Title: How to boot on a BTRFS snapshot

	Title: How to boot on a BTRFS snapshot
	Author: Solène
	Date: 04 January 2023
	Tags: linux gentoo btrfs
	Description: In this article, you will learn how to boot from BTRFS
	snapshots

	# Introduction

	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.

	# Setup

	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:

	```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:

	```script
	$ 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.

	# Booting on a snapshot

	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.

	# Be careful

	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.

	# Conclusion

	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.

	# Going further

	There is a project grub-btrfs which can help you adding BTRFS snapshots
	as boot choices in GRUB menus.

	grub-btrfs GitHub project page