Hello, as it took me a while to find a solution to this problem, I though I might post here in case someone else happens to struggle with a similar situation 
My dev environment is based on Debian 10 with a ZFS root filesystem located on a pool (rpool) whose layout is as follows:
$ sudo zfs list -r rpool NAME USED AVAIL REFER MOUNTPOINT rpool 247G 652G 96K / rpool/ROOT 40.2G 652G 96K none rpool/ROOT/debian-10 40.2G 652G 10.5G / rpool/ROOT/debian-10/opt 4.36G 652G 1.16G /opt rpool/ROOT/debian-10/var 11.1G 652G 96K /var rpool/ROOT/debian-10/var/cache 11.0G 652G 11.0G /var/cache rpool/ROOT/debian-10/var/log 18.5M 652G 18.5M legacy rpool/ROOT/debian-10/var/tmp 47.4M 652G 47.4M legacy rpool/home 139G 652G 92.4G /home rpool/home/root 1.93M 652G 948K /root rpool/swap 68.0G 715G 4.60G - rpool/tmp 620K 652G 620K legacy
When I started using Docker, I discovered that it was creating a lot of datasets right under the root dataset:
$ sudo zfs list -r rpool NAME USED AVAIL REFER MOUNTPOINT rpool 246G 653G 96K / rpool/ROOT 39.2G 653G 96K none rpool/ROOT/debian-10 39.2G 653G 10.3G / rpool/ROOT/debian-10/0594a.... 5.04M 653G 53.9M legacy rpool/ROOT/debian-10/094e0.... 560K 653G 56.0M legacy rpool/ROOT/debian-10/0a248.... 200K 653G 44.9M legacy rpool/ROOT/debian-10/12dc5.... 568K 653G 56.0M legacy rpool/ROOT/debian-10/17829.... 108K 653G 56.0M legacy rpool/ROOT/debian-10/1a6ce.... 160K 653G 78.3M legacy rpool/ROOT/debian-10/1c27c.... 1.62M 653G 55.4M legacy rpool/ROOT/debian-10/20f13.... 144K 653G 57.6M legacy rpool/ROOT/debian-10/254ed.... 44.8M 653G 44.8M legacy rpool/ROOT/debian-10/2997e.... 7.79M 653G 51.9M legacy rpool/ROOT/debian-10/2e725.... 20.9M 653G 76.4M legacy ... ...
This is obviously quite annoying and it would be much preferable to have all Docker related datasets grouped under a dedicated one.
I immediately jumped to the Docker documentation, which BTW is really well done, and found the [Docker ZFS Storage page] (docs.docker .com/storage/storagedriver/zfs-driver/) giving a lot of detailed explanations about ZFS itself and stating that creating a dataset that gets mounted under /var/lib/docker should be the solution.
I did apply the recommendation, verifying that the ZFS dataset was well mounted (/var/lib/docker has to be empty!), only to discover that, unfortunately, even though I followed the procedure, Docker still referenced the root dataset as its Parent Dataset:
$ sudo zfs list -r rpool NAME USED AVAIL REFER MOUNTPOINT rpool 246G 653G 96K / rpool/ROOT 39.2G 653G 96K none rpool/ROOT/debian-10 39.2G 653G 10.3G / rpool/ROOT/debian-10/opt 4.36G 652G 1.16G /opt rpool/ROOT/debian-10/var 11.1G 652G 96K /var rpool/ROOT/debian-10/var/cache 11.0G 652G 11.0G /var/cache rpool/ROOT/debian-10/var/log 18.5M 652G 18.5M legacy rpool/ROOT/debian-10/var/tmp 47.4M 652G 47.4M legacy rpool/docker 96K 652G 96K /var/lib/docker ... $ docker info Client: Debug Mode: false Server: ... Server Version: 19.03.9 Storage Driver: zfs Zpool: rpool Zpool Health: ONLINE Parent Dataset: rpool/ROOT/debian-10 ... Kernel Version: 4.19.0-9-amd64 Operating System: Debian GNU/Linux 10 (buster) OSType: linux Architecture: x86_64 ... Docker Root Dir: /var/lib/docker Debug Mode: false Registry: https://index.docker.io/v1/ ...
Digging deeper in the Docker documentation I finally found out that the docker daemon can be configured using an /etc/docker/daemon.json file (as described here), while all the possible options are listed on the dockerd page…
Thus, to specify the Parent Dataset to be used by the Docker daemon for its own storage, one needs to first stop the docker service, create/modify the /etc/docker/daemon.json file content and then start the docker daemon again, as follows:
$ sudo systemctl stop docker $ sudo nano /etc/docker/daemon.json { "storage-opts": [ "zfs.fsname=rpool/docker"] } $ sudo systemctl start docker
And now, docker will create its legacy datasets at the desired location:
$ docker info ... Parent Dataset: rpool/docker $ sudo zfs list -r rpool/docker NAME USED AVAIL REFER MOUNTPOINT rpool/docker 287M 652G 96K /var/lib/docker rpool/docker/0225c9b... 192K 652G 44.9M legacy rpool/docker/0e3f3f5... 20.8M 652G 64.6M legacy rpool/docker/28b0bc9... 144K 652G 178M legacy rpool/docker/2ee79f0... 1.62M 652G 43.7M legacy ... ...
Hopefully this information will be useful to others!