How do you know if your disaster recovery solution is as strong as you need it to be? It's usually measured in two ways: RPO (Recovery Point Objective) and RTO (Recovery Time Objective). RPO is how much data you're willing to lose when things go wrong, and RTO is how long you're willing to go without service after a disaster. For a large enterprise running SANs, the RTO and RPO targets are an hour or less: the more you pay, the lower the numbers. “Google Apps customers don't need to worry about any of this for the data they create and store within Google Apps. They get best-in-class disaster recovery for free, no matter their size. For Google Apps customers, our RPO design target is zero, and our RTO design target is instant failover. We do this through live or synchronous replication: every action you take in Gmail is simultaneously replicated in two data centers at once, so that if one data center fails, we nearly instantly transfer your data over to the other one that's also been reflecting your actions,” noted Google.
More info: Disaster Recovery by Google