This article has a lot of good insights about data warehouse.
… Too many people misunderstand and think, Agile is just Fast. But Fast without “Continuous Improvement”, isn’t what Agile is about. If you’re having too much churn and repeating the same task without improvement, that’s called “being a hamster on a wheel”.
… Top down starts from the sources, and bottom-up starts from the requirements, but betting the entire architecture on either of these causes serious issues and gaps. What if the architecture was top-down, but the implementation was bottom-up? The only way you can achieve this is if you de-couple the archival area from the user-facing areas, at least logically.
… The key for your project or program to succeed is to produce “what you want” from “what you have” ,and do it as soon as possible to give the organization the information, in the format required in a timely manner (the sooner the better).