Google can store roughly 500k * 8G = 4 petabytes of data in memory on their cluster.
Four petabytes. How much is that?
It is twice the size of the entire Internet Archive, a historical copy of much of the Web. It is the same as the estimated size of all the data Google had in 2003 on disk.
It is a staggering amount of data. And it is all accessible at speeds orders of magnitude faster than those with punier clusters.