--- title: Configure default boot image in Grub2 date: 2024-02-08 slug: config-default-grub2-boot-img authors: - lunik description: Simple guide to configure the default boot image in Grub2 tags: - linux - grub - boot - grub2 - initramfs - kernel - image --- ![cover](/blog/img/posts/2024-02-08-config-default-grub2-boot-img/cover.jpg) Today I have upgraded my [Fedora][fedoraproject] server from version `33` to `38` but all did not go as planned. I have [OpenZFS][openzfs-github] installed to handle my storage and the current released version of [OpenZFS][openzfs-github] ([2.2.2][zfs-release-2.2.2]) is not compatible with the latest [kernel][kernel-website] (`6.7`). I was stuck with my new [kernel][kernel-website] and unable to access my data. For reference here is the error message I was getting: ```shell zfs list ``` Output: ```shell The ZFS modules cannot be auto-loaded. Try running 'modprobe zfs' as root to manually load them. ``` And: ```shell modprobe zfs ``` Output: ```shell modprobe: FATAL: Module zfs not found in directory /lib/modules/6.7.3-100.fc38.x86_64 ``` ## Troubleshooting The first thing I did was to check what [kernel][kernel-website] version is supported by the latest ZFS release ([2.2.2][zfs-release-2.2.2]) and I found out that the latest supported [kernel][kernel-website] is `6.6`. I missed it by one minor version. Well that's not a big deal, I can just install the previous [kernel][kernel-website] and boot on it, right? Let's see what kernels are available: ```bash dnf --releasever=38 --showduplicates list kernel ``` Output: ```shell Paquets disponibles kernel.x86_64 6.2.9-300.fc38 fedora kernel.x86_64 6.7.3-100.fc38 pdates ``` ## Install previous kernel Not really what I was expecting. I was hoping to see the previous [kernel][kernel-website] version `6.6` but it's not there. Let's try to install the `6.2.9` version and see what happens: ```shell dnf install kernel-6.2.9-300.fc38 ``` Let's reboot and see if it works. ```shell uname -r ``` Output: ```shell 6.7.3-100.fc38.x86_64 ``` After rebooting I was still booting on the latest [kernel][kernel-website] `6.7`. Let's plug a monitor and see what's happening during the boot process. ## Grub2 During the boot process, I got the Grub2 menu and I was able to select the previous [kernel][kernel-website] version `6.2.9` but it was not the default boot image. I had to manually select it every time I rebooted the server. ### Troubleshooting again Let's have a look at the Grub2 configuration to see what's the default configuration: ```shell cat /etc/default/grub ``` Output: ```shell GRUB_TIMEOUT=5 GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)" GRUB_DEFAULT=saved GRUB_DISABLE_SUBMENU=true GRUB_TERMINAL_OUTPUT="console" GRUB_CMDLINE_LINUX="resume=/dev/mapper/fedora_nubs-swap rd.lvm.lv=fedora_nubs/root rd.lvm.lv=fedora_nubs/swap rhgb quiet" GRUB_DISABLE_RECOVERY="true" GRUB_ENABLE_BLSCFG=true ``` Hum. `GRUB_DEFAULT=saved`. Loooks like the default boot image is saved somewhere. Let's have a look at the Grub2 configuration file: ```shell grep saved /etc/grub2.cfg ``` Output: ```shell set default="${saved_entry}" if [ "${prev_saved_entry}" ]; then set saved_entry="${prev_saved_entry}" save_env saved_entry set prev_saved_entry= save_env prev_saved_entry function savedefault { saved_entry="${chosen}" save_env saved_entry ``` The grub2 configuration file is quite complex and I don't want to mess with it but it seeams that it sets the default boot image to the `saved_entry` variable. I found out there is a binary to work with grub2 environment variables: `grub2-editenv`. ```shell grub2-editenv list ``` Output: ```shell saved_entry=8bfb6e63623d4ee6aab1634ebafdd5d4-5.6.15-200.fc31.x86_64 kernelopts=root=/dev/mapper/fedora_nubs-root ro resume=/dev/mapper/fedora_nubs-swap rd.lvm.lv=fedora_nubs/root rd.lvm.lv=fedora_nubs/swap rhgb quiet systemd.unified_cgroup_hierarchy=0 boot_success=0 boot_indeterminate=1 ``` `5.6` ! That's an old kernel version. I guess my previous upgrade