Checkpointing roll back and recovery is a common technique to insure data integrity, to increase availability and to improve the performance of transaction oriented database systems. Parameters such as the Checkpointing frequency and the system load have an impact on the overall performance and it is important to develop accurate models of the systems under study. We find expressions for the system availability from a model that, unlike previous analytical work, takes into account the dependency among the recovery times between two checkpointing. Furthermore, our model can incorporate details concerning the contention for the systems resources.