We are witnessing the rapid development of Cloud Computing techniques and services. Cloud Storage is widely applied in nowadays research and industry community. One of the most hottest topic in cloud storage is the data reliability. To ensure reliability under the condition of inexpensive hardware, several techniques are discussed and applied. We simplify and analyze the data reliability by a markov model, which includes the discussion of correlated failures and rack-aware placement. Our model aims to tell the effect of rack-aware placement for cloud storage systems.