---
title: Restoring Root Filesystem from BtrFS Snapshot
description: Undo your grave mistakes quickly and efficiently. Yay!
published: true
date: 2022-04-30T20:10:46.302Z
tags: btrfs, recovery, rootfs
editor: markdown
dateCreated: 2021-10-20T05:15:59.558Z
---

If you frigged up *really* bad, you likely cannot read this, which is ironic.

However, I'll write it down so it hopefully sinks in the ol' noggin.

# Booting into snapshots
I configured the system to use `snapper`, a tool from OpenSUSE, which takes and manages BtrFS snapshots. Thanks to the `grub-btrfs` plugin, these root filesystem snapshots are accesable from the GRUB2 boot menu.

First, select whichever snapshot (they are neatly dated) by following the `Gentoo snapshots` option in the boot menu.

# Making snapshots writable

By default, snapshots are read only. They should still boot neatly, but lots of things will not work (like NTP and most compiles).

Take a good look around - do you like this snapshot? is this a nice snapshot? would you like to settle down with this snapshot?

If so, run:
`btrfs property set -ts / ro false`
when `/` is the path the snapshot is currently mounted at (use `findmnt -nt btrfs` to find out).

> Once a snapshot is writeable, consider it compromised - booting from it again if you change your mind may not be as smooth!
{.is-warning}

# Setting snapshot as new root
First, find the snapshot ID by runnnig `findmnt -nt btrfs` - which will give you something like this:

```
root@Takahe] ~ (takahe-root) 
❯ findmnt -nt btrfs                                
/                     /dev/nvme0n1p2[/.snapshots/232/snapshot] btrfs  rw,noatime,compress=zstd:11,ssd,d
```

Now, `grep` the path in the square brackets from the output of `btrfs snapshots list /`:
```
[root@Takahe] ~ (takahe-root) 
❯ btrfs subvolume list / | grep .snapshots/232/snapshot                                              
ID 8624 gen 2199024063871 top level 2740 path .snapshots/232/snapshot
```
Noticed how I removed the leading `/`.

Now, take the `ID` of the snapshot, and set it as the new default:
```
[root@Takahe] ~ (takahe-root) 
❯ btrfs subvolume set-default 8624 /
```

> The BtrFS default is built into the filesystem - it overrides the value on `/etc/fstab`.
{.is-info}

Finally, update the bootloader to enable the default entry to boot into your snapshot:
`grub-mkconfig -o /boot/grub/grub.cfg`

This should be it - but you can double check if you're really keen by reading that file and looking for something like this in the kernel parameters:
```
linux   /vmlinuz-5.14.10-gentoo-x86_64 root=UUID=d9699f1e-6825-4b38-9e3a-c228763b07cc ro rootflags=subvol=.snapshots/232/snapshot
```
See the `rootflags`? that's us, baby.


Happy compiling!