In ZFS, it is not possible to add one more disk on the root pool for the reason that it is the root disk, for a senior system engineer, this logic is pretty straight forward.
Imagine if the root pool has multiple disks, if one disk was removed for some reason, the system will become unbootable.
What if you get a request to extend the rpool, because for some reason, the project manager has put /var, /usr or /home in the same rpool, these directories usually fills up fast and highly suggested to be a separate filesystem from root filesystem.
There is only one way to extend the rpool space. For an instance the rpool size is 20GB and they want you to extend to 50GB.
You can do the below steps:
- Add a bigger size root hard disk, in this case 50GB
- Mirror the rpool using the new 50GB hard disk
- Installboot block on the new disk
- Detach the old disk from rpool