High Availability, Scalable Storage, Dynamic Peer Networks: Pick Two

Peer-to-peer storage aims to build large-scale, reliable and available storage from many small-scale unreliable, low-availability distributed hosts. Data redundancy is the key to any data guarantees. However, preserving redundancy in the face of highly dynamic membership is costly. We apply a simple resource usage model to measured behavior from the Gnutella file-sharing network to argue that large-scale cooperative storage is limited by likely dynamics and cross-system bandwidth — not by local disk space. We examine some bandwidth optimization strategies like delayed response to failures, admission control, and load-shifting and find that they do not alter the basic problem. We conclude that when redundancy, data scale, and dynamics are all high, the requisite cross-system bandwidth is beyond reasonable expectations.