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There are many papers that deal with interesting ways to deal with elasticity beyond turning servers on and off when needed, such as Blink (http://none.cs.umass.edu/papers/pdf/asplos2011.pdf) and Powernap (http://web.eecs.umich.edu/~twenisch/papers/asplos09.pdf), which 'smartly' use power state S3 (sleeping) to minimize power usage when load is low and can quickly become available when needed.

Some techniques used are to have all servers maintain a percentage of time that they are on (staggered so there is never a time they are all sleeping) and have a master modify their sleeping percentage based on workload.

Other power saving policies have whole clusters of nodes sleep at once and have data intelligently striped and replicated across clusters so if any piece of data is on a sleeping cluster it can be retrieved in a timely manner from an awake one and if needed that whole cluster can be waken up.