Ask HN: What versioning system do GDrive/Dropbox use?

I'm curious as to how Google Drive and Dropbox work under the hood but I couldn't find any information. Anybody in HN has some insight of what version control system or architecture they use? Do they make use of any of the industry VCSs? (I've been researching version control systems for quite a while and this has been a long burning curiosity of mine)