Verification of Crashsafe Caching in a Virtual File System Switch

When developing file systems, caching is a common technique to achieve a performant implementation. Integrating write-back caches is not primarily a problem for functional correctness, but is critical for proving crash safety. Since parts of written data are stored in volatile memory, special care has to be taken when integrating write-back caches to guarantee that a power cut during a running operation leads to a consistent state. This article shows how non-order-preserving caches can be added to a virtual file system switch (VFS) and gives a novel crash-safety criterion matching the characteristics of such caches. Broken down to individual files, a power cut can be explained by constructing an alternative run, where all writes since the last synchronization of that file have written a prefix. VFS caches have been integrated modularly into Flashix, a verified file system for flash memory, and both functional correctness and crash-safety of this extension have been verified with the interactive theorem prover KIV.