Tips and tricks for managing your btrfs submodules.
Mount the top-level subvolume (which is of ID 5):
sudo mount /dev/mapper/archlinux -o subvolid=5 /mnt
Create the subvolume:
sudo btrfs subvolume create /mnt/@my-new-subvolume
Now that your subvolume has been created, get its ID:
sudo btrfs subvolume list / | grep my-new-subvolume
You can add it to /etc/fstab if need be.
This is done automatically already on a daily basis by btrfs-balance.service.
But if you want to run it manually, you can use the following command:
sudo btrfs balance start -dusage=50 -dlimit=2 -musage=50 -mlimit=4 /
Where / is the path to the subvolume to balance.
Official documentation there.
Do not use the nodatacow mount option, it wont work !! Use chattr +C instead.
From btrfs(5):
within a single file system, it is not possible to mount some subvolumes with nodatacow and others with datacow. The mount option of the first mounted subvolume applies to any other subvolumes.
Setting chattr +C on an existing folder is undefined behavior.
To workaround that, first make sure the folder is not in use (for system directories you will have to boot into a live CD).
Let's say we want to disable CoW on /var which is the mount point of the subvolume @var.
Mount the top-level volume from your LiveCD:
mount /dev/mapper/archlinux -o subvolid=5 /mnt
Rename the folder you want to disable CoW for:
mv /mnt/@var /mnt/@old_var
Create the new @var subvolume:
btrfs subvolume create /mnt/@var
Disable CoW:
chattr +C /mnt/@var
Copy the old content the new subvolume:
cp -a --reflink=never /mnt/@old_var/. /mnt/@var
Remove the old subvolume:
btrfs subvolume delete /mnt/@old_var
And wait for it to complete:
btrfs subvolume sync /mnt
You must now change /etc/fstab to match the new subvolid.
These instructions were given for subvolumes, but the same logic also applies to regular folders. Except no subvolume has to be created/deleted.