Use Btrfs on Fedora Installed from a Raw Image

Oct 8, 2020 8 minutes to read

Fedora is moving to Btrfs for its default file system on its “desktop variants” in Fedora 33. This change is welcomed by some users because of some benefits Btrfs is offering. Other Fedora variants still default to ext4, although users may elect to use Btrfs during system installation with the Anaconda installer. But for non-desktop-variant raw image distributions like the Minimal aarch64 image, you do not have the chance to choose a file system other than ext4, because you are applying the image directly instead of using Anaconda. This post will show you how to use Btrfs on a Fedora installation derived from such image by converting the file system after the image is applied.

Reasons to Use Btrfs

Some advantages of Btrfs are already outlined in the Fedora Magazine article linked above. Personally, after using a Btrfs-based Fedora installation for a while on my laptop, I have found these benefits of Btrfs:

There will be no more headache of adjusting partition sizes when one partition is almost full but another one still has plentiful space.

Imagine if your /home partition is running out of space, but the / partition has some spare capacity, so you wish to shrink / and expand /home. In standard partitioning scheme, resizing partitions can be very hard. If LVM is used, adjusting partitions (i.e. logical volumes) can be done but still requires running some commands.

For Btrfs, you can create multiple subvolumes in a Btrfs partition for different mount points. /home and / will be placed in two subvolumes in the same Btrfs partition, but they can be managed like if they are on two real and different partitions. The coolest thing here is that /home and / can share the entire Btrfs partition’s space, thus the barrier between them no longer exists, and you will never find the need to shrink one partition to make space for another.
Btrfs allows you to take snapshots of subvolumes. Snapshots can be sent to another place, effectively allowing full-disk and full-system backups. The snapshots can be taken incrementally, which enables incremental backups that save spaces.

Disclaimer

I will try my best to ensure that the instructions in this post are accurate, but this can never be guaranteed. Before you proceed, please back up all the data you cannot afford to lose, and make sure you are aware of the procedure to correctly recover the backup. I will not be responsible for any data loss and hardware damage, even if they are caused by you accurately following the steps listed in this post.

Overview

The following instructions assume you have already installed a Fedora raw image to a disk, and you can operate on its partitions and files when Fedora on the disk is not running. For example, you can boot into another OS installed at a different location to operate on the Fedora installation from the same computer; or, if the disk is a removable storage device, you can plug it into another computer.

To use Btrfs, these tasks need to be completed:

Convert the root file system from ext4 to Btrfs
Modify OS files for the new file system
Relabel files on the root file system for SELinux

Convert File System to Btrfs

First, find out the device for the root partition of the disk. You may run lsblk -p to find out the correct device.

$ lsblk -p
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
/dev/sda      8:0    0 232.9G  0 disk
├─/dev/sda1   8:1    0   100M  0 part /boot/efi
├─/dev/sda2   8:2    0   512M  0 part /boot
├─/dev/sda3   8:3    0  71.2G  0 part /home
└─/dev/sda4   8:4    0 161.1G  0 part
/dev/sdb      8:16   0 238.5G  0 disk
├─/dev/sdb1   8:17   0   100M  0 part
├─/dev/sdb2   8:18   0    16M  0 part
├─/dev/sdb3   8:19   0 237.9G  0 part
└─/dev/sdb4   8:20   0 512.3M  0 part
/dev/sdc      8:32   1  29.8G  0 disk
├─/dev/sdc1   8:33   1   600M  0 part /run/media/leo/BD95-A5EF
├─/dev/sdc2   8:34   1     1G  0 part /run/media/leo/f25c31eb-a67b-46bb-a8b6-280
└─/dev/sdc3   8:35   1  28.2G  0 part /run/media/leo/b9c84f8b-74cc-4615-b8bb-59e
/dev/zram0  252:0    0     4G  0 disk [SWAP]

In the output of lsblk, find the disk entry whose size is similar to the disk containing Fedora installed from the image. In the sample output shown above, the disk is /dev/sdc. The root partition is the last and largest partition in the disk, which is /dev/sdc3.

With this information, you are ready to convert the root partition to Btrfs using the btrfs-convert program. Indicate the device for that partition in the program’s argument. If the device for the root partition is not shown as /dev/sdc3 in the output of lsblk, please change the argument accordingly. In addition, you need superuser privilege to use btrfs-convert, so you might need to invoke it with sudo.

# btrfs-convert /dev/sdc3

If you see an error saying the device is mounted, simply unmount it with this command, and run btrfs-convert again:

# umount /dev/sdc3

Modify OS Files

The file system for root partition is now Btrfs. However, because the file system conversion changes the partition’s UUID, operating system files that refer to the root partition by its UUID need to be modified.

