Side-topic: the way you described how blocks and snapshots work is _exactly_ how git works, with references and blobs: as long as a blob can be reached by a ref

Side-topic: the way you described how blocks and snapshots work is _exactly_ how git works, with references and blobs: as long as a blob can be reached by a reference (branch or tag), it will not be garbage-collected. Turns out a DAG, as is used in both ZFS and git, is a good data structure in many use cases.