Building OpenZFS for Fedora CoreOS

build.sh

#!/bin/bash

FEDORA_MAJOR="38"
ARCH="x86_64"
KERNEL_VERSION="6.3.8-200.fc38"
ZFS_VERSION="2.1.12"

podman run \
    --interactive \
    --tty \
    --rm \
    -v $(pwd):/data:z \
    -e FEDORA_MAJOR=${FEDORA_MAJOR} \
    -e ARCH=${ARCH} \
    -e KERNEL_VERSION=${KERNEL_VERSION} \
    -e ZFS_VERSION=${ZFS_VERSION} \
    fedora:${FEDORA_MAJOR} \
    /data/internal_build.sh

internal_build.sh

#!/bin/bash

set -Eeuo pipefail
set -x

KERNEL_FULL_VERSION="${KERNEL_VERSION}.${ARCH}"

ZFS_NAME="zfs-${ZFS_VERSION}"
ZFS_FILENAME="zfs-${ZFS_VERSION}.tar.gz"

WORKDIR=$(mktemp -d)

cd $WORKDIR

dnf install -y \
    koji
koji download-build --arch=${ARCH} --rpm kernel-devel-${KERNEL_FULL_VERSION}
dnf install -y \
    ./kernel-devel-${KERNEL_FULL_VERSION}.rpm

dnf install -y \
    wget
wget https://github.com/openzfs/zfs/releases/download/${ZFS_NAME}/${ZFS_FILENAME}
tar xfv ${ZFS_FILENAME}
cd ${ZFS_NAME}

# https://openzfs.github.io/openzfs-docs/Developer%20Resources/Custom%20Packages.html#get-the-source-code
dnf install -y --skip-broken \
    epel-release \
    gcc \
    make \
    autoconf \
    automake \
    libtool \
    rpm-build \
    kernel-rpm-macros \
    libtirpc-devel \
    libblkid-devel \
    libuuid-devel \
    libudev-devel \
    openssl-devel \
    zlib-devel \
    libaio-devel \
    libattr-devel \
    elfutils-libelf-devel \
    kernel-devel-$(uname -r) \
    python3 \
    python3-devel \
    python3-setuptools \
    python3-cffi \
    libffi-devel \
    ncompress
dnf install -y --skip-broken --enablerepo=epel --enablerepo=powertools \
    python3-packaging \
    dkms

./configure --with-linux=/usr/src/kernels/${KERNEL_FULL_VERSION}

# kmodtool patch is obsolete somewhere from zfs 2.1.6
# /lib/modules/${kernel_uname_r}/System.map ${kernel_uname_r} branch is already present
#dnf install -y \
#    patch
#patch -i /data/patches/*.patch -p 1

make -j1 rpm-utils rpm-kmod

cp kmod-zfs-${KERNEL_FULL_VERSION}-${ZFS_VERSION}-1.fc${FEDORA_MAJOR}.x86_64.rpm /data/packages/

patches___0001-zfs-kmodtool-system-map-path.patch

From: Mikhail Cheshkov <[email protected]>
Date: Sat Aug  7 17:15:45 UTC 2021
Subject: Adjust kmodtool %post script to generate System.map in /lib/modules

diff -u a/scripts/kmodtool b/scripts/kmodtool
--- a/scripts/kmodtool	2021-07-28 19:13:53.373619061 +0300
+++ b/scripts/kmodtool	2021-07-28 19:15:24.643052684 +0300
@@ -182,9 +182,9 @@
 %{?KmodsRequires:Requires: %{KmodsRequires}-uname-r = ${kernel_uname_r}}
 %{?KmodsRequires:BuildRequires: %{KmodsRequires}-uname-r = ${kernel_uname_r}}
 %post          -n kmod-${kmodname}-${kernel_uname_r}
-${prefix}${depmod_path} -aeF /boot/System.map-${kernel_uname_r} ${kernel_uname_r} > /dev/null || :
+${prefix}${depmod_path} -aeF /lib/modules/${kernel_uname_r}/System.map ${kernel_uname_r} > /dev/null || :
 %postun        -n kmod-${kmodname}-${kernel_uname_r}
-${prefix}${depmod_path} -aF /boot/System.map-${kernel_uname_r} ${kernel_uname_r} &> /dev/null || :
+${prefix}${depmod_path} -aF /lib/modules/${kernel_uname_r}/System.map ${kernel_uname_r} &> /dev/null || :
 
 EOF
 	else

You should have a packages dir with a file named like kmod-zfs-6.3.8-200.fc38.x86_64-2.1.12-1.fc38.x86_64.rpm
Install it as any other RPM on your system, for example dnf install on Fedora/CentOS/RHEL and rpm-ostree install on Silverblue/CoreOS

How do you handle os updates?

I only used this with CoreOS, so no idea how to properly upgrade regular Fedora.

Updates that doesn't touch kernel should be happy with simple rpm-ostree upgrade.
But if there's a new kernel version it will fail with message like this:
package kmod-zfs-%OLD_KMOD_VERSION% from @commandline requires kernel-uname-r = %OLD_KERNEL_VERSION%, but none of the providers can be installed

Basically, you need to remove old ZFS kmod and install new together with kernel upgrade. One way to do this is something like this:

1. Get current package name, i.e. from that error message, or from rpm-ostree status output
2. Get new kernel version, i.e. from rpm-ostree upgrade --preview, look for kernel package.
3. Build kmod for new kernel.
4. Run rpm-ostree upgrade --uninstall kmod-zfs-%OLD_KMOD_VERSION% --install /var/local/packages/zfs/kmod-zfs-%NEW_KMOD_VERSION%.rpm