Move the root from one subvolume to another in BTRFS

Bob

3/3/24, 7:26 AM

I want to install Ubuntu in the BTRFS @root subvolume, but the Ubuntu installer does not allow changing the default subvolumes and installs in @ and @home.

I'm trying to create a @root subvolume after installation, but I have a problem. I need to edit fstab and GRUB. GRUB needs to know where to find the new root. I've read that the community edits /boot/grub/grub.cfg but some people say it's unsafe because /boot/grub/grub.cfg will be overwritten on the next kernel update or when you run update-grub.

How can I move the root from one subvolume to another?

562

1 + 4

grub2

btrfs

Pilot6

3/3/24, 7:47 AM

Do you want to keep separate `@home`? In this case it is much easier.

Bob

3/3/24, 8:38 AM

@Pilot6 It's not matter. The main goal move root to new subvolume `@root`. In general I think about structure `@root`, `@home`, `@swap`.

Pilot6

3/3/24, 9:40 AM

So the only point is to change the name. You can simply rename the subvolume and update-grub as mentioned in my answer. No need to copy or move anything. But it makes no sense. You'll have `/home` at `@home`, `/swap` at `@swap` and `/` at `@root`. it makes no sense and is confusing, because there is `/root` too.

Bob

3/3/24, 9:52 AM

@Pilot6 Not really. I will have [Flat Layout](https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Flat:~:text=in%20the%20filesystem.-,Layout,-There%20are%20several). But it like rename. Thanks.

Score:2

Ubuntu

Pilot6

3/3/24, 7:40 AM

It is not very complicated if you know what you are doing.

Boot from a LiveUSB.
Mount the disk to /mnt and create @root subvolume.
Copy using cp -a everything from @ and @home to @root. You can use mv, but it is safer to keep the old subvolumes before all settles.
Update /etc/fstab with subvol=@root option. Don't forget to remove the /home mount if you don't want a separate /home. (Why?)

Mount the disk to e.g. /mnt with -O subvol=@root. Also run

 sudo mount --bind /dev /mnt/dev
 sudo mount --bind /proc /mnt/proc
 sudo mount --bind /sys  /mnt/sys

Chroot to /mnt and update grub.

+ 2

Bob

3/3/24, 8:25 AM

Do you want to say I don't need edit GRUB file? Only edit `fstab` and use `update-grub` ?

Pilot6

3/3/24, 9:39 AM

`update-grub` will create a correct file.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Move the root from one subvolume to another in BTRFS

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