From Fedora Project Wiki

Description

This test case creates a dual boot system with two Fedoras of different release versions, using btrfs snapshots, works as expected

Setup

  • Install Fedora 36. Any desktop, using Automatic partitioning.
  • Freshen your backups, just in case
  • Backup /boot/efi, just in case, e.g.
  1. cd /boot
  2. sudo tar -acf bootefibackup.tar efi

Actions

Make snapshots

  1. Use the df command to find the /dev node+partition for your / mount point, e.g. $BTRFSPOOL = "/dev/nvme0n1p5"
  2. mount $BTRFSPOOL /mnt; cd /mnt; ls -l
  3. You should see two directories, root and home. These are Btrfs subvolumes, which we can snapshot.
  4. btrfs subvolume snapshot root root37 #any name will do

Create a bootloader entry using grubby

  1. grubby --default-kernel
  2. grubby --default-title
  3. Using the information above about your system, construct a command using the example below:

grubby --add-kernel=/boot/vmlinuz-5.18.16-200.fc36.x86_64 --copy-default --title="Fedora Linux (5.18.16-200.fc36.x86_64) 36 (Workstation Edition) UPGRADE" --make-default

  1. There will be a message indicating an entry for this kernel exists and another is being created with ~custom appended to the name. You can confirm the new drop-in file with ls -l /boot/loader entries.
  2. grubby --remove-args="rootflags=subvol=root" --update-kernel=/boot/vmlinuz-5.18.16-200.fc36.x86_64 --config-file=/boot/loader/entries/64db766697e04179a6d3f7a67c2941e1-5.18.16-200.fc36.x86_64.0~custom.conf
  3. grubby --args="rootflags=subvol=root37" --update-kernel=/boot/vmlinuz-5.18.16-200.fc36.x86_64 --config-file=/boot/loader/entries/64db766697e04179a6d3f7a67c2941e1-5.18.16-200.fc36.x86_64.0~custom.conf
  • Optional 1: The gist is to dup the most recent /boot/loader/entries snippet, giving it a $NEWFILENAME, edit it and give it a new title, e.g. add "UPGRADE" to the end, and change the options line rootflags=subvol=root to whatever name you used for the snapshot, e.g. root37. So you can either do it CLI style or use grubby; or
  • Optional 2: You could even just reveal the hidden GRUB menu at boot time, and manually edit the default entry. You only need to do it twice. One boot to boot the snapshot and initiate the upgrade download. And the next boot to actually run the (offline) upgrade. The upgrade will install a new kernel package and its BLS snippet will contain the proper reference to root37 as the root subvolume to use (or whatever name you used).

One bootloader

As each Fedora performs updates, it'll step on the bootloader in /boot/efi. It shouldn't be a problem unless there are bugs and then "wheee!" So you could consider adding an exclude=grub2-* in the test instance's dnf.conf.

How to boot

  1. GRUB menu shows boot options: variant+version+kernelversion for switching between the two; or
  2. Use grubby --set-default=/boot/vmlinuz-5.18.15-200.fc36.x86_64

Limitations

  1. /boot is only 1GiB by default, so this is the limiting factor right now, how many Fedoras you can have installed at one time. Two is safe. Three is iffy unless
    1. configure the test Fedora instances' dnf.conf such exclude=kernel-*;
    2. consider deleting the "rescue" initramfs and kernel for the test instances, also removing dracut-config-rescue-056-1.fc36.x86_64 from them so these files aren't recreated; or
    3. put /boot on a Btrfs subvolume.
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