[nix23]

dd says "no space left" and kills itself, "df" says 100% full, "zpool list" says 98% 80GB free...it's like black magic that ZFS knows to keep some space for itself ;)

And to other comments:

It was a DC-Harddisk, and NO, not even root should be capable destroying the filesystem by simply write to it, it's not 1970 anymore.

Calculating the "to reserve" Metadata-block should be rather trivial since it's ONE big file. And it's not dd that is the problem, it's btrfs that cannot handle a process that writes ONE BIG file.