From ArchWiki
Revision as of 01:04, 19 February 2012 by Demizer (talk | contribs) (Rewrite article.)
Jump to: navigation, search

This template has only maintenance purposes. For linking to local translations please use interlanguage links, see Help:i18n#Interlanguage links.

Local languages: Català – Dansk – English – Español – Esperanto – Hrvatski – Indonesia – Italiano – Lietuviškai – Magyar – Nederlands – Norsk Bokmål – Polski – Português – Slovenský – Česky – Ελληνικά – Български – Русский – Српски – Українська – עברית – العربية – ไทย – 日本語 – 正體中文 – 简体中文 – 한국어

External languages (all articles in these languages should be moved to the external wiki): Deutsch – Français – Română – Suomi – Svenska – Tiếng Việt – Türkçe – فارسی

Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary text Template:Article summary heading Template:Article summary link Template:Article summary link Template:Article summary link Template:Article summary link Template:Article summary link Template:Article summary end

btrfs provides a number of features (such as checksumming, snapshots, and subvolumes) that make it an excellent candidate for using it as the root partition in an Arch Linux installation. For a tour of btrfs features see A tour of btrfs. As is common with FLOSS, there are a number of ways to configure btrfs as the root filesystem.

This article assumes your are doing a fresh install from the Arch Linux installation media. It is also possible to create a backup of an existing Arch Linux installation and restore it to a fresh btrfs configuration. This technique is also shown in this article.

Note: This article is for more advanced Arch Linux users, but if you are a newbie and are one with the command line, you shouldn't have any trouble as long as you are determined
Warning: btrfs is considered experimental, with caveats. The filesystem itself is considered stable, but as of February 17, 2012 there is still no reliable way to recover from errors as the fsck tool is under heavy development. [1]. Chris Mason, the creator and main developer of btrfs suggests the fsck tool should be ready by the end of February 2012 [2] [3]


To create a btrfs filesystem you will need to install the btrfs-progs from Official Repositories. If using Archboot, it's already available.

# pacman -S btrfs-progs


To get rollback features on your btrfs root device, you will need to install the mkinitcpio-btrfsAUR package from the Arch User Repository. This example uses Yaourt, as it is the easiest to setup:

# yaourt -S mkinitcpio-btrfs

For reference, here are the internal variables used by mkinitcpio-btrfsAUR:

Note: 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.

Backup an existing Arch Linux installation

Skip this section if you are doing a fresh installation.

Having a complete backup strategy before messing with the partitioning of storage drives is vitally important. So before doing anything you must backup, and you must also know how to recover from a backup. There are of course a number of ways to do this. But the easiest by far is to mount a spare backup drive and use rsync:

# rsync -avhHPS --exclude={dev,sys,proc,mnt,tmp,media} / <dest>/`date +%Y-%m-%d-%T`/

To restore from an rsync backup, just reverse the paths. It is also possible to use tar:

# tar -cvpPJf $BACKUPDIR/$FILENAME --exclude="/proc/*" --exclude="/sys/*" \
--exclude="/mnt/*" --exclude="/media/*" --exclude="/dev/*" --exclude="/tmp/*" /

and to restore from a tar backup:

# tar -xvpJf <backup_file> -C <root>

Boot into a live image

It is recommended that you boot and install from the latest Archlinux Netinstall Image, or and up-to-date Archlinux installation, 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.

Preparing the root drive

Now it is time to actually install the btrfs filesystem. Please make sure that btrfs-progs is installed and functioning correctly:

# btrfs --help
Note: 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.
Note: It is possible to allow btrfs to use the entire hard drive without using a partitioning scheme, but that is not recommended. [4]. The method used for the syslinux directions in this article do not use a partitioning system. This portion of the article will be updated in the future, but until then you can study the GUID Partition Table + syslinux installation method here.
Note: syslinux and GRUB2 allow booting from a drive without a separate boot partition. However, syslinux does not currently support boot directories within a btrfs subvolume. Both syslinux and GRUB2 allow the use of modern GPT partitioning.

