Having recently purchased a new laptop, it was a good opportunity to try out a new filesystem, btrfs. Just to complicate things I wanted to secure my system with whole drive encryption, so today we're throwing dm-crypt (LUKS) into the mix!


Installation media

First, download and write the installation image.

Cleaning hard drive

As the drive will be encrypted, we'll want to first fill it with random data. This is so that the encrypted areas are not easily identifiable.

cryptsetup open --type plain /dev/sda container
dd if=/dev/zero of=/dev/mapper/container

This took about 3hrs on my 500Gb drive

cryptsetup luksClose container


I'm going to be using a 500Mb EFI partition mounted as /boot and the rest of the disk for btrfs mounted on a dm-crypt partition. I have plenty of RAM so won't be using a swap partition for this setup (which also neatly avoids any issues with swap encryption).

gdisk /dev/sda
  • /dev/sda1 512Mb for /boot
  • /dev/sda2 remainder for btree filesystem

Filesystem and encryption

Set up dm-crypt container

I'm encrypting using sha512 instead of the default, sha1. Remember to increase iteration time when doing this, as each iteration will take longer with a stronger hash.

cryptsetup --hash sha512 --iter-time 2000 --use-random luksFormat /dev/sda2

Open the new encrypted container. I'm using the name "btree" which will open the container in /dev/mapper/btree.

cryptsetup luksOpen /dev/sda2 btree

Create filesystem

Create the filesystems on both partitions. I'm labelling my btrfs partition "honey". Bee tree. Get it? Never mind.

mkfs.fat -F32 /dev/sda1
mkfs.btrfs -L honey /dev/mapper/btree

Mount root of btrfs partition for use. I'm using the same mount options that I intend to use in fstab.

mkdir /broot
mount -o autodefrag,compress=lzo,noatime,space_cache /dev/mapper/btree /broot

Set up subvolumes

Enter the root of the btrfs filesystem:

cd /broot

There are no difinitive rules (as yet) around subvolume heirarchy. There seem to be two popular methods:

  • using __active (or __current) and __snapshots directories
  • using @ for root subvolume, @home for home subvolume, etc. directly in the root of the device

The first way seems to favour using __active as the root, so all subvolumes can be created underneath and therefore will be auto-mounted. The issue with this comes when you want to restore a snapshot of / (root) -- you need to move all the subvolumes first. The suggestion in the official wiki is to create both the root filesystem and other subvolumes on the same level, then mount the subvolumes separately in fstab, which is closer to the second method listed above.

I like both approaches: the subdirectories I find neater, but the flat structure allows for better root filesystem restores. I don't see why I can't combine both approaches -- so I will! First I'll create directories to hold the active and snapshot subvolumes (these don't need to be subvolumes themselves as I don't intend to mount them anywhere):

mkdir active snapshots

One thing to note is that when taking snapshots, we probably want to keep some paths out of the backup. The Suse documentation discusses each of these paths and the reason to have them as a separate subvolume. Of course, not all of these will be applicable to everyone and due to the nature of the COW in btrfs, we shouldn't worry too much about paths which aren't modified often (or by much). I've taken a look at the modified dates on these paths and determined that I ought to keep tmp, run, and var/log separate, along with /home of course. Arch creates tmpfs mounts for /run and /tmp so we don't need to worry about those.

btrfs subvolume create active/root
btrfs subvolume create active/home
btrfs subvolume create active/log

Mount subvolumes

Mount the root filesystem subvolume:

mount -o subvol=active/root /dev/mapper/btree /mnt

Create mount points and mount the boot partition and other subvolumes:

mkdir -p /mnt/{boot,home,var/log}
mount /dev/sda1 /mnt/boot
mount -o subvol=active/home /dev/mapper/btree /mnt/home
mount -o subvol=active/log /dev/mapper/btree /mnt/var/log

Install and config

Base system

pacstrap /mnt base btrfs-progs
genfstab -p /mnt >> /mnt/etc/fstab

Edit fstab if needs be -- in my case I had to change relatime to noatime.

Now arch-chroot into new system and set hostname, time zone, locale, etc.

Initial RAM disk

Add encrypt to the MODULES array in /etc/mkinitcpio.conf and create a new initial RAM disk:

mkinitcpio -p linux


systemd-boot (previously gummiboot) is my bootloader of choice and is already installed with systemd.

bootctl install
vi /boot/loader/loader.conf
  default  arch
vi /boot/loader/entries/arch.conf
  title          Arch Linux
  linux          /vmlinuz-linux
  initrd         /initramfs-linux.img
  options        cryptdevice=/dev/sda2:btree root=/dev/mapper/btree rootflags=subvol=active/root ro


Exit chroot, unmount partitions and reboot

umount -R /mnt