Merging semantics for conflict updates in geo-distributed file systems

Proceedings of the 13th ACM Workshop on Hot Topics in Storage and File Systems

et al. 2021

Self Cite

Building scalable and highly available geo-replicated file systems is hard. These systems need to resolve conflicts that emerge in concurrent operations in a way that maintains file system invariants, is meaningful to the user, and does not depart from the traditional file system interface. Conflict resolution in existing systems often leads to unexpected or inconsistent results. This paper introduces ElmerFS, a geo-replicated, truly concurrent file system designed with the aim of addressing these challenges. ElmerFS is based on two key ideas: (1) the use of Conflict-Free Replicated Data Types (CRDTs) for representing file system structures, which ensures that replicas converge to a correct state, and (2) conflict resolution rules, which are determined by the choice of CRDT types and their composition, designed with the principle of being intuitive to the user. We argue that if the state of the file system after resolving a conflict conveys to the user the resolved conflict in an intuitive way, the user can complement or reverse it using traditional file system operations. We discuss the challenges in the design of geo-replicated weakly consistent file systems, and present the design of ElmerFS.

Section: Related Workmentioning

confidence: 99%

CRDTs for truly concurrent file systems

Vaillant¹,

Vasilas

Proceedings of the 13th ACM Workshop on Hot Topics in Storage and File Systems

et al. 2021

Self Cite

“…For instance, in Fig. 6, the cycle between directories u and v is removed by making copies of the directories u and v. This merge semantics is used in real file systems, such as geoFS [45].…”

Section: Fully-asynchronous File Systemmentioning

confidence: 99%

“…The only exception is that concurrent move operations may violate the tree invariant, resulting in a disconnected cyclic component. To fix this issue, our design follows the geoFS system [45]: if a cycle would occur, it effectively replaces move with a copy-delete, which preserves the tree invariant but might duplicate the directories that would otherwise end up in a cycle.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Co-Design and Verification of an Available File System

Lecture Notes in Computer Science

Eugster

2017

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Abstract. Distributed file systems play a vital role in large-scale enterprise services. However, the designer of a distributed file system faces a vexing choice between strong consistency and asynchronous replication. The former supports a standard sequential model by synchronising operations, but is slow and fragile. The latter is highly available and responsive, but exposes users to concurrency anomalies. In this paper, we describe a rigorous and general approach to navigating this trade-off by leveraging static verification tools that allow to verify different file system designs. We show that common file system operations can run concurrently without synchronisation, while still retaining a semantics reasonably similar to Posix hierarchical structure. The one exception is the move operation, for which we prove that, unless synchronised, it will have an anomalous behaviour.

“…Existing systems are cloud-based, sometimes providing adhoc application-level caching. Consistency violations are common, baffling users and vexing application developers [10,18,19,50,63]. Support for offline operation is spotty.…”

Section: Introductionmentioning

confidence: 99%

Highly-available and consistent group collaboration at the edge with colony

Toumlilt

Sutra

Proceedings of the 22nd International Middleware Conference

2021

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Edge applications, such as gaming, cooperative engineering, or in-the-field information sharing, enjoy immediate response, autonomy and availability by distributing and replicating data at the edge. However, application developers and users demand the highest possible consistency guarantees, and specific support for group collaboration. To address this challenge, Colony guarantees Transactional Causal Plus Consistency (TCC+) globally, strengthened to Snapshot Isolation within edge groups. To help with scalability, fault tolerance and security, its logical communication topology is forestlike, with replicated roots in the core cloud, but with the flexibility to migrate a node or a group. Despite this hybrid approach, applications enjoy the same semantics everywhere in the topology. Our experiments show that local caching and peer groups improve throughput and response time significantly, performance is not affected in offline mode, and that migration is seamless. CCS Concepts: • Computer systems organization → Peer-to-peer architectures; • Networks → Cloud computing; • Information systems → Key-value stores; Distributed storage; • Computing methodologies → Distributed algorithms.