GPT partitioning for GRUB2

To get started partitioning, we must install the gptfdisk package from the Official Repositories:

# pacman -S gptfdisk

GRUB2 requires a +2M boot partition at the front of the drive [5]. You can create that with gdisk:

# gdisk /dev/sda

Once gdisk has loaded, you can begin entering commands into the command prompt. Type ? to see the available commands. Press o to create a new partition table. Just use the help command and create two partitions and then use p to verify the output:

GPT fdisk (gdisk) version 0.8.2

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): p
Disk /dev/sda: 234441648 sectors, 111.8 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): C75A01F9-D34E-4895-9AAB-2C8118118211
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 234441614
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048            6143   2.0 MiB     EF02  BIOS boot partition
   2            6144       234441614   111.8 GiB   8300  Linux filesystem

Command (? for help):
Note: Notice the EF02 hex code for the partition type for the 2Mb GRUB2 boot partition. This is needed to allow GRUB2 to install the core.img file into that partition for booting

Once you have created the partitions, we will need to set the boot flag on the btrfs partition. Press x for advanced options and then press a to set attributes. Press the number for the partition you want to alter, in this example it is 2. Now press 2 to set the legacy boot flag:

Command (? for help): x

Expert command (? for help): a
Partition number (1-2): 2
Known attributes are:
0: system partition
1: hide from EFI
2: legacy BIOS bootable
60: read-only
62: hidden
63: do not automount

Attribute value is 0000000000000000. Set fields are:
  No fields set

Toggle which attribute field (0-63, 64 or <Enter> to exit): 2
Have enabled the 'legacy BIOS bootable' attribute.
Attribute value is 0000000000000004. Set fields are:
2 (legacy BIOS bootable)

Toggle which attribute field (0-63, 64 or <Enter> to exit):

Finally, press w to write the data to the disk.

GPT partitioning for Syslinux

This section hasn't been written yet. For a quick tutorial go here;

Configure the btrfs filesystem

Note: This section makes reference to btrfs snapshots. Please be advised that the methods described herein for working with snapshots are [unconfirmed].
Warning: Since the initial rewrite of this article, the details regarding syslinux with btrfs are [unconfirmed].

With the drive partitioned and ready for a filesystem, make sure btrfs-progs is installed and create the filesystem on the root partition:

# mkfs.btrfs -L btrfs-root /dev/sda2

Now it is time to mount the new filesystem. First create the mount directory:

# mkdir /mnt/btrfs-root

and mount the partition:

# mount -o defaults,noatime,discard,ssd /dev/sda2 /mnt/btrfs-root && cd /mnt/btrfs-root
Note: If you are not using an SSD drive, remove discard,ssd from the mount options.

Now you should be in the newly mounted btrfs filesystem! If you plan on using the rollback features of btrfs, create a __snapshot directory for containing snapshots:

# mkdir __snapshot
Note: __snapshot is chosen for integration with current mkinitcpio-btrfsAUR. See the variable reference above, or the source mkinitcpio-btrfs package.

If you plan on using Syslinux as your boot loader, you will need to create a boot directory:

# mkdir boot
Note: Syslinux currently does not support boot directories within btrfs subvolumes. For this reason a boot directory must be created in the root directory of the btrfs filesystem and bound to a boot directory within the subvolume.

Next we will create the subvolumes. To see all the commands available to manage subvolumes with btrfs:

# btrfs subvolume --help

To allow us to easily snapshot the root of our Arch Linux installation, it is recommended it be contained in a subvolume named __active:

Note: __active is chosen for integration with current mkinitcpio-btrfsAUR. See the variable reference above, or the source mkinitcpio-btrfs package.
# btrfs subvolume create __active && cd __active

Now we will create separate subvolumes for important FHS directories. This would allow us to monitor and adjust the size allocations for each subvolume:

# btrfs subvolume create home
# btrfs subvolume create var
# btrfs subvolume create usr

To see your handy work:

# btrfs subvolume list -p .
ID 256 parent 5 top level 5 path __active
ID 258 parent 256 top level 5 path __active/home
ID 259 parent 256 top level 5 path __active/usr
ID 260 parent 256 top level 5 path __active/var

Fix the permissions:

chmod 755 ../\__active var usr home

Now that the subvolumes are created and we have a layout, lets mount the root subvolume directly:

# mkdir /mnt/btrfs-active
# mount -o defaults,discard,noatime,ssd,subvol=__active /dev/sda2 /mnt/btrfs-active && cd /mnt/btrfs-active
Note: If you are not using an SSD drive, remove discard,ssd from the mount options.