Get Partition UUID After the Conversion

You need to get the new UUID in order to update those OS files. To do this, invoke lsblk -o +UUID.

$ lsblk -o +UUID
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT UUID
sda      8:0    0 232.9G  0 disk
├─sda1   8:1    0   100M  0 part /boot/efi  6122-EBE7
├─sda2   8:2    0  71.7G  0 part /home      b2c7f4cf-d75c-4f49-9a24-5f001e30d695
└─sda3   8:3    0 161.1G  0 part            E01A56741A564824
sdb      8:16   0 238.5G  0 disk
├─sdb1   8:17   0   100M  0 part            6CBE-049D
├─sdb2   8:18   0    16M  0 part
├─sdb3   8:19   0 237.9G  0 part            8A36C90E36C8FBE7
└─sdb4   8:20   0 512.3M  0 part            6CA8C978A8C940F6
sdc      8:32   1  29.7G  0 disk
├─sdc1   8:33   1   600M  0 part            3EC1-8F5F
├─sdc2   8:34   1     1G  0 part            5c7fb847-cc50-4d5c-92e8-0aa3ca60cc8d
└─sdc3   8:35   1  28.1G  0 part            368d0ec0-5f81-45f3-8dd1-786bd2e58c52
zram0  252:0    0     4G  0 disk [SWAP]

The UUID of the root partition sdc3 shown in this sample output is 368d0ec0-5f81-45f3-8dd1-786bd2e58c52. The partition UUID on your disk will be a different one, so please remember to replace this example UUID with the actual one you get when you are following the instructions below.

Update Partition UUID in OS Files

Once you know the new UUID, you can update the files as instructed below. Here are some things you should be wary of:

You need superuser privilege to modify those files, even though they are files that belong to another operating system which is off-line.
Make sure you are not editing the files of the currently running system. Do not use absolute paths like /etc/fstab if you are editing the files from a terminal.
Since the root partition was unmounted for Btrfs conversion before, you should remount it to edit files in it. For example:
```
# mkdir /mnt/rootfs
# mount /dev/sdc3 /mnt/rootfs
```

The modifications are:

etc/fstab under root partition

Edit the entry for / as follows:

Update UUID
Change file system type from ext4 to btrfs
Change the last two numbers at the end of line to 0 0

- UUID=80ddf2b2-a7c5-4e16-b520-b41a3d4feb3d /                       ext4    defaults        1 1
+ UUID=368d0ec0-5f81-45f3-8dd1-786bd2e58c52 /                       btrfs   defaults        0 0
  UUID=5c7fb847-cc50-4d5c-92e8-0aa3ca60cc8d /boot                   ext4    defaults        1 2
  UUID=3EC1-8F5F          /boot/efi               vfat    umask=0077,shortname=winnt 0 2

Any .conf file under the loader/entries directory in the partition for /boot, which is the second partition on the disk:

Change the value after root=UUID= under options
```
  title Fedora (5.8.15-301.fc33.aarch64) 33 (Thirty Three)
  version 5.8.15-301.fc33.aarch64
  linux /vmlinuz-5.8.15-301.fc33.aarch64
  initrd /initramfs-5.8.15-301.fc33.aarch64.img
- options root=UUID=80ddf2b2-a7c5-4e16-b520-b41a3d4feb3d ro rhgb quiet console=tty0
+ options root=UUID=368d0ec0-5f81-45f3-8dd1-786bd2e58c52 ro rhgb quiet console=tty0
  grub_users $grub_users
  grub_arg --unrestricted
  grub_class kernel
```
Please make sure the /boot partition is mounted too before editing this file. If the root partition is /dev/sdc3, then this partition will be /dev/sdc2.

If you have updated the Linux kernel before, there will be multiple .conf files under loader/entries, each for a single kernel version. You may choose to edit only one of them, like the one for the latest kernel for example, as long as you do not boot into other kernels whose .conf file is not modified.

Relabel File System for SELinux

The pointers to the root file system are updated, but at this stage, the system boot process still cannot complete because of issues with SELinux.

To resolve the issue, trigger a relabeling of files in the root file system by creating a file .autorelabel directly under the root file system:

# touch .autorelabel

The operating system, now residing in a Btrfs partition, is ready to boot at this moment. On the first boot after the file system conversion, you might see some systemd units that are failed to start, which is OK for the first time. Then, you should find a Starting Relabel all filesystems message, and this is when the files are being relabeled for SELinux.

$Boot message indicating file systems are being relabeled for\nSELinux$

This process may take a while. On my Raspberry Pi 4, it would take a few minutes to relabel all files in a fresh Fedora installation. After it completes, the system reboots automatically and should no longer emit errors when booting.

Enjoy Fedora installed from raw image on Btrfs!