Introduction

For quite some time I want to play around with a PI Cluster. Of course “cluster” can mean many things. A Cluster can be combining all the CPU power of the nodes, to get more processing power. A cluster can also mean having couple of nodes to build a scalable platform like OpenStack. In this case, the purpose of the cluster isn’t that important. The thing I’m mainly interested in is build a frame which can hold a couple of PI’s. And the frame should be easy to extend, so that when there is a need for more PI nodes in the cluster, the frame is can be easily extended. At the end I came up with a working frame. However this implementation might not be suitable for everyone… More on that later on.

How this all started

With a friend of my we started to build a 20 node PI cluster. The cluster was split into two: 10 Nodes lived in my home, and the other 10 nodes lived by my friends house. We used a VPN to connect the cluster nodes together. This worked great. However when we used PI3 nodes, we needed cooling. And the current frame didn’t provide that. After a attempt to alter the frame to add cooling FAN’s I realized that it might be better to start from scratch. The reason for that was during the cluster was operational, we discovered the current frame could be improved by adding some features.

Designing the ultimate cluster

Well the “ultimate” is maybe a bit strong.. but since I wanted to redesign a cluster from scratch I decided that at least the following features must be implemented:

• • The nodes must be easy to remove or easy to be inserted into the cluster
  • The frame itself should be 19 inch so it could fit in a standard network rack
  • The nodes should be powered from a own power supply, and should NOT rely on PoE
  • The frame must be easy to extend
  • Each node should have proper cooling
  • It would be nice that each node has some LED’s to display status (preferred RGB LED’s)

Tackling the power requirements

Powering each node from a power supply is the hardest part to implement. After thinking about it I came to the conclusion that I could do this by developing a back-plane. This back-plane is then used to distribute the power from one Power Supply (PSU) to all the other nodes. This sound like a great idea. However it introduces a new problem:

How can I make a easy extensible frame when there is a back-plane. The answer to that problem was quite easily actually: by splitting the back-plane up into smaller back-planes. This is how I came up with a back-plane design which can hold two PI nodes, and can be extended by adding more back-planes together. And as it turns out: splitting the back-plane up into smaller back-planes also makes the manufacturing of the PCB easier.

By using a back-plane design it’s easy to come up with a PCB for each node , which would provide the interconnect between the PI node and the back-plane. Which makes it possible to remove or insert a PI node.

Tackling the cooling

Next thing problem to tackle was: How to keep the PI node cool, so that the CPU won’t overheat and throttle. I soon came with the idea to develop a PI Cluster HAT. This Cluster HAT solves a couple of other problems:

• The HAT can be used to hold a FAN to cool the CPU (PWM controlled)
• The HAT can be used to distribute the 5V to the back-plane
• The HAT can also be used for other features:
  • Hold the connections for 3 RGB LED’s so they can be controlled by GPIO pins
  • Break-out I2C
  • Break out serial RX and TX
  • Break out 3V

Developing the cluster frame

Within three months I had the first version of a 10 node cluster working. It would take me another six months to get it to a a workable version. The final design uses 3 PCB’s:

• A back-plane PCB with can power 2 PI Nodes and which are extensible.
• A PI Cluster HAT with a lot of features (EEProm, PWN controlled FAN, 3 RGB LEDS, I2C, 3V, 5V power lines)
• A Power board which connects the power of the PI node to the back-plane
• A LED Board which holds the 3 RGB LED, which connects to the PI Cluster HAT

The frame of the cluster has mainly two parts: The frame which holds the PI tray. And the PI tray which holds the PI, the PI Hat with FAN, the Power Board and Led Board. The whole design is modular.

The downside of this is: To build this cluster frame, a lot of parts are needed, and the PCB’s must be soldered. So that’s why this might not be for everyone. However…

OpenSource is the way to go

Did I mention this whole design is OpenSource? No?? Well it is. And it’s on github for everyone to download. All the 3D models, Gerber files , schematics, a full hardware assembly guide is available. You can find it here