We introduce POROS that is a new solution for proof of data reliability. In addition to the integrity of the data outsourced to a cloud storage system, proof of data reliability assures the customers that the cloud storage provider (CSP) has provisioned sufficient amounts of redundant information along with original data segments to be able to guarantee the maintenance of the data in the face of corruption. In spite of meeting a basic service requirement, the placement of the data repair capability at the CSP raises a challenging issue with respect to the design of a proof of data reliability scheme. Existing schemes like Proof of Data Possession (PDP) and Proof of Retrievability (PoR) fall short of providing proof of data reliability to customers, since those schemes are not designed to audit the redundancy mechanisms of the CSP. Thus, in addition to verifying the possession of the original data segments, a proof of data reliability scheme must also assure that sufficient redundancy information is kept at storage. Thanks to some combination of PDP with time constrained operations, POROS guarantees that a rationale CSP would not compute redundancy information on demand upon proof of data reliability requests but instead would store it at rest. As a result of bestowing the CSP with the repair function, POROS allows for the automatic maintenance of data by the storage provider without any interaction with the customers.