Multiple Operating Systems On Single Btrfs Partition

February 24, 2025

The goal

My scenario is that I want to “reinstall” my ArchLinux, but I don’t want to completely replace my installation, instead I want to add another instance of said ArchLinux. This method doesn’t involve external drives. We will add new btrfs subvolume and install new instance of ArchLinux there.

The process

Analyzing partition situation

Before we start installing, let’s examine present setup. In this case I have 2 partitions:

EFI Partition - where bootloader is located
Root partition - where our OS is installed

# fdisk -l
Disk /dev/nvme0n1: 1.82 TiB, 2000398934016 bytes, 3907029168 sectors
Disk model: [...]                       
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 329DE734-DECF-40F1-A227-5C13DE22E5D7

Device           Start        End    Sectors  Size Type
/dev/nvme0n1p1    2048    1230847    1228800  600M EFI System
/dev/nvme0n1p2 1230848 3907028991 3903700992  1.8T Linux filesystem

Create new btrfs subvolume

Mount the /dev/nvme0n1p2 device on /mnt folder

# mount /dev/nvme0n1p2 /mnt
$ ls /mnt
home root var

Notice the existing home root and var btrfs subvolumes.

Create new subvolume

# btrfs subvolume create /mnt/newroot
# umount /dev/mnt

Install ArchLinux

Mount newly created subvolume along with boot partition and home subvolume and proceed with instructions on ArchLinux wiki.

# mount /dev/nvme0n1p2 -o subvol=newroot,compress=zstd /mnt
# mkdir /mnt/home /mnt/boot
# mount /dev/nvme0n1p1 /mnt/boot
# mount /dev/nvme0n1p2 -o subvol=home,compress=zstd /mnt/home

At the end, don’t forget to change your bootloader’s parameters. In my case, I copied the contents of /boot/loader/entries/arch.conf file into /boot/loader/entries/arch2.conf. And modified the subviolume target in boot options as seen below.

Before

title	Arch Linux
linux	/vmlinuz-linux-zen
initrd	/intel-ucode.img
initrd	/initramfs-linux-zen.img
options	root=UUID=d0aeca77-5f3e-4122-b523-8bd9d18b186f rootflags=subvol=/@root quiet rw

After

title	Arch Linux
linux	/vmlinuz-linux-zen
initrd	/intel-ucode.img
initrd	/initramfs-linux-zen.img
options	root=UUID=d0aeca77-5f3e-4122-b523-8bd9d18b186f rootflags=subvol=/@newroot quiet rw

which could be simplified by this command

# sed 's/root/newroot/g' /boot/loader/entries/arch.conf > /boot/loader/entries/arch2.conf'

Conclusion

You should have now two fully functioning installations of your operating systems. This setup didn’t require creating new partitions nor preparing external drive. All of it was done from existing ArchLinux installation.