Abstract
As more and more data generated every moment, more space is needed to store these data. After the appearance of private cloud and public cloud, Hybrid cloud system becomes more and more famous recently. Here we make the hybrid cloud being the third choice for data storage. We build a cost model to prove that with appropriate settings, hybrid cloud has lower cost than private cloud in a long run. We also introduce an availability model with erasure code. With this model, hybrid cloud can guarantee higher availability and security with lower data redundancy. In the end we combine both models to prove that hybrid cloud is beneficial for all the features.