Using cloud technologies, it is possible to provision HPC services on-demand. Customers of the service are able to provision virtual HPC systems in a self-service portal and deploy and execute their specific application without operator intervention. The business model foresees to only charge the amount of resources actually used. There remain open questions in the area of performance optimization, advanced resource management, and fault tolerance. The Open Cirrus cloud computing testbed offers an environment in which we can treat these problems.