This paper proposes a comparative study of cloud dependability between two methods - bug analysis and fault injection for assessing the impact of component failure on cloud service availability. We focus on the IaaS cloud with open source platform Open Stack. The actual bug data are analyzed to show numerical examples of dependability assessment. A fault injection tool has also been developed to create failures of components and then observe their effects on services. The comparison analysis between two methods shows that bug analysis method has richer features for analyzing but not as precise as fault injection.