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boot-from-snapshot.md

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+---
+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!