Los Alamos National Laboratory put together a 750 Raspberry Pi cluster. https://web.archive.org/web/20180825200515/https://www.lanl....
If I have my story straight, it was network booted and I believe is source of another piece of software they developed, called Kraken, that helped manage the finicky-ness of a Raspberry Pi cluster. https://github.com/kraken-hpc/kraken
Kraken is a state engine. In the case of the Pi cluster Kraken provided network images, installed and configured nodes, rebooted nodes, and manipulated their state as needed to keep things running.
Another fun piece of research on Pis out of LANL is that at their altitude, 7000ft, they estimate that on average a Raspberry Pi will crash once every 2 years due to bit flips caused by cosmic rays.