A quite old, but very educative post by James Golick dissecting the mythical extra server capacity:
There’s this idea floating around that we can scale out our data services “just in time”. Proponents of cloud computing frequently tout this as an advantage of such a platform. Got a load spike? No problem, just spin up a few new instances to handle the demand. It’s a great sounding story, but sadly, things don’t quite work that way.
This is the Mythical Man-Month of the IT department.
Original title and link: The Myth of Auto Scaling as a Capacity Planning Approach ( ©myNoSQL)