It's a system within Facebook called Warm Storage. There have been some public presentations about it, so I'm comfortable mentioning the name, but unfortunately

It's a system within Facebook called Warm Storage. There have been some public presentations about it, so I'm comfortable mentioning the name, but unfortunately I can't provide many other details about architecture or scale. I'll just warn people that the public information on it is way out of date. Most of it seems to be from 2014, and what it describes is really a separate system from what we have now.