on a ZFS volume.The official
> ZFS documentation, which has been linked in a footnote, explains
> that creating a swap partition on a ZFS volume could lead to
> dangerous and undefined behavior.
> 
> Signed-off-by: Alexander Abraham <a.abraham@proxmox.com>

Shaping up nicely now! A few more comments/nits below.

> ---
>  local-zfs.adoc | 26 +++++++++++++++-----------
>  1 file changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/local-zfs.adoc b/local-zfs.adoc
> index c64fb27..d411222 100644
> --- a/local-zfs.adoc
> +++ b/local-zfs.adoc
> @@ -622,14 +622,9 @@ 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.
> +[[creating_swap_partitions]]
> +Creating SWAP Partitions
> +~~~~~~~~~~~~~~~~~~~~~~~~
>  

This is not related to ZFS, so a second patch could move this section to
a different place, see Alexander Zeidler's suggestion in v2:
https://lore.proxmox.com/pve-devel/D7ITC3NDGSV5.1S3BV4L2Q49LF@proxmox.com/

>  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
> @@ -637,18 +632,18 @@ 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"]
>  |===========================================================

Is there a specific reason behind these whitespace changes or were they
done by accident?

> @@ -663,6 +658,15 @@ improve performance when sufficient memory exists in a system.
>  | vm.swappiness = 100 | The kernel will swap aggressively.
>  |===========================================================
>  
> +[[zfs_swap]]
> +SWAP on ZFS
> +~~~~~~~~~~~
> +
> +It is strongly recommended to not use a ZFS volume for creating a swap partition

Missing a comma before "because"

> +because this could lead to deadlocks. These deadlocks could cause the affected 

Nit: I'd replace "could" with "can" both times

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

Nit: I'd move the footnote link on its own line, then it will overshoot
the line length a bit less.

> +warns about using a ZFS volume for creating a swap partition.
> +
>  [[zfs_encryption]]
>  Encrypted ZFS Datasets
>  ~~~~~~~~~~~~~~~~~~~~~~



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