  Peer-to-peer content distribution systems have been enjoying great
popularity, and are now gaining momentum as a means of disseminating video
streams over the Internet. In many of these protocols, including the popular
BitTorrent, content is split into mostly fixed-size pieces, allowing a client
to download data from many peers simultaneously. This makes piece size
potentially critical for performance. However, previous research efforts have
largely overlooked this parameter, opting to focus on others instead. This
paper presents the results of real experiments with varying piece sizes on a
controlled BitTorrent testbed. We demonstrate that this parameter is indeed
critical, as it determines the degree of parallelism in the system, and we
investigate optimal piece sizes for distributing small and large content. We
also pinpoint a related design trade-off, and explain how BitTorrent's choice
of dividing pieces into subpieces attempts to address it.
