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A reasonable model at present is to maintain this level for the next two years as projects such as LSST, LBNE et al start their planning. These are the average needs. We should build in headroom for peak usage - add 20%, to 109k units. Cluster slot usage over the past couple of months is shown here: 
illustrating that the cluster is on average heavily used. Week 27 is when Fermi reprocessing began, so we expect full usage for some time to come.
Purchase Record of Existing Hardware
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