Synthesis-Aided Crash Consistency for Storage Systems

Reliable storage systems must be crash consistent - guaranteed to recover to a consistent state after a crash. Crash consistency is non-trivial as it requires maintaining complex invariants about persistent data structures in the presence of caching, reordering, and system failures. Current programming models offer little support for implementing crash consistency, forcing storage system developers to roll their own consistency mechanisms. Bugs in these mechanisms can lead to severe data loss for applications that rely on persistent storage. This paper presents a new synthesis-aided programming model for building crash-consistent storage systems. In this approach, storage systems can assume an angelic crash-consistency model, where the underlying storage stack promises to resolve crashes in favor of consistency whenever possible. To realize this model, we introduce a new labeled writes interface for developers to identify their writes to disk, and develop a program synthesis tool, DepSynth, that generates dependency rules to enforce crash consistency over these labeled writes. We evaluate our model in a case study on a production storage system at Amazon Web Services. We find that DepSynth can automate crash consistency for this complex storage system, with similar results to existing expert-written code, and can automatically identify and correct consistency and performance issues

Caching, crashing & concurrency - verification under adverse conditions

The formal development of large-scale software systems is a complex and time-consuming effort. Generally, its main goal is to prove the functional correctness of the resulting system. This goal becomes significantly harder to reach when the verification must be performed under adverse conditions. When aiming for a realistic system, the implementation must be compatible with the “real world”: it must work with existing system interfaces, cope with uncontrollable events such as power cuts, and offer competitive performance by using mechanisms like caching or concurrency. The Flashix project is an example of such a development, in which a fully verified file system for flash memory has been developed. The project is a long-term team effort and resulted in a sequential, functionally correct and crash-safe implementation after its first project phase. This thesis continues the work by performing modular extensions to the file system with performance-oriented mechanisms that mainly involve caching and concurrency, always considering crash-safety. As a first contribution, this thesis presents a modular verification methodology for destructive heap algorithms. The approach simplifies the verification by separating reasoning about specifics of heap implementations, like pointer aliasing, from the reasoning about conceptual correctness arguments. The second contribution of this thesis is a novel correctness criterion for crash-safe, cached, and concurrent file systems. A natural criterion for crash-safety is defined in terms of system histories, matching the behavior of fine-grained caches using complex synchronization mechanisms that reorder operations. The third contribution comprises methods for verifying functional correctness and crash-safety of caching mechanisms and concurrency in file systems. A reference implementation for crash-safe caches of high-level data structures is given, and a strategy for proving crash-safety is demonstrated and applied. A compatible concurrent implementation of the top layer of file systems is presented, using a mechanism for the efficient management of fine-grained file locking, and a concurrent version of garbage collection is realized. Both concurrency extensions are proven to be correct by applying atomicity refinement, a methodology for proving linearizability. Finally, this thesis contributes a new iteration of executable code for the Flashix file system. With the efficiency extensions introduced with this thesis, Flashix covers all performance-oriented concepts of realistic file system implementations and achieves competitiveness with state-of-the-art flash file systems

PerSeVerE: persistency semantics for verification under ext4.

Although ubiquitous, modern filesystems have rather complex behaviours that are hardly understood by programmers and lead to severe software bugs such as data corruption. As a first step to ensure correctness of software performing file I/O, we formalize the semantics of the Linux ext4 filesystem, which we integrate with the weak memory consistency semantics of C/C++. We further develop an effective model checking approach for verifying programs that use the filesystem. In doing so, we discover and report bugs in commonly-used text editors such as vim, emacs and nano

Collecting ground truth annotations for drum detection in polyphonic music

In order to train and test algorithms that can automatically detect drum events in polyphonic music, ground truth data is needed. This paper describes a setup used for gathering manual annotations for 49 real-world music fragments containing different drum event types. Apart from the drum events, the beat was also annotated. The annotators were experienced drummers or percussionists. This paper is primarily aimed towards other drum detection researchers, but might also be of interest to others dealing with automatic music analysis, manual annotation and data gathering. Its purpose is threefold: providing annotation data for algorithm training and evaluation, describing a practical way of setting up a drum annotation task, and reporting issues that came up during the annotation sessions while at the same time providing some thoughts on important points that could be taken into account when setting up similar tasks in the future