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: After the system is upgraded, you can find this ~custom.conf file and delete it.
  • Optional 2: You could choose to reveal the hidden GRUB menu, and edit the default entry, modifying rootflags=subvol=root to match the name you used for the snapshot, e.g. root37. You'd do this twice: once to boot the root37 snapshot to download the upgrade RPMs; and the following boot after that so it boots root37 thus initiating the offline upgrade.

Reboot

  • If you used grubby, the --make-default option already made the "UPGRADE" menu entry the default so all you need to do is reboot.
  • If you didn't use grubby, use grub2-set-default 64db766697e04179a6d3f7a67c2941e1-5.18.16-200.fc36.x86_64.0~custom which happens to be the filename of the /boot/loader/entries you want to use without the *.conf ending
  • Once you reboot, confirm / is actually using the snapshot you made: mount | grep btrfs e.g.

/dev/vda3 on / type btrfs (rw,relatime,seclabel,compress=zstd:1,space_cache=v2,subvolid=257,subvol=/root) you should see subvol=/root37 or whatever you named the snapshot. If you see root as in this example, oops! You're still working on your original root, figure out what went wrong.

One bootloader

As each Fedora performs updates, it'll occasionally update the bootloaders on /boot/efi. This shouldn't be a problem. But then there's sometimes bugs, so if you don't want to be testing Rawhide bootloaders, consider addingexclude=grub2-* in the test instance's dnf.conf. The time to do this is after the first boot into root37 snapshot, before starting upgrade download.

Initiate the system upgrade

Either of these, just like you normally would:

Switching between the two roots

  • You'll need to reveal the GRUB hidden menu to make manual choices, OR
  • ls -l /boot to see a list of kernels so you can complete the path based on which release version you want to boot, e.g.
  • grubby --set-default=/boot/vmlinuz-5.18.15-200.fc36.x86_64 to boot Fedora 36
  • grubby --set-default=/boot/vmlinuz-5.19.0-65.fc37.x86_64 to boot Fedora 37

Limitations

  • /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.

How to delete the test instance

  1. Boot the menu entry for the instance you want to keep
  2. Confirm you've booted it mount | grep btrfs does the subvolume name match what you want to keep?
  3. btrfs sub list -t / ##find the ID in the left column that matches the subvolume snapshot name you want to delete
  4. btrfs sub del --subvolid $ID / ##Note that / is the mountpoint for the file system, it's not going to delete your currently running root, it'll delete the ID you specify.
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