Btrfs has separate allocation pools for data and metadata. If you delete files, the freed-up space is returned to the data pool but that does not make it genera

Btrfs has separate allocation pools for data and metadata. If you delete files, the freed-up space is returned to the data pool but that does not make it generally-available for use in the metadata pool. So in the case where the entire disk space is already allocated to one of the allocation pools, none of the pools can grow beyond their current size. One of the effects of balancing the tree is to release unused space from the allocation pools to general availability, solving that problem.