A filesystem-level storage cloud offers network-filesystem access to multiple customers at low cost over the Internet. In this paper, we investigate two alternative architectures for achieving multi-tenancy securely and efficiently in such storage cloud services. They isolate customers in virtual machines at the hypervisor level and through mandatory access-control checks in one shared operating-system kernel, respectively. We compare and discuss the practical security guarantees of these architectures. We have implemented both approaches and compare them using performance measurements we obtained.