In this chapter, we present our data reliability assurance solution for big data in the Cloud with a huge amount of data files during the data maintenance stage, which is a novel cost-effective data reliability assurance mechanism named Proactive Replica Checking for Reliability (PRCR). PRCR is able to maintain Cloud data with minimum replication and hence, incurs the lowest storage cost. Compared to the storage using the conventional three-replica strategy, our PRCR can reduce the storage cost between two-thirds and one-third, while the running overhead for PRCR itself is negligible