Subvolumes are not flat but hierarchical. btrfs subvolume list / provides you with the information. man btrfs-subvolume also provides you with information how subvolumes are defined or managed. From the man page of btrfs-balance: From kernel 3.3 onwards, btrfs balance can limit its action to a subset of the whole filesystem, and can be used to change the replication con- figuration (e.g. moving data from single to RAID1).