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Abstract:
This paper considers replication strategies for storage systems that aggregate
the disks of many nodes spread over the Internet. Maintaining replication in
such systems can be prohibitively expensive, since every transient network or
host failure could potentially lead to copying a server's worth of data over
the Internet to maintain replication levels.
The following insights in designing an efficient replication algorithm emerge
from the paper's analysis. First, durability can be provided separately from
availability; the former is less expensive to ensure and a more useful goal for
many wide-area applications. Second, the focus of a durability algorithm must
be to create new copies of data objects faster than permanent disk failures
destroy the objects; careful choice of policies for what nodes should hold what
data can decrease repair time. Third, increasing the number of replicas of each
data object does not help a system tolerate a higher disk failure probability,
but does help tolerate bursts of failures. Finally, ensuring that the system
makes use of replicas that recover after temporary failure is critical to
efficiency.
Based on these insights, the paper proposes the Carbonite replication algorithm
for keeping data durable at a low cost. A simulation of Carbonite storing 1 TB
of data over a 365 day trace of PlanetLab activity shows that Carbonite is able
to keep all data durable and uses 44% more network traffic than a hypothetical
system that only responds to permanent failures. In comparison, Total Recall
and DHash require almost a factor of two more network traffic than this
hypothetical system.
