The paper is dedicated to the properties of the workload process in multi-server systems, i.a. in the presence of heavy tails. The moment properties of the workload process are analyzed. A model of a high performance cluster as a multi-server stochastic system is suggested. A connection between the model of the cluster and the classical multi-server system models is discussed in detail. The results of numerical experiments are provided, demonstrating an agreement between the model and the cluster performance.