Dávid, Róbert

Vincze, Gábor

Dávid Róbert

Vincze Gábor

University of Szeged

Improving DHT-Based File System Performance with B-treesRóbert Dávid and Gábor VinczeIn this paper, we present a way of improving the performance of network file systems basedon Distributed Hash Tables (DHT) by incorporating B-tree variant into the file system.System StructureThe proposed system has four subsystem layers:• File System (user interface, top layer)• Search trees, indexing• Reliable data block storage• DHT (data block location and routing)The top layer is a conventional file system interface: it should show to users files, attributes,directories, and should provide tools to add, modify, copy, delete files and their metadata.DHTs offer a scalable and completely decentralized storage mechanism [1, 2, 3, 4]. However,DHTs only solve the problem of Decentralized Object Location and Routing (DOLR), and donot provide any means for indexing and searching contents of the network. Many solutionshave been proposed to build an indexing layer on top of the DHT, and offer file systemfunctionality [5, 6]. In the Cooperative File System, a DHash layer provides redundancy andcaching of file system data and metadata blocks, on top of which a File System layer providesthe file system interface to users. In our system, we replace the simple replication of DHashwith a more efficient block-coding algorithm, like IDA [7] or Reed- Solomon block coding. Inour system, we add a search tree/indexing layer between the reliable data block storage andthe File System. Modern conventional file systems use B-trees to improve performance [8],and outperform non-B-tree file systems by a factor of 10-15x in certain usage scenarios. Wehope to use B-trees in DHT-based file systems to reduce the number of routing steps necessaryto locate data blocks, which can have a significant impact in the case of a globally distributednetwork.Search TreesThe search tree we propose is a B+-tree, we call C-tree, C for content addressing. Takingadvantage of content addressing, we can increase the branching factor, while node sizesin bytes can stay the same. The concept is the following: Let the address of tree-nodes becalculated from the first and last index value, one can reach using the node in question. Iproposed a hash function to spread nodes data uniformly over the DHT network. Nowchild nodes can be addressed without a pointer, we need only the "lowerthan- node" and"higher-than-node" index values the B+ algorithm already stores, and use the hash function.A little downside is that we must store the low and high limit of the node, but it is outweighted by the benefit of not having to store node addresses implicitly. An extra advantageis that the root blocks address will be always well known, because it must be the hash of theindex of all zero bits and all one bits. To install multiple file systems over one DHT can beguaranteed to use of keyed cryptographic hash functions, encrypted storage, user specific keys.ConclusionBy incorporating B-trees into DHT-based file systems, we hope to take DHT-based file sys-tems beyond the simple initial architectures used in CFS or Pastis.37References[1] I. Stoica, R. Morris, D. Karger, M.F. Kaashoek, and H. Balakrishnan. Chord: A scalable peer-to-peer lookup service for internet applications, In Proc. ACM SIGCOMM (San Diego, aug.2001).[2] S.P. Ratnasamy. A scalable content-addressable network (2002)[3] P. Druschel and A. Rowstron. Past: Persistent and anonymus storage in a peer-to-peer net-working environment, In Proceedings of the 8th IEEE workshop on hot topics in operating sys-temy (HOTOS 2001), (Elmau/Oberbayern, Germany, may 2001), PP. 65-70.[4] B.Y. Zhao, J. Kubiatowicz, and A.D. Joseph. TAPESTRY: An infrastructure for fault-tolerantwide area location routing, (2001)[5] F. Dabek. A cooperative file system, (2001)[6] J.-M. Busca, F. Picconi, and P. Sens. PASTIS: A highly-scalable multi-user peer-to-peer filesystem, (2004)[7] M.O. Rabin. Efficient dispersal of information for security, Load balancing, and fault toler-ance, JACM, Vol. 36, no. 2, april 1989.[8] REISERFS4, http://www.namesys.com/V4/V4.html38

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Improving DHT-based file system performance with B-trees

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