Comment by tw04

This is the case with any parity based raid, they just hide it or lie to you in various ways. If you have two 6TB dives and two 12TB drives in a single raid-6 array, it is physically impossible to have two drive parity once you exceed 12TB of written capacity. BTRFS and bcachefs can’t magically create more space where none exists on your 6TB drives. They resort to dropping to mirror protection for the excess capacity which you could also do manually with ZFS by giving it partitions instead of the whole drive.