Verifying a high-performance crash-safe file system using a tree specification

DFSCQ is the first file system that (1) provides a precise specification for fsync and fdatasync, which allow applications to achieve high performance and crash safety, and (2) provides a machine-checked proof that its implementation meets this specification. DFSCQ's specification captures the behavior of sophisticated optimizations, including log-bypass writes, and DFSCQ's proof rules out some of the common bugs in file-system implementations despite the complex optimizations. The key challenge in building DFSCQ is to write a specification for the file system and its internal implementation without exposing internal file-system details. DFSCQ introduces a metadata-prefix specification that captures the properties of fsync and fdatasync, which roughly follows the behavior of Linux ext4. This specification uses a notion of tree sequences---logical sequences of file-system tree states---for succinct description of the possible states after a crash and to describe how data writes can be reordered with respect to metadata updates. This helps application developers prove the crash safety of their own applications, avoiding application-level bugs such as forgetting to invoke fsync on both the file and the containing directory. An evaluation shows that DFSCQ achieves 103 MB/s on large file writes to an SSD and durably creates small files at a rate of 1,618 files per second. This is slower than Linux ext4 (which achieves 295 MB/s for large file writes and 4,977 files/s for small file creation) but much faster than two recent verified file systems, Yggdrasil and FSCQ. Evaluation results from application-level benchmarks, including TPC-C on SQLite, mirror these microbenchmarks.