Modular Integration of Crashsafe Caching into a Verified Virtual File System Switch

Abstract

AbstractWhen developing file systems, caching is a common technique to achieve a performant implementation. Integrating write-back caches into a file system does not only affect functional correctness but also impacts crash safety properties of the file system. As parts of written data are only stored in volatile memory, special care has to be taken when integrating write-back caches to guarantee that a power cut during running operations leads to a consistent state. This paper 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.KeywordsPOSIX-compliant File SystemsWrite-Back CachingCrash-SafetyRefinementInteractive Verification