Difference between revisions of "Mkinitcpio-btrfs"

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(Preparing the root drive: : as of 2012.06.04 syslinux can boot from btrfs subvolume)
(Back up an existing Arch Linux installation)
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  # rsync -aAXv --exclude={dev,sys,proc,mnt,tmp,media} / <dest>/`date +%Y-%m-%d-%T`/
  # rsync -aAXv --exclude={dev,sys,proc,mnt,tmp,media} / <dest>/`date +%Y-%m-%d-%T`/
{{Warning| This is not correct! It will not copy all files which have the excluded strings *somwhere* in their path!}}
To restore from an rsync backup, just reverse the paths. It is also possible to use tar:
To restore from an rsync backup, just reverse the paths. It is also possible to use tar:

Revision as of 14:15, 18 March 2013

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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.


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


To get rollback features on your btrfs root device, you will need to install the mkinitcpio-btrfsAUR package from the Arch User Repository.

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

Note: The btrfs_advanced mkinitcpio hook (provided by mkinitcpio-btrfs package) is needed specifically for rollback and possibly other advanced features. The standard btrfs hook is enough to get multi-device (RAID) support. The kernel is capable of booting a single-device btrfs root on its own.

Back up an existing Arch Linux installation

Tip: 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 back up, 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 -aAXv --exclude={dev,sys,proc,mnt,tmp,media} / <dest>/`date +%Y-%m-%d-%T`/
Warning: This is not correct! It will not copy all files which have the excluded strings *somwhere* in their path!

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

# tar -cvJf <backup_file> --exclude="/proc/*" --exclude="/sys/*" \
--exclude="/mnt/*" --exclude="/media/*" --exclude="/dev/*" --exclude="/tmp/*" /

and to restore from a tar backup:

# tar -xvJf <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
Warning: 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. GRUB2#Install_to_Partition_or_Partitionless_Disk.The method used for the syslinux directions in this article does 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.
Warning: As of January 7,2012, Syslinux does not support multidevice btrfs root!!
Note: syslinux and GRUB2 allow booting from a drive without a separate boot partition. 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 GRUB2#GPT_specific_instructions. 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

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

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

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: please use the first argument of the template to provide a brief explanation. (Discuss in Talk:Mkinitcpio-btrfs#)

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

Note: The AIF is no longer supported. Installing manually is now the official way. See the Installation Guide for more info
Note: You may need to uncomment a mirror from /mnt/btrfs-active/etc/pacman.d/mirrorlist, and/or make other adjustments to /mnt/btrfs-active/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 base, btrfs-progs, and a bootloader (grub-bios, grub-efi, syslinux, etc). Switch <boot_loader> to your preferred boot loader.

Also install base-devel to build mkinitcpio-btrfs from the AUR.

# pacman -r . -Sy base btrfs-progs mkinitcpio-btrfs <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 /mnt/btrfs-active/etc/fstab:

# nano /mnt/btrfs-active/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.


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

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

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

Optional bind an empty folder over the /var/lib/btrfs-root/__active folder to prevent apps such as 'find' from complaining of any loops.


Download the mkinitcpio-btrfs tarball:

# wget -O /mnt/btrfs-active/root/mkinitcpio-btrfs.tar.gz https://aur.archlinux.org/packages/mk/mkinitcpio-btrfs/mkinitcpio-btrfs.tar.gz

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

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

Once you've chroot'ed into the btrfs installation, first install mkinitcpio-btrfs:

# cd /root
# tar -xvzf mkinitcpio-btrfs.tar.gz
# cd mkinitcpio-btrfs
# makepkg
# pacman -U mkinitcpio-btrfs-${version}.pkg.tar.xz

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 GRUB for advanced instructions. Once you have GRUB configured, and you 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 -o bind /dev/pts dev/pts
# mount -t proc /proc proc/
# mount -t sysfs /sys sys/
# chroot . /bin/bash

And now:

# grub-install --directory=/usr/lib/grub/i386-pc --target=i386-pc --boot-directory=/boot --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.


Syslinux now has an automatic installer that creates the necessary folders, sets bootflags, installs MBR etc. See Syslinux#Automatic_Install for more info. Make sure you have /mnt bind mounted then run:

# syslinux-install_update -iam

Then edit the syslinux.cfg:

Sample config:


LABEL arch
        LINUX ../vmlinuz-linux
        APPEND root=/dev/disk/by-label/btrfs-root rootflags=subvol=__active ro
        INITRD ../initramfs-linux.img

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 back up your kernel + initramfs before a snapshot. Upcoming releases of mkinitcpio-btrfsAUR 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.

Slow meta-data after installation

Do a defragmentation of / after the installation is done to fix the slow metadata issue. (a simple ls may take seconds)

# btrfs filesystem defrag /