dekay.org

"As we tackle this challenge, we are starting to see a third general model of scaling emerging over and over again. This third model can be conveniently characterized as “scaling diagonally”. In diagonal scaling, a small fraction of the total number of servers (say 1% or 0.1%) are highly stateful and highly reliable. The other 99% or 99.9% are very low-cost (inherently unreliable) commodity machines that carry out the bulk of the work, but are not responsible for maintaining complex metadata or complex state."