I always wanted to have a simple rollback method on Linux systems, NixOS gave me a full featured one, but it wasn't easy to find a solution for other distributions.
Fortunately, with BTRFS, it's really simple thanks to snapshots being mountable volumes.
You need a Linux system with a BTRFS filesystem, in my examples, the root subvolume (where `/` is) is named `gentoo`.
I use `btrbk` to make snapshots of `/` directly in `/.snapshots`, using the following configuration file:
snapshot_preserve_min 30d
volume /
snapshot_dir .snapshots
subvolume .
With a systemd service, it's running once a day, so I'll have for 30 days of snapshots to restore my system if needed.
This creates snapshots named like the following:
$ ls /.snapshots/ ROOT.20230102 ROOT.20230103 ROOT.20230104
A snapshot address from BTRFS point of view looks like `gentoo/.snapshots/ROOT.20230102`.
I like btrbk because it's easy to use and configure, and it creates easy to remember snapshots names.
When you are in the bootloader (GRUB, systemd-boot, Lilo etc..), edit the command line, and add the new option (replace if already exists) with the following, the example uses the snapshot `ROOT.20230102`:
rootflags=subvol=gentoo/.snapshots/ROOT.20230103
Boot with the new command line, and you should be on your snapshot as the root filesystem.
When you are on a snapshot, this mean any change will be specific to this volume.
If you use a separate partition for `/boot`, an older snapshot may not have the kernel (or its module) you are trying to boot.
This is a very simple but effective mecanism, more than enough to recover from a bad upgrade, especially when you need the computer right now.
There is a project grub-btrfs which can help you adding BTRFS snapshots as boot choices in GRUB menus.