Argosy: verifying layered storage systems with recovery refinement

Storage systems make persistence guarantees even if the system crashes at any time, which they achieve using recovery procedures that run after a crash. We present Argosy, a framework for machine-checked proofs of storage systems that supports layered recovery implementations with modular proofs. Reasoning about layered recovery procedures is especially challenging because the system can crash in the middle of a more abstract layer’s recovery procedure and must start over with the lowest-level recovery procedure. This paper introduces recovery refinement, a set of conditions that ensure proper implementation of an interface with a recovery procedure. Argosy includes a proof that recovery refinements compose, using Kleene algebra for concise definitions and metatheory. We implemented Crash Hoare Logic, the program logic used by FSCQ, to prove recovery refinement, and demonstrated the whole system by verifying an example of layered recovery featuring a write-ahead log running on top of a disk replication system. The metatheory of the framework, the soundness of the program logic, and these examples are all verified in the Coq proof assistant.