> I've never worked on anything bigger than 10 machines or so, but is this really true? I would think the difficulty of scaling is not any kind of smooth function, but very easy or very hard depending on the bottlenecks you hit at any particular point.
You are right it's not a smooth function. This isn't just about numerical aspect, it's figuring how to get over a hump: going from one datacenter to two is much harder (not even twice as hard) than adding a rack to two datacenters or adding a fourth datacenter when there are five.
In other words, when you're at scale you have tools available to operate at that scale. When you find yourself having to scale out without having done so in the past, you have to build those tools.
You are right it's not a smooth function. This isn't just about numerical aspect, it's figuring how to get over a hump: going from one datacenter to two is much harder (not even twice as hard) than adding a rack to two datacenters or adding a fourth datacenter when there are five.
In other words, when you're at scale you have tools available to operate at that scale. When you find yourself having to scale out without having done so in the past, you have to build those tools.