Unlike an OLTP system, which gives a snapshot of the present, a data warehouse is meant to provide a complete picture of the data over time. Practical Considerations Time and money. The average ROI over three years, cumulative, was about 400 percent, with a higher ROI for data marts. Clearly, building a data warehouse can be profitable, but it's not a quick hit. Your company should be aware of the amount of investment likely before any payback can be expected.
Data warehouses require lots of disk space.
When estimating how much storage space you'll need, don't look at just the current production systems. Remember that you'll be building a system of historical data. Most organizations will want to keep at least a year's worth of data, and if you want to do any kind of trend analysis, you will need several years. Also, reports and analysis usually need more than one index. Be generous in your estimates of disk space.