my backup server is running btrfs (kernel wiki) with raid1 (mirror), but i’m running out of space and don’t want to upgrade. And raid1 isn’t even necessary, it is the backup after all. To free up some space, well half of it, i used this command: btrfs balance start -dconvert=single / you can live migrate between ‘profiles’, for example from ‘single’ (data is saved only once on the btrfs filesyste), to raid1