Flexible replica placement for optimized P2P backup on heterogeneous, unreliable machines

P2P architecture is a viable option for enterprise backup. In contrast to dedicated backup servers, nowadays, a standard solution, making backups directly on organization's workstations should be cheaper as existing hardware is used, more efficient as there is no single bottleneck server, and more reliable as the machines can be geographically dispersed. We present an architecture of a P2P backup system that uses pairwise replication contracts between a data owner and a replicator. In contrast to a standard P2P storage system using directly a distributed hash table (DHT), the contracts allow our system to optimize replicas' placement depending on a specific optimization strategy and so to take advantage of the heterogeneity of the machines and the network. Such optimization is particularly appealing in the context of backup: replicas can be geographically dispersed, the load sent over the network can be minimized, or the optimization goal can be to minimize the backup/restore time. However, managing the contracts, keeping them consistent and adjusting them in response to dynamically changing environment is challenging. We built a scientific prototype and ran experiments on 150 workstations in our university's computer laboratories and, separately, on 50 PlanetLab nodes. We found out that the main factor affecting the performance of the system is the availability of the machines. Yet, our main conclusion is that it is possible to build an efficient and reliable backup system on highly unavailable machines, as our computers had just 13% average availability. Copyright © 2015 John Wiley & Sons, Ltd.