NSDI '08 – Abstract
Pp. 133–146 of the Proceedings
UsenetDHT: A Low-Overhead Design for Usenet
Emil Sit, Robert Morris, and M. Frans Kaashoek, MIT CSAIL
Abstract
Usenet is a popular distributed messaging and file sharing
service: servers in Usenet flood articles over an overlay network
to fully replicate articles across all servers. However,
replication of Usenet's full content requires that each server pay
the cost of receiving (and storing) over 1 Tbyte/day. This paper
presents the design and implementation of UsenetDHT, a Usenet
system that allows a set of cooperating sites to keep a shared,
distributed copy of Usenet articles. UsenetDHT consists of
client-facing Usenet NNTP front-ends and a distributed hash table
(DHT) that provides shared storage of articles across the wide
area. This design allows participating sites to partition the
storage burden, rather than replicating all Usenet articles at all
sites.
UsenetDHT requires a DHT that maintains durability despite
transient and permanent failures, and provides high storage
performance. These goals can be difficult to provide
simultaneously: even in the absence of failures, verifying adequate
replication levels of large numbers of objects can be resource
intensive, and interfere with normal operations. This paper
introduces Passing Tone, a new replica maintenance algorithm for
DHash that
minimizes the impact of monitoring replication levels on memory and
disk resources by operating with only pairwise communication.
Passing Tone's implementation provides performance by using data
structures that avoid disk accesses and enable batch
operations.
Microbenchmarks over a local gigabit network demonstrate that
the total system throughput scales linearly as servers are added,
providing 5.7 Mbyte/s of write bandwidth and 7 Mbyte/s of
read bandwidth per server. UsenetDHT is currently deployed on a
12-server network at 7 sites running Passing Tone over the
wide-area: this network supports our research laboratory's live
2.5 Mbyte/s Usenet feed and 30.6 Mbyte/s of synthetic
read traffic. These results suggest a DHT-based design may be a
viable way to redesign Usenet and globally reduce costs.
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