Abstract
Cloud computing is an emerging paradigm that consists of hosting and delivering computing services across the web. The availability is one of the security features such as integrity and confidentiality. Certainly endorsing high availability by the improvement of fault tolerance techniques is one of the major concerns of the cloud. Elsewhere we cannot afford to directly evaluate new approaches for cost reason. For this reason we introduce in this paper a probabilistic model for simulation based on the principle of "Large Numbers Law". The idea is to simulate a scenarios of cloud virtual environment in which faults can occur in a random way following failure occurrence probabilities. The global unavailability measured is faithful to unavailability average known of Cloud providers. The model allows live virtual machine migration in order to evaluate proactive fault tolerance approaches based on preemptive migration.