In this paper we present WALTy (Web Application Load-based Testing tool), a set of tools that allows the performance analysis of web applications by means of a scalable what-if analysis on the test bed. The proposed approach is based on a workload characterization generated from information extracted from log files. The workload is generated by using of Customer Behavior Model Graphs (CBMG), that are derived by extracting information from the web application log files. In this manner the synthetic workload used to evaluate the web application under test is representative of the real traffic that the web application has to serve. One of the most common critics to this approach is that synthetic workload produced by web stressing tools is far from being realistic. The use of the CBMGs might be useful to overcome this critic.