README.md

@@ -109,6 +109,30 @@ In cases where a variable is both exported prior to script execution and specifi
 
 Known options are as follows.
 
+### Kernel downgrade
+
+By default we install newest `linux` and `linux-headers` packages into a `chroot`. Once we're in that `chroot` we then install newest [AUR zfs-dkms package](https://aur.archlinux.org/packages/zfs-dkms). You may want to override `linux` and `linux-headers` versions we're installing to ensure you end up with a compatible mix between them and `zfs-dkms`.
+
+For example:
+```
+export ARCHZBM_KERNEL_VER=6.5.9.arch2
+```
+
+In our `chroot` this will trigger execution of:
+```
+downgrade --ala-only 'linux=6.5.9.arch2' 'linux-headers=6.5.9.arch2' --ignore always
+```
+
+Where `downgrade` is the [AUR downgrade package](https://aur.archlinux.org/packages/downgrade). This will downgrade `linux` and `linux-headers` and will add a setting to your `/etc/pacman.conf`:
+```
+[options]
+IgnorePkg = linux linux-headers
+```
+
+Setting `ARCHZBM_KERNEL_VER` to an empty string `''` or keeping it undefined are both valid and will retain newest versions instead of downgrading.
+
+Also read [Kernel selection](#kernel-selection) for details.
+
 ### Compression
 
 By default we create a zpool with ZFS property `compression=on`. If the `lz4_compress` pool feature is active this will by default enable `compression=lz4`. See `man 7 zfsprops` for example in ZFS 2.1.9 for details. See `zpool get feature@lz4_compress <pool>` to check this feature's status on your `<pool>`.
@@ -296,6 +320,24 @@ When it comes to the point that your SSH client sends an are-your-still-there me
 
 This effectively configures your SSH client to remain connected even through somewhat lossy hops to the Dropbear daemon; and to cleanly disconnect 3 seconds and some change after you've executed whatever you needed to do in ZFSBootMenu.
 
+# Kernel selection
+
+This script compiles ZFS via Arch Linux' [Dynamic Kernel Module Support](https://wiki.archlinux.org/title/Dynamic_Kernel_Module_Support) (DKMS). Not all kernels allow for successful compilation, in some instances a particularly recent kernel version may change APIs to such a degree that ZFS compilation simply fails.
+
+We strongly suggest to that you:
+
+- Firstly, refer to a resource such as the [Arch Linux Archive package version list](https://archive.archlinux.org/packages/l/linux/) to find out what newest kernel version this script will install.
+- Secondly, research if newest [AUR zfs-dkms package](https://aur.archlinux.org/packages/zfs-dkms) is compatible with that kernel. Two reasonable points of contact are AUR and its comments section for `zfs-dkms` where users quickly report issues; and the [github.com/openzfs/zfs issues list](https://github.com/openzfs/zfs/issues).
+
+An example for this is that `linux-6.6.1.arch1-1-x86_64` released on Wednesday, November 8, 2023 at a time when newest `zfs-dkms` package version [was 2.2.0](https://aur.archlinux.org/cgit/aur.git/commit/?h=zfs-dkms&id=da1b6372c57b16f2781a7fda2b95971bb392c5ee) which did not compile against `linux` 6.6.x.
+
+You'd then set for example:
+```
+export ARCHZBM_KERNEL_VER=6.5.9.arch2
+```
+
+Where any 6.5.x version is known to work well with `zfs-dkms`. See also [Kernel downgrade](#kernel-downgrade) for details on how to configure this.
+
 # Flavor choices
 
 We make the following opinionated flavor choices. Feel free to change them to your liking.

setup.sh

@@ -68,6 +68,10 @@ function setup_env_vars () {
     echo '----------------------------------------'
     echo
 
+    read -u3 -p 'Please type kernel version to use, leave empty for latest, confirm with <Enter>: ' -s ARCHZBM_KERNEL_VER
+    echo
+    echo
+
     echo 'Do you want compressed datasets?'
     select arg_compressed in "Compressed" "Uncompressed"; do
         case "${arg_compressed}" in
@@ -301,7 +305,7 @@ function setup_env_vars () {
         fi
     fi
 
-    for env_var in 'ARCHZBM_ZFSPROPS_NO_COMPRESSION' 'ARCHZBM_ZFSPROPS_NO_ENCRYPTION' 'ARCHZBM_ZPOOL_PASSWORD' 'ARCHZBM_ROOT_PASSWORD' 'ARCHZBM_NET_AUTOCONF' 'ARCHZBM_NET_DEVICE' 'ARCHZBM_NET_CLIENT_IP' 'ARCHZBM_NET_NETMASK' 'ARCHZBM_NET_GATEWAY_IP' 'ARCHZBM_SSH_PORT' 'ARCHZBM_SSH_KEEPALIVE_INTVL' 'ARCHZBM_SSH_AUTH_KEYS' 'ARCHZBM_OS_CLIENT_IP' 'ARCHZBM_OS_GATEWAY_IP' 'ARCHZBM_OS_DNS_IP' 'ARCHZBM_OS_NTP_IP'; do
+    for env_var in 'ARCHZBM_KERNEL_VER' 'ARCHZBM_ZFSPROPS_NO_COMPRESSION' 'ARCHZBM_ZFSPROPS_NO_ENCRYPTION' 'ARCHZBM_ZPOOL_PASSWORD' 'ARCHZBM_ROOT_PASSWORD' 'ARCHZBM_NET_AUTOCONF' 'ARCHZBM_NET_DEVICE' 'ARCHZBM_NET_CLIENT_IP' 'ARCHZBM_NET_NETMASK' 'ARCHZBM_NET_GATEWAY_IP' 'ARCHZBM_SSH_PORT' 'ARCHZBM_SSH_KEEPALIVE_INTVL' 'ARCHZBM_SSH_AUTH_KEYS' 'ARCHZBM_OS_CLIENT_IP' 'ARCHZBM_OS_GATEWAY_IP' 'ARCHZBM_OS_SSH_AUTH_KEYS' 'ARCHZBM_OS_DNS_IP' 'ARCHZBM_OS_NTP_IP'; do
         if [[ "${!env_var}" ]]; then
             printf -- '%s='"'"'%s'"'"'\n' \
                 "${env_var}" "${!env_var}" \