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Evaluating the Price of Consistency in Distributed File Storage Services

Type of publication Peer-reviewed
Publikationsform Proceedings (peer-reviewed)
Author Valerio Jose, Sutra Pierre, Rivière Etienne, Felber Pascal,
Project Trustworthy Cloud Storage
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Proceedings (peer-reviewed)

Title of proceedings 13th International IFIP Conference on Distributed Applications and Interoperable Systems
Place Florence, Italy

Abstract

Distributed file storage services (DFSS) such as Dropbox, iCloud, SkyDrive, or Google Drive, offer a filesystem interface to a distributed data store. DFSS usually differ in the consistency level they provide for concurrent accesses: a client might access a cached version of a file, see the immediate results of all prior operations, or temporarily observe an inconsistent state. The selection of a consistency level has a strong impact on performance. It is the result of an inherent tradeoff between three properties: consistency, availability, and partition-tolerance. Isolating and identifying the exact impact on performance is a difficult task, because DFSS are complex designs with multiple components and dependencies. Furthermore, each system has a different range of features, its own design and implementation, and various optimizations that do not allow for a fair comparison. In this paper, we make a step towards a principled comparison of DFSS components, focusing on the evaluation of consistency mechanisms. We propose a novel modular DFSS testbed named FlexiFS, which implements a range of state-of-the-art techniques for the distribution, replication, routing, and indexing of data. Using FlexiFS, we survey six consistency levels: linearizability, sequential consistency, and eventual consistency, each operating with and without close-to-open semantics. Our evaluation shows that: (i) as expected, POSIX semantics (i.e., linearizability without close-to-open semantics) harm performance; and (ii) when close-to-open semantics is in use, linearizability delivers performance similar to sequential or eventual consistency.
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