How To Convert An ext3/ext4 Root File System To btrfs

http://www.howtoforge.com/how-to-convert-an-ext3-ext4-root-file-system-to-btrfs-on-ubuntu-12.10


ext3 and ext4 file systems can be converted to btrfs. For non-root file systems, this can be done online (i.e., without reboot), while for root file systems we need to boot into some kind of rescue system or Live CD. This guide explains how to convert an ext3 or ext4 root file system into btrfs on Ubuntu 12.10 and how to roll back to ext3/ext4 again if desired.
I do not issue any guarantee that this will work for you!

1 Preliminary Note

I'm using a system here with one large / partition (i.e., no /boot partition) and without LVM. During initial installation, it was installed with the option Guided - Use entire disk. For different partition schemes, the procedure might differ.
My hard drive is named /dev/sda in this tutorial, my system partition is /dev/sda1.
I will use an Ubuntu 12.10 Desktop Live-CD as the rescue system throughout this tutorial.
I will show two ways of doing the conversion: one where we simply convert the system partition and change /etc/fstab, and one where we create the subvolumes @ and @home according to Ubuntu's btrfs partition layout (see https://help.ubuntu.com/community/btrfs) - this is slightly more complicated, but a must if you want to use apt-btrfs-snapshot which requires this subvolume layout.
A note for Ubuntu users:
Because we must run all the steps from this tutorial with root privileges, we can either prepend all commands in this tutorial with the string sudo, or we become root right now by typing

2 Install btrfs-tools

On the original system, before we boot into the rescue system, install btrfs-tools so that the package is available when we chroot to the system partition in the rescue system (it is possible that we don't have a network connection for installing the package when we are chrooted into the system partition in the rescue system, that's why we should install it now):
Now we must boot into some kind of rescue system with btrfs support. For example, you can insert the Ubuntu 12.10 Desktop CD into the CD drive (make sure it has the same architecture - i386 or x86_64 - as this system) and reboot:

3 Doing A Simple Conversion (No @ And @home Subvolumes)

In the rescue system, log in as root. Make sure that btrfs-tools are installed:
Do a file system check...
... and then run the conversion tool:
Next we mount our system partition and chroot to it:
As you see, there's now a folder called ext2_saved which contains an image of our system partition before the conversion (with the original ext3 or ext4 file system). This image can be used to do a rollback later on.
Run
We need the UUID from the output for modifying /etc/fstab:
Comment out the old / partition line and add a new one. Replace the UUID with the UUID from the blkid output, then replace ext4 (or ext3) with btrfs, and finally replace the mount options (e.g. errors=remount-ro) with the string defaults:

# /etc/fstab: static file system information. # # Use 'blkid' to print the universally unique identifier for a # device; this may be used with UUID= as a more robust way to name devices # that works even if disks are added and removed. See fstab(5). # # # / was on /dev/sda1 during installation #UUID=ad50ef37-797d-44ea-a8fa-ae61abe4d00f / ext4 errors=remount-ro 0 1 UUID=63accb30-95b9-4268-ae1e-6d0ad3ef3a9d / btrfs defaults 0 1 # swap was on /dev/sda5 during installation UUID=4dc578f3-c65c-4013-b643-72e70455b21b none swap sw 0 0

Next open /etc/grub.d/00_header...
... and comment out line 93 (if [ -n "\${have_grubenv}" ]; then if [ -z "\${boot_once}" ]; then save_env recordfail; fi; fi):

[...] function recordfail { set recordfail=1 #if [ -n "\${have_grubenv}" ]; then if [ -z "\${boot_once}" ]; then save_env recordfail; fi; fi } [...]

If you don't do this, you will get the error...
... when you boot from the btrfs file system, and you have to press ENTER to proceed with the boot process (see Ubuntu 12.10 + btrfs: error: sparse file not allowed).
Next run
and exit from the chroot:
Reboot into the normal system (make sure you remove the Live CD from the CD drive):
If everything goes well, the system should come up without problems, now running on btrfs.
As you see, there's still the ext2_saved folder with the image of the original system in case you want to do a rollback:
In fact, this is not a folder, it's a btrfs subvolume:
If you are sure you want to stay with btrfs and don't want to do a rollback, you can delete that subvolume to free up some space:
Afterwards, the image should be gone:

