Decision support systems (DSS) and online transaction processing applications (OLTP) are crucial for several organizations and frequently require high levels of availability. Many organizations moved their systems to the virtualized environment aiming at improving system availability. Despite the flexibility and manageability features provided by virtualization, a question arises on what policies to apply in order to achieve high availability. Usual approaches highlight redundancy as a strategy for high availability. Still, a concern persists on what components we should consider for redundancy. This paper proposes a hierarchical availability model for evaluating different redundancy allocations for DSS and OLTP systems in virtualized environments. We present three case studies investigating only-Virtual Machine (VM) redundancy and physical machine redundancy strategies. The results provide an overview of the availability impact due to each strategy. We noticed that the physical machine failure rate limits the maximum availability obtained from only-VM redundancy. We exercise our model with a genetic algorithm to find alternatives for high availability. The presented models and results may bring insights when designing availability policies.