Customers can have hundreds of variants in their IT environment. This sprawl is often the result of lack of governance, lack of standardization, and a bottoms-up/best-of-breed mentality that resulted in "configuration pollution" (a wide variance among arguably-similar stack configurations). Managing such an environment is expensive and inefficient.
Compare that complexity to the service catalog of most public cloud providers. For example, the Oracle Database Cloud Service has only a few offerings. Not 50, or 500 or 5,000. Your private cloud service catalog should look and sound more like a public provider's catalog. Doing that requires making choices about standardization and consolidation. Sometimes these choices are politically challenging, but they need to be made nonetheless or your private cloud will not provide the cost optimization that it should.
* Is chargeback necessary? Chargeback/showback is the idea of passing consumption costs back to the consumer either via internal automated transfer costs (chargeback) or simply via reporting (showback). It sounds great on paper, and is a relatively simple matter to execute technically with a fully-integrated cloud management regime (since the software that does the automated provisioning knows who's using what at all times). But the transparency it provides is truly transformational to an organization, and therefore has political and human consequences. A well-implemented private PaaS cloud will automatically have all the information IT needs to make costs transparent, but making those costs visible is an organizational decision.
* PaaS versus IaaS? To reiterate, if your goal is agility and cost reduction, PaaS gives you more flexibility, more efficiency and more value than infrastructure-as-a-service (IaaS). The raw shared compute and storage (think hypervisors, guest OSs, etc.) that IaaS provides are simply containers that then need to be installed, configured and managed, and that cost lives somewhere (either in the provider or the consumer's bucket).
Furthermore, because most organizations pass the configuration effort onto the consumers, the tendency for "bottoms-up" configuration pollution continues to be a problem. I call this "cost shifting". PaaS, on the other hand, provides instantly-consumable services (database, middleware, presentation layer, etc.) in standardized configurations that can be managed with minimum effort on an individual basis and with maximum efficiency on an enterprise scale. IaaS provides efficiency but primarily just shifts costs around. PaaS doesn't just shift costs around, it eliminates a substantial portion of them outright.
* How to succeed? The most successful model I've seen to introduce PaaS to an organization is to start small, with a well-defined scope. Pick a service, or two services, and a defined user base (say, a particular LOB development organization in a "grow the business" activity) and let them see what PaaS can do for them.
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