I have a desktop machine with two sets of disks - small SSDs for things I need to be quick and large HDDs for things I need to be big.

I have set up the small SSDs into a pool called "rpool" and successfully followed the instructions to use it as Ubuntu's root filesystem - it even boots from it. <https://github.com/zfsonlinux/zfs/wiki/Ubuntu-16.04-Root-on-ZFS>

ZFS will not let me create a second pool ("hpool" for the HDDs) with a root of `/`. To work around this I've used a root of `/hdd` for hpool.

This means that all of my hdd datasets are mounting under `/hdd`, not under `/` where I want them to be - e.g. /hdd/tmp instead of /tmp.

How can I fix this? (Other than by a mess of symlinks.)