$ zfs list -o space,refratio -rtall filling/.../debian-kfreebsd
NAME                                                 AVAIL   USED  USEDSNAP  USEDDS  REFRATIO
filling/.../debian-kfreebsd                          10.6T   487M      106M    381M     2.27x
filling/.../debian-kfreebsd@orig-install-buster-sid      -   106M         -       -     2.36x
filling/.../debian-kfreebsd@upgraded-sid-20230705        -     0B         -       -     2.27x


The debian-kfreebsd@orig-install-buster-sid image was obtained with
  qemu-system-x86_64 -enable-kvm debian-kfreebsd -cdrom debian-unofficial-kfreebsd-amd64-NETINST-1.iso -smp 6 -m 2g
(the installation ISO is https://people.debian.org/~jrtc27/debian-unofficial-kfreebsd-amd64-NETINST-1.iso)

debian-kfreebsd@upgraded-sid-20230705 is upgraded directly from the last archive snapshot that had kfreebsd-amd64:
  http://snapshot.debian.org/archive/debian-ports/20230705T142129Z/

Login nab:hehe.


debian-kfreebsd@orig-install-buster-sid and debian-kfreebsd@upgraded-sid-20230705
are ZFS sends (latter is incremental over the former).

debian-kfreebsd.zst is a compressed image of debian-kfreebsd@upgraded-sid-20230705
(it's 40G, you want to pipe through dd conv=sparse bs=64k).