zfsuseradd.sh

#!/bin/bash -ex
# Hack to add user USERDATA zpool to an Ubuntu 19.10 system.
# I have this at /usr/local/bin/zfsuseradd.sh
user="${1}"
[[ -n "$user" ]] || (echo "User not specified." && exit 1)
zfs_user_suffix=$(mount | grep rpool/USERDATA/root_ | awk '{print $1}' | sed 's/rpool\/USERDATA\/root_//')
[[ -n "$zfs_user_suffix" ]] || (echo "Can't get Ubuntu ZFS user suffix." && exit 1)
zfs_system_suffix=$(mount | grep rpool/ROOT/ubuntu_ | awk '{print $1}' | head -1 | sed 's/rpool\/ROOT\/ubuntu_//')
[[ -n "$zfs_system_suffix" ]] || (echo "Can't get Ubuntu ZFS system suffix." && exit 1)

echo "creating rpool/USERDATA/""${user}""_""${zfs_user_suffix}"""
[[ -e /"${user}"_"${zfs_user_suffix}" ]] && (echo "mount point already exists" && exit 1)
zfs create rpool/USERDATA/"${user}"_"${zfs_user_suffix}"
zfs set mountpoint=/"${user}"_"${zfs_user_suffix}" \
rpool/USERDATA/"${user}"_"${zfs_user_suffix}"
# Set local properties created by ubuntu installer for userdata zpools
# set canmount=on since otherwise these don't appear to get mounted at boot.
# canmount=on is default. In theory this can be set to on by:
# zfs inherit -S canmount rpool/USERDATA/$user_$zfs_user_suffix
# zfs set canmount=on \
# rpool/USERDATA/"${user}"_"${zfs_user_suffix}"
zfs set com.ubuntu.zsys:bootfs-datasets=rpool/ROOT/ubuntu_"${zfs_system_suffix}" \
rpool/USERDATA/"${user}"_"${zfs_user_suffix}"

rsync -haHAX /home/"${user}"/ /"${user}"_"${zfs_user_suffix}"
rm -r /home/"${user:?}"
chown "${user}"."${user}" /"${user}"_"${zfs_user_suffix}"
zfs set mountpoint=/home/"${user}" rpool/USERDATA/"${user}"_"${zfs_user_suffix}"

As per discussion here: https://discourse.ubuntu.com/t/enhancing-our-zfs-support-on-ubuntu-19-10-an-introduction/12130/117