To set up ZFS on the spinning discs, followed steps derived from these sources (but without encryption):
* https://ipetkov.dev/blog/installing-nixos-and-zfs-on-my-desktop/
* https://cheat.readthedocs.io/en/latest/nixos/zfs_install.html

```
# DISK=<disk from /dev/disk/by-id>
# POOL=<a name for the pool>
# zpool create "${POOL}" $DISK
# zfs create -o compression=on -o mountpoint=legacy "${POOL}/main"
```

then added the ZFS filesystem to `hardware-configuration.nix` (use the zfs
created label as the device) and rebooted.

Don't forget to `chown` the mounted system so that non-root can read/write there.