from [Fedora][fedoraproject] 31 was not as clean as I thought. How can I change the default boot image ? And what is this long hexadecimal string at the beginning of the `saved_entry` variable ? ### Changing the default boot image We first need to find the list of available boot images. I found another handy binary to do that `grubby`. ```shell grubby --info ALL ``` Output: ```shell index=0 kernel="/boot/vmlinuz-6.7.3-100.fc38.x86_64" args="ro resume=/dev/mapper/fedora_nubs-swap rd.lvm.lv=fedora_nubs/root rd.lvm.lv=fedora_nubs/swap rhgb quiet systemd.unified_cgroup_hierarchy=0" root="/dev/mapper/fedora_nubs-root" initrd="/boot/initramfs-6.7.3-100.fc38.x86_64.img" title="Fedora Linux (6.7.3-100.fc38.x86_64) 38 (Server Edition)" id="8bfb6e63623d4ee6aab1634ebafdd5d4-6.7.3-100.fc38.x86_64" index=1 kernel="/boot/vmlinuz-6.5.12-100.fc37.x86_64" args="ro resume=/dev/mapper/fedora_nubs-swap rd.lvm.lv=fedora_nubs/root rd.lvm.lv=fedora_nubs/swap rhgb quiet systemd.unified_cgroup_hierarchy=0" root="/dev/mapper/fedora_nubs-root" initrd="/boot/initramfs-6.5.12-100.fc37.x86_64.img" title="Fedora Linux (6.5.12-100.fc37.x86_64) 37 (Server Edition)" id="8bfb6e63623d4ee6aab1634ebafdd5d4-6.5.12-100.fc37.x86_64" index=2 kernel="/boot/vmlinuz-6.2.9-300.fc38.x86_64" args="ro resume=/dev/mapper/fedora_nubs-swap rd.lvm.lv=fedora_nubs/root rd.lvm.lv=fedora_nubs/swap rhgb quiet systemd.unified_cgroup_hierarchy=0" root="/dev/mapper/fedora_nubs-root" initrd="/boot/initramfs-6.2.9-300.fc38.x86_64.img" title="Fedora Linux (6.2.9-300.fc38.x86_64) 38 (Server Edition)" id="8bfb6e63623d4ee6aab1634ebafdd5d4-6.2.9-300.fc38.x86_64" index=3 kernel="/boot/vmlinuz-0-rescue-8bfb6e63623d4ee6aab1634ebafdd5d4" args="ro resume=/dev/mapper/fedora_nubs-swap rd.lvm.lv=fedora_nubs/root rd.lvm.lv=fedora_nubs/swap rhgb quiet systemd.unified_cgroup_hierarchy=0" root="/dev/mapper/fedora_nubs-root" initrd="/boot/initramfs-0-rescue-8bfb6e63623d4ee6aab1634ebafdd5d4.img" title="Fedora (0-rescue-8bfb6e63623d4ee6aab1634ebafdd5d4) 30 (Thirty)" id="8bfb6e63623d4ee6aab1634ebafdd5d4-0-rescue" ``` Wait a minute. The output list is really similar to what I have on my monitor during the boot process. And the `id` field as the same format as the `saved_entry` variable. I guess if I change the `saved_entry` variable to the `id` of the boot image I want to boot on, it should work. ```shell grub2-editenv - set saved_entry='8bfb6e63623d4ee6aab1634ebafdd5d4-6.2.9-300.fc38.x86_64' ``` Checking the `saved_entry` variable: ```shell grub2-editenv list ``` Output: ```shell saved_entry=8bfb6e63623d4ee6aab1634ebafdd5d4-6.2.9-300.fc38.x86_64 kernelopts=root=/dev/mapper/fedora_nubs-root ro resume=/dev/mapper/fedora_nubs-swap rd.lvm.lv=fedora_nubs/root rd.lvm.lv=fedora_nubs/swap rhgb quiet systemd.unified_cgroup_hierarchy=0 boot_success=1 boot_indeterminate=1 ``` Looks good. Time for the final test. Rebooting the server... I see a the Grub2 menu and the default selected image is the one I set. Don't move ... 5 ... 4 ... 3 ... 2 ... 1 ... 0. It boots ! Last check: ```shell uname -r ``` Output: ```shell 6.2.9-300.fc38.x86_64 ``` And my ZFS ? ```shell zfs list ``` Output: ```shell The ZFS modules cannot be auto-loaded. Try running 'modprobe zfs' as root to manually load them. ``` Argh. I forgot to reinstall the ZFS modules for the new [kernel][kernel-website]. Let's do that. ```shell dnf install zfs ``` ```shell zfs list ``` Output: ```shell NAME USED AVAIL REFER MOUNTPOINT poule 3.64T 3.47T 141K /poule ... ``` It's alive ! ## Conclusion I hope this guide will help you to configure the default boot image in Grub2. It's a simple task but it can be tricky if you don't know where to look. Also be very careful when playing with Grub2, you can easily break your system. Always have a backup plan (I didn't ... YOLO). [zfs-release-2.2.2]: https://github.com/openzfs/zfs/releases/tag/zfs-2.2.2 [fedoraproject]: https://fedoraproject.org [openzfs-github]: https://github.com/openzfs/zfs [kernel-website]: https://www.kernel.org