Raspberry Pi Cluster — “Why To” Guide

This will be more of a “why to” guide and not so much of a “how to”, and certainly not a step-by-step on the setup of an individual pi. A good place to start for setting up and individual pi is the Raspberry Pi sit at: https://www.raspberrypi.org

A single Raspberry Pi v3 has enough compute power to be a decent general purpose Linux box, run a Linux Apache MySQL PHP (LAMP) stack to run a website, or be a streaming media server by running OSMC. But what a single Pi can’t do is all of the above at the same time handling a respectable workload.

Back in the day, techies would run a few Linux boxes in their den  or closet, usually re-purposed desktop PCs that were once very powerful desktop or gaming systems but then became inadequate for those purposes. While you can still do this today, you have to consider the cost of powering a full desktop or gaming PC 24/7. When you consider that, plus the cost of the air conditioning, and the noise, and the physical footprint these antiquated PCs take up, it no longer makes sense to do that.

Also, in the event of an extended power outage — such as what we experienced with Hurricane Sandy — you need your infrastructure to take up as little electrical power as possible. Running your services on low powered Pis versus antiquated desktops, you can run longer on UPS; and if you end up living on generator for weeks, your stack of Pi’s will take up negligible load on your generator.

I should clarify that the word “cluster” in the context of multiple Pi’s doesn’t end up being one big Linux box that runs across the Pi’s. It is simply a handful of Pi’s with services distributed across them.

I run three Pi’s, or now I suppose four as of today, with the workloads split out as follows:

  1. One Pi V3 running OSMC acts as the streaming media server for the living room entertainment center.
  2. One Pi V2 running a LAMP stack, plus wordpress, and is what is serving up this page right now for you.
  3. One RP V2 running RasPBX , which is a distribution that specializes in making your Pi run Asterisk and FreePBX  so you can have a full enterprise level phone system, complete with voicemail, conference bridges, etc.
  4. My newest Pi V3 will be a general purpose box and playpen.

Usually you acquire multiple Pis over time buying one, then another, then another, etc. Over time you end up with a pile of Pis just hanging about and a mess of cables. Or perhaps you bought a case for each one, but you find they don’t stack well and still are a mess that is hard to manage.

To solve that problem, I recommend the Dog Bone case by Geaux Robot, sold at Amazon here: https://www.amazon.com/GeauxRobot-Raspberry-Model-4-layer-Enclosure/dp/B00MYFAAPO

This case will accommodate four (4) Raspberry Pi’s model 2 or 3. They also offer a 2-pi or 3-pi version. This will keep your Pi’s stacked neatly and nicely as a single unit, while providing good airflow for cooling and it looks nice and techy.

Next, you need to consolidate all those power cords. If you bought a power cord for each Pi, then you find that they don’t work nicely with a UPS power strip and take up too many slots. Now, you do need a UPS, but there will be more on that later. If you buy a powered USB hub that can provide 2A of power to each port, then you can plug that USB hub into your UPS, and plug the pi’s into that via UPS cords. The net result is you end up with only one power port on your UPS for the entire stack of Pis.

I used this, which is a power port designed for delivering power, not a USB hub: https://www.amazon.com/gp/product/B0115MVRO4

I condensed my stack of Pi’s onto this power source as well as three NetGear switches, so that all of those devices consume only one port on my UPS.

Why do you need a UPS? Because the Raspberry Pi ‘s SD card can become corrupt if power is cut without a graceful system shutdown. What that means is if you lose power, your Pi may not boot and you have to spend time recovering the image — or starting with a new one and reconfiguring your services.  Plus, your Pi’s will become an integral part of your home infrastructure which you don’t want to lose service if it can be avoided.

Before I move off the topic of power, it is important to point out that stable power is critical. If your power source cannot provide at least 15.A  to each Pi, you run the risk of the Pi locking up, crashing, rebooting, etc. Again, this would run the risk of corrupting the SD card of the Pi, because the reboot would be without a graceful shutdown.

For the network, you really should get a 5 or 6 port gigabit switch. That would be 4 ports for the Pi’s and another that uplinks to your other switches. You can, of course, just plug the Pi’s into an existing switch if you happen to have enough ports available. But segregating the Pi’s onto their own small switch is cleaner because you can strap the dog bone case to the switch and handle and manage as a single unit. Usually you buy, or make your own, very short patch cords.

A word about wireless — avoid it if you can. You can use it of course, but each Pi will lose signal every so often, or you will end up with your wireless net oversaturated. Over the past couple years I have re-worked my home infrastructure to put all infrastructure back on wired Gig-E, as well as networked cameras and desktop PCs and gaming rigs, and the only things consuming wireless are the mobile devices and everyone is happy and fast.

I recommend using the “Raspian Lite” image instead of the default image, which includes the X windows GUI and desktop environment. If you install that, you will play with it long enough to realize it isn’t a viable desktop environment for everyday use, and then you spend time figuring out how to uninstall all that bloat. I recommend to install the lite version and then only install on top of that the services you really want and need.

Typically the only thing I install, other than the packages required for the services planned (Apache Httpd, MySQL, etc.) is webmin in order to  manage and administer the system.

You may have picked up that I do not have firewall services running on any of the Pis. I love pfSense, but as far as I am aware it still is next to impossible to make run on an ARM based Pi. It might be possible these days, but just not something I want to spend time on. I run pfSense on a dedicated Zotac C Series CI323, which is a mini x86 system that is fanless and draws little power.

None of my Pi’s are directly on the public Internet, but I do have firewall rules to pass the traffic that is appropriate to each service on the pfSense firewall.  Therefore I do not talk much about security here, but it is very important to consider. I personally would not put a Pi directly onto the public Internet, but that doesn’t mean it can’t or shouldn’t be done. But if you go that route, be extremely diligent managing and applying patches and hotfixes, strong password, and some form of protection like fail2ban.