What you say might be true for CPU usage, maybe even for network usage, but definitely not for I/O. In that case it's definitely noisy neighbors causing problems for others, but in a way it doesn't matter. Whether it's noisy neighbors or hard resource limits, the variability is still there. For single-machine stuff you just have to assume the worst case. For things that are distributed among large enough numbers of nodes you can do a certain amount of averaging, but high variability still forces you close to the worst-case assumption.
I've run a lot of tests in this area, and even presented some of the results at LISA last month. When it comes to I/O, EC2 is considerably less consistent than many others[1] and that's what really hurts their users. Their prices might be only 2x someone else's if you're only looking at the average case, but it's more like 10x if you consider their extreme variability as well.
[1] Side note: this is a hint that Amazon has an unusually high oversubscription ratio. The same practice has been evident in shared web hosting, with the same effect, since that industry was created.
I've run a lot of tests in this area, and even presented some of the results at LISA last month. When it comes to I/O, EC2 is considerably less consistent than many others[1] and that's what really hurts their users. Their prices might be only 2x someone else's if you're only looking at the average case, but it's more like 10x if you consider their extreme variability as well.
[1] Side note: this is a hint that Amazon has an unusually high oversubscription ratio. The same practice has been evident in shared web hosting, with the same effect, since that industry was created.