File systems not only have to be functionally correct, they also have to be crash-safe: a power cut while an operation is running must be guaranteed to lead to a consistent state after restart that loses as little information as possible. Specification and verification of crash-safety is particularly difficult for non-redundant write-back caches. This paper defines a novel crash-safety criterion that facilitates specification and verification of order-preserving caches. A power cut is basically observationally equivalent to a retraction of a few of the last executed operations. The approach is modular: It gives simple proof obligations for each individual component and for each refinement in the development. The theory is supported by our interactive theorem prover KIV and proof obligations for crash-safety have been verified for the Flashix flash file system.
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