I assumed they would punt persistent storage for the first release, kinda like they launched Cloud Filestore with only NFSv3 support and without snapshots. As you say, it's a tough problem to tackle. If they to ship with something, I'd expect them to go for Ceph. As far as I can tell, if you squint hard enough, its lowest layers are the ones that resemble the most those at Google (Colossus/D).