I was researching BitTorrent and noticed in the Specification that it said that the typical length of a Piece was 250 kilobytes long. That made me curious so I decided to perform some quick research to determine the extent of variation.
First, I modified the BitTorrent Parser that I had created previously to append data to a “statistics.csv” file. The getfiles.sh script downloaded a 279 files from www.legittorrents.com of which 169 were unique.
After pulling the raw data into MS Excel, de-duplicating it, and graphing it, I came up with the chart below:
Note that the Y-Axis is Log-2.
Fourty-Four percent of the sampled Metainfo files were for a torrent that was under 1 Gb. Of these 76 Metainfo files that correspond to smaller torrents, all but 5 had a piece size of 250Kb and these 5 files were all for torrents that are under ½ Gb in size.
At the other end of the spectrum, 7% of the Metainfo files correspond to torrents that are larger than 10 Gb. All of the Metainfo files in this group had piece sizes that were 2 Mb or larger.
According to its Metainfo file, the largest torrent in the sample was 61.5 Gb and its piece size is 16 Mb.
The Metainfo file in the sample with the largest number of pieces (39,975) is for a torrent that is 9.76 Gb in size, and its piece size is that standard 250 Kb.
Conclusion
Although this sample is too small to prove anything other than that there is indeed variation of piece size for Metainfo files in the wild, it does indicate that Metainfo files may decrease the piece size for small torrents to get more pieces for the swarm to exchange and conversely, increase the piece size for very large torrents to keep the number of pieces to a reasonable number for the swarm.