Hi,
I'm not sure if there are many people that really want to build their own cluster of Raspberry Pi,
but I built one recently. Just for fun (possibly some experimentation with MPI).
If you are interested, I posted the full details on blog: blog www.waioeka.com , I'd post the details here (not trying to advertise the blog), but there is quite a bit of detail. I'll also add some details on MPI, which is pretty simple to get setup. Nothing fancy with the cluster, no custom power-etc.
My next goal is to get Spark running on Yarn, but think memory could be an issue.
Out of interest, has anyone managed to get Spark running?