| 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.
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