Mkinitcpio-btrfs

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This article explains how to install Archlinux on a single btrfs device without a separate boot partition by using the extlinux bootloader. It notes the lack of swap file support. The mkinitcpio-btrfs initramfs hook providing rollback support is graced.

This setup does not support btrfs RAID because a separate boot partition/device is needed, ie. only one drive allowed. Pending releases of mkinitcpio-btrfs will attempt to provide solutions.

Warning: btrfs is experimental; article provided for academic use only.

Preparation

System to use for installation

It is recommended to boot and install using the lastest Archlinux image, another Archlinux system, or Archboot. The latter is capable of installing to a btrfs partition natively but does not support the unified setup detailed here; it is however a good option for this procedure because it includes recent builds of all the necessary packages.

btrfs-progs-unstable

To create a btrfs filesystem you will need the btrfs-progs-unstable package from [extra]. If using Archboot, it's already available.

Prepare hard drive

Create a partition for the installation, or allow btrfs to consume an entire device [recommended]. If you need swap support, either make a partition, or use the loop method detailed below. DO NOT simply use a swap file! Doing so will corrupt your btrfs filesystem. Mounting via loop device *probably* prevents this [unconfirmed].

Create/prepare the btrfs filesystem

Note: The following assumes you have dedicated an entire device to btrfs.
Tip: __active and __snapshot chosen for integration with current mkinitcpio-btrfs.
mkfs.btrfs -L btrfs-root /dev/sda
mkdir /btrfs
mount /dev/sda /btrfs
btrfs subvolume snapshot /btrfs /btrfs/__active
mkdir /btrfs/boot
mkdir /btrfs/__snapshot
mkdir /btrfs/__active/boot
mount -o subvol=__active /dev/sda /mnt
mkdir -p /mnt/var/lib/btrfs-root
mount --move /btrfs /mnt/var/lib/btrfs-root
mount --bind /mnt/var/lib/btrfs-root/boot /mnt/boot

Take your time, it's not difficult but everything must succeed... you now have a btrfs subvolume (__active, your system vol) mounted at /mnt, for final use as /. The last two steps --move the btrfs root to it's final location, and then --bind mounts (btrfs-root)/boot to /mnt/boot. For now, /boot MUST NOT be within a subvolume; syslinux cannot directly boot, or properly look within, subvolumes.

Installing the system

My favorite way of installing Archlinux, the manual installation :-)

Base

Note: You may need to uncomment a mirror from /mnt/etc/pacman.d/mirrorlist, and/or make other adjustments to /mnt/etc/pacman.conf.
cd /mnt
mkdir -p dev proc sys var/lib/pacman
mount -o bind /dev dev
mount -t proc none proc
mount -t sysfs none sys
pacman -r . -Sy base syslinux --ignore grub
# --ignore is currently broken for groups, use ...
# pacman -r . -Rd grub
# ... to remove grub

btrfs-progs-unstable

Don't forget to also install the btrfs-progs-unstable package onto the new system. Without doing so, users will not be able to anything interesting with the filesystem.

AUR Packages

If you like, build mkinitcpio-btrfs and install with:

pacman -r /mnt -U mkinitcpio-btrfs

After installing, add btrfs_advanced to the HOOKS section of /mnt/etc/mkinitcpio.conf, then rebuild the initramfs:

nano /mnt/etc/mkinitcpio.conf
chroot /mnt mkinitcpio -p kernel26
Tip: btrfs_advanced is needed specifically for rollback or multi-device (RAID) support, and the standard btrfs hook is needed only for multi-device support. The kernel is capable of booting a single-device btrfs root on it's own (see below).

/etc/fstab

Tip: Locations chosen for integration with upcoming release of mkinitcpio-btrfs

Add everything to /mnt/etc/fstab ... (btrfs-root)/boot will be --bind mounted so kernel upgrades work:

/dev/disk/by-label/btrfs-root  /                    btrfs  defaults,noatime             0  0
/dev/disk/by-label/btrfs-root  /var/lib/btrfs-root  btrfs  defaults,noatime,subvolid=0  0  0
/var/lib/btrfs-root/boot       /boot                none   bind                         0  0

A properly functional fsck.btrfs is not available yet.

It is not necessary to add a subvol=XX option for /, as it must be handled by either the initramfs [btrfs_advanced] or via the kernel boot line [extlinux], and it may not be accurate anyways (e.g. rollback mode). subvolid=0 corresponds to the btrfs root, and guarantees it will always be mounted correctly, even if the default is changed.

Installing the bootloader

cd /mnt
mkdir boot/extlinux
cp usr/lib/syslinux/*.c32 boot/extlinux
cat usr/lib/syslinux/mbr.bin > /dev/sda
chroot . extlinux --install /var/lib/btrfs-root/boot/extlinux
nano boot/extlinux/extlinux.conf

Sample extlinux.conf:

PROMPT 0
TIMEOUT 0
DEFAULT arch

LABEL arch
    KERNEL /boot/vmlinuz26
    INITRD /boot/kernel26.img
    APPEND root=/dev/disk/by-label/btrfs-root rootflags=subvol=__active ro

If you ARE using the mkinitcpio hook, remove rootflags=subvol=__active from the APPEND line.

Final

Almost done... be sure to edit /mnt/etc/rc.conf, set the root password, and similar. Good luck on reboot!

cd /
umount /mnt/{dev,proc,sys,boot,var/lib/btrfs-root}
umount /mnt
reboot
Warning: Rollback support, while fully functional, currently has one important caveat... it cannot handle kernel rollbacks because the kernel is not a part of the snapshot. This is due to the limitations of the bootloader described above. Until bootloaders support subvolumes, you must remember to manually backup your kernel + initramfs before a snapshot. Upcoming releases of mkinitcpio-btrfs will attempt to address this shortcoming.

Known problems

  • syslinux operates successfully through times, but sometimes fails