4 Doing A Conversion According To Ubuntu Subvolume Layout (With @ And @home Subvolumes)

This is a bit more complicated and took me some time to figure out. It is absolutely necessary to follow each step in the same order as described!
In the rescue system, log in as root. Make sure that btrfs-tools are installed:
Do a file system check...
... and then run the conversion tool:
Next we mount our system partition and chroot to it:
As you see, there's now a folder called ext2_saved which contains an image of our system partition before the conversion (with the original ext3 or ext4 file system). This image can be used to do a rollback later on.
Run
We need the UUID from the output for modifying /etc/fstab:
Comment out the old / partition line and add a new one. Replace the UUID with the UUID from the blkid output, then replace ext4 (or ext3) with btrfs, and finally replace the mount options (e.g. errors=remount-ro) with the string defaults,subvol=@ - this makes the system boot from the subvolume @ (which is to e created yet) instead of the top-level volume:

# /etc/fstab: static file system information. # # Use 'blkid' to print the universally unique identifier for a # device; this may be used with UUID= as a more robust way to name devices # that works even if disks are added and removed. See fstab(5). # # # / was on /dev/sda1 during installation #UUID=ad50ef37-797d-44ea-a8fa-ae61abe4d00f / ext4 errors=remount-ro 0 1 UUID=d6c9b57b-caa1-4a88-b659-930c130b337f / btrfs defaults,subvol=@ 0 1 # swap was on /dev/sda5 during installation UUID=4dc578f3-c65c-4013-b643-72e70455b21b none swap sw 0 0

Next open /etc/grub.d/00_header...
... and comment out line 93 (if [ -n "\${have_grubenv}" ]; then if [ -z "\${boot_once}" ]; then save_env recordfail; fi; fi):

[...] function recordfail { set recordfail=1 #if [ -n "\${have_grubenv}" ]; then if [ -z "\${boot_once}" ]; then save_env recordfail; fi; fi } [...]

If you don't do this, you will get the error...
... when you boot from the btrfs file system, and you have to press ENTER to proceed with the boot process (see Ubuntu 12.10 + btrfs: error: sparse file not allowed).
Before we update the GRUB boot loader, we must make sure that it will include the boot option rootflags=subvol=@. Check the output of
If it looks like this (i.e., it contains rootflags=subvol=@), everything is ok, and you can proceed with the update-grub command a few lines below:
But if the output looks as follows (no appearance of rootflags=subvol=@)...
... we must modify /etc/grub.d/10_linux:
Comment out lines 67 and 68 and add rootsubvol="@" in line 69:

[...] case x"$GRUBFS" in xbtrfs) #rootsubvol="`make_system_path_relative_to_its_root /`" #rootsubvol="${rootsubvol#/}" rootsubvol="@" if [ "x${rootsubvol}" != x ]; then GRUB_CMDLINE_LINUX="rootflags=subvol=${rootsubvol} ${GRUB_CMDLINE_LINUX}" fi;; xzfs) rpool=`${grub_probe} --device ${GRUB_DEVICE} --target=fs_label 2>/dev/null || true` bootfs="`make_system_path_relative_to_its_root / | sed -e "s,@$,,"`" LINUX_ROOT_DEVICE="ZFS=${rpool}${bootfs}" ;; esac [...]

Now we continue with updating the GRUB boot loader:
Now we create the @ subvolume as a snapshot of the top level volume:
Then we create the @home subvolume...
... and copy the contents from /home to it and make sure that /home and /@/home (which contains the same contents as /home because /@ is a snapshot of /) are empty so that the @home subvolume can be mounted to /home when we reboot the system:
Then open /etc/fstab...
... and add the line UUID=d6c9b57b-caa1-4a88-b659-930c130b337f /home btrfs defaults,subvol=@home 0 2 (make sure you use the same UUID as for the / partition!) to it:

# /etc/fstab: static file system information. # # Use 'blkid' to print the universally unique identifier for a # device; this may be used with UUID= as a more robust way to name devices # that works even if disks are added and removed. See fstab(5). # # # / was on /dev/sda1 during installation #UUID=ad50ef37-797d-44ea-a8fa-ae61abe4d00f / ext4 errors=remount-ro 0 1 UUID=d6c9b57b-caa1-4a88-b659-930c130b337f / btrfs defaults,subvol=@ 0 1 UUID=d6c9b57b-caa1-4a88-b659-930c130b337f /home btrfs defaults,subvol=@home 0 2 # swap was on /dev/sda5 during installation UUID=4dc578f3-c65c-4013-b643-72e70455b21b none swap sw 0 0

Next copy the modifed fstab to our @ subvolume:
Leave the chroot:
Unmount /mnt and mount the @ subvolume to it:

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