Persistent Caching Systems Don't Mean Crash Consistency

Byte-addressable non-volatile memory (NVM) proposes a new opportunity to enable file systems with better performance and durability by adding a new persistent caching layer. However, crash consistency of caching layers and compatibility with diverse reliability features of file systems remains unknown. This paper conducts a crash consistency case study of Open CAS, a popular block-level caching system. Through careful and thorough crash consistency experiments, we show that Open CAS cannot always maintain crash consistency in the persistent caching layer. We also demonstrate some reliability features of file systems are not compatible with Open CAS. Our analysis reveals the importance of a systematic crash consistency test to caching systems and reliability feature co-design with file systems in the construction of a reliable end-to-end file system.