They're almost definitely copying 500GB databases between machines. More than likely your 500GB database is running across 3 machines, and if 1 fails they bring

They're almost definitely copying 500GB databases between machines. More than likely your 500GB database is running across 3 machines, and if 1 fails they bring a new one up and copy your database over. They wouldn't be able to provide that SLA otherwise. 500GB copies reasonable quickly on the in-datacenter links Google runs their servers on, afaik.