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How to rescue a broken Arch Linux install (btrfs edition)

A couple of days ago I had an issue with my Arch install: the system ran out of space on the boot drive while upgrading the kernel, and I was stuck with a broken system that wouldn't even be seen by the UEFI bios. Here's how I fixed it :D

A TL;DR is available at the bottom of the article! ♥︎

Prerequisites

  • An Arch Linux live USB (which you can get from the Arch Linux download page).
  • A working internet connection on the live environment, if you need to rerun post-install hooks.

Step 1: Boot from the Live USB

Sounds easy enough, you probably don't need me to tell you how to handle this :D

Step 2: Mount the arch install

First off, list all your drives with fdisk -l, so that you know what to pick.

$ fdisk -l
Disk /dev/sda: 223.57 GiB, 240057409536 bytes, 468862128 sectors
Disk model: KINGSTON SA400S3
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 3AF5C031-FC67-4A57-8786-2C6872DD8C6B

Device         Start       End   Sectors   Size Type
/dev/sda1       2048   1048575   1046528   511M EFI System
/dev/sda2    1050624 435306495 434255872 207.1G Linux filesystem
/dev/sda3  435306496 468860927  33554432    16G Linux swap

In this case, /dev/sda2 is the btrfs main partition (which will be mounted on /mnt), while /dev/sda1 is the EFI partition (which we'll mount on /mnt/boot).

This is where, with btrfs, it gets a bit tricky. See, normally you'd be able to mount the drive with mount /dev/sda2 /mnt... but with btrfs, you have to specify which subvolume to use. This can be done with the -o subvol=@ flag.

Thus, our final mount commands are as follows:

mount -o subvol=@ /dev/sda2 /mnt
mount /dev/sda1 /mnt/boot

Side note!

It might be useful to mount some additional virtual filesystems to allow docker to work. In that case, you can use the following mount commands:

mount -t proc /proc /mnt/proc
mount --rbind /sys /mnt/sys
mount --rbind /dev /mnt/dev
mount --rbind /run /mnt/run

We can then normally proceed by chrooting inside our installation with arch-chroot /mnt.

You'll probably want to run pacman -S linux to make it run all the postinstall hooks (mkinitcpio, etc...).

Wrapping it all up〔TL;DR〕

# Check which partitions to mount (in this case, /dev/sda1 and /dev/sda2)
fdisk -l

# Mount the aforementioned partitions (with -o subvol=@)
mount -o subvol=@ /dev/sda2 /mnt
mount /dev/sda1 /mnt/boot

# (Optional) Mount virtual FSes to allow things like Docker to work
mount -t proc /proc /mnt/proc
mount --rbind /sys /mnt/sys
mount --rbind /dev /mnt/dev
mount --rbind /run /mnt/run

# Chroot into the install
arch-chroot /mnt

# Run post-install hooks again
pacman -S linux

I hope this was useful to someone! :)