And that's it! You should be ready to install a fresh copy of Arch Linux or restore from a backup. For example, if you made a tar backup with the techniques described above, simply:

# tar -xvpJf <backup_file> -C .

or if using rsync:

# rsync -avhHPS --exclude={dev,sys,proc,mnt,tmp,media} <backup_dir> .

btrfs and syslinux

Warning: This section is [unconfirmed].

In order to continue the installation (or restoration), boot in the root of the btrfs filesystem must be bound to the boot directory contained in the __active subvolume. First we should create a boot directory within the subvolume:

# mkdir /mnt/btrfs-active/boot

and then bind the two directories using mount:

# mount --bind /mnt/btrfs-root/boot /mnt/btrfs-active/boot

Installing the base system

Warning: This section is [unconfirmed].

We will not be relying on AIF to do the install automatically and instead perform it manually.

Note: You may need to uncomment a mirror from /mnt/etc/pacman.d/mirrorlist, and/or make other adjustments to /mnt/etc/pacman.conf.

Lets begin by creating the necessary directories:

# mkdir -p dev proc sys var/lib/pacman

Now we bind the system directories to our installation directory:

# mount -o bind /dev dev/
# mount -t proc /proc proc/
# mount -t sysfs /sys sys/

Install the base and bootloader (grub2-bios, grub2-efi, syslinux). Switch <boot_loader> to your preferred boot loader:

# pacman -r . -Sy base <boot_loader> --ignore grub


Once the you have successfully configured btrfs and installed (or recovered) your Arch Linux, it is important that we tweak some settings to get everything working together so when you reboot everything will work correctly.


This section has two different configurations depending on the bootloader you selected. We can begin by editing /etc/fstab:

# nano /etc/fstab


Add the following line, supplement <uuid> for the UUID of the btrfs root partition:

UUID=<uuid> / btrfs   defaults,noatime,discard,ssd,subvol=__active 0 0
Note: If you are not using an SSD drive, remove discard,ssd from the mount options.


Warning: Since the initial rewrite of this article, the details regarding syslinux are [unconfirmed].
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

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.


Add btrfs_advanced to the HOOKS section of /mnt/etc/mkinitcpio.conf:

# nano /mnt/etc/mkinitcpio.conf
HOOKS="[...] btrfs_advanced"

then rebuild the initramfs:

# mkinitcpio -p linux

Installing the bootloader

If everything went smoothly (when does it ever?), you should have the great pleasure of installing your bootloader into the mbr of your root drive.


See GRUB2 for advanced instructions. Once you have grub configured, and you arch chrooted into your install directory, install the boot loader:


If you decided that you liked your current Arch Linux and are restoring from a backup, you will need to chroot into your installation after doing all the necessary steps:

# cp /etc/resolv.conf etc/
# mount -o bind /dev dev/
# mount -t proc /proc proc/
# mount -t sysfs /sys sys/
# chroot . /bin/bash

and now:

# grub2_bios-install --boot-directory=/boot --no-floppy --recheck /dev/sda

Don't forget to update /boot/grub/grub.cfg:

# grub-mkconfig -o /boot/grub/grub.cfg

You should now be able to reboot into grub, provided there were no problems encountered doing the steps above.


Warning: This section is [unconfirmed].
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:


LABEL arch
    KERNEL /boot/vmlinuz-linux
    INITRD /boot/initramfs-linux.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 configuration

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

# reboot

Known issues

Rollback support

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.}}


  • syslinux operates successfully through times, but sometimes fails.
  • Installing syslinux with compression enabled, or editing the configuration file after enabling compression, causes boot to fail because syslinux cannot read compressed files. This probably also affects installing a new kernel with btrfs enabled but I haven't tested this.