Do You Know Where Your Cloud Files Are?

Clients of storage-as-a-service systems such as Amazon's S3 want to be sure that the files they have entrusted to the cloud are available now and will be available in the future. Using protocols from previous work on proofs of retrievability and on provable data possession, clients can verify that their files are available now. But these protocols do not guarantee that the files are replicated onto multiple drives or multiple datacenters. Such tests are crucial if cloud storage is to provide resilience to natural disasters and power outages as well as improving the network latency to different parts of the world. In this paper, we study the problem of verifying that a cloud storage provider replicates the data in diverse geolocations. We provide a theoretical framework for verifying this property. Our model accurately determines which Amazon CloudFront location serves content for Planetlab nodes across the continental US. Our work is complementary to the recent paper of Bowers et al., which uses different techniques to verify that files are replicated across multiple drives in a single datacenter.