Re: Back in the day...
"While obviously "renting" spare capacity of bespoke systems is not really viable unless everything is "cloud""
Back in the day when people knew how to do these things right, when cloud was still called "bureau timesharing", there were companies that had their DR kit in a freight container (or maybe several, across several sites) that could be readily deployed to anywhere with the required connectivity. It might be a container+kit they owned or it might be a container+kit rented from a DR specialist or whatever. And they'd know how it worked and how to deploy when it was needed (not "if").
That's probably a couple of decades ago, so it's all largely been forgotten by now, and "containers" means something entirely different.
E.g. here's one that Revlon prepared earlier (article from 2009), but my recollection is that this had already been going on elsewhere for years by that time:
http://www.computerworld.com/article/2524681/business-intelligence/revlon-creates-a-global-it-network-using--mini-me--datacenters.html
Biting the hand that feeds IT
