The previous smallest unit was a "cluster" - imagine for sake of example that it is the same number of racks as 3 pods. Some time ago, clusters were somewhat a

The previous smallest unit was a "cluster" - imagine for sake of example that it is the same number of racks as 3 pods. Some time ago, clusters were somewhat arbitrarily limited in size by a few things - human understanding was definitely one, management software and visualization, network layout and port density issues, and so forth. However, each cluster had a bunch of overhead associated with it that outweigh the benefits, including the primary one - failure domain. If we only needed one more pod-worth of servers, we would have to add a cluster with 3 pods worth of racks. I don't know the actual strategy (I work in a nearby team, but my focus is mostly on load balancing and CDN infrastructure), but one could imagine that in future it may be more normal to augment existing clusters/failure domains (say, add one pod) rather than building whole new ones.