This is an example of a commercially usable Ubuntu Pi Cluster.
The control Node will utilize a USB 3.0 storage controller and a hard drive to act as storage for all nodes.
You will also require a usb ethernet adapter to act as the external communication port.
The onboard ethernet will act as the internal communication port to offer the best possible bandwidth for NFS and other internal communication.
Each Processing Node will utilize the SD card to boot from the NFS server and to act as Swap Space only. We will utilze the SD card for booting instead of TFTP as TFTP requires one to continually update the kernel by manually copying the kernel files to the TFTP directory. I have found this to be a pain and at times this has caused booting issues that require a person onsite to reboot the node. We will be lowering swappiness to utilize swap as a last resort.
As each Processing Node will run from NFS, you will be able to easily backup and restore Nodes easily from the Control Node.