Eliminating Cross-server Operations in Scalable File Systems

Carnegie Mellon University Parallel Data Lab Technical Report CMU-PDL-06-105, May 2006.

James Hendricks, Shafeeq Sinnamohideen, Raja R. Sambasivan, Gregory R. Ganger

Carnegie Mellon University
Pittsburgh, PA 15213


Distributed file systems that scale by partitioning files and directories among a collection of servers inevitably encounter crossserver operations. A common example is a RENAME that moves a file from a directory managed by one server to a directory managed by another. Systems that provide the same semantics for cross-server operations as for those that do not span servers traditionally implement dedicated protocols for these rare operations. This paper suggests an alternate approach that exploits the existence of dynamic redistribution functionality (e.g., for load balancing, incorporation of new servers, and so on). When a client request would involve files on multiple servers, the system can redistribute those files onto one server and have it service the request. Although such redistribution is more expensive than a dedicated cross-server protocol, the rareness of such operations makes the overall performance impact minimal. Analysis of NFS traces indicates that cross-server operations make up fewer than 0.001% of client requests, and experiments with a prototype implementation show that the performance impact is negligible when such operations make up as much as 0.01% of operations. Thus, when dynamic redistribution functionality exists in the system, cross-server operations can be handled with little additional implementation complexity.

KEYWORDS: Object-based storage, OSD, metadata scalability, cross-server operations, dynamic redistribution, trace analysis, failure recovery, atomicity in distributed systems, mitigating complexity

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