1. A method for performance modeling for an information technology (IT) system having one or more servers for performing a number of types of transactions, comprising: receiving data for system topology and transaction flows; receiving performance measurement data for the IT system; clustering the performance measurement data into multiple regimes based on similarities; deriving equations for the performance measurement data; and flexibly predicting service demand and network delay parameters based on the equations and the clustered performance measurement data using a processor, such that online performance models are dynamically created and updated.