So what are these padding files?
For every file in a multi-file torrent, BitComet includes a padding file by default. This overcomes the problem of ending one file and beginning another on the same BitTorrent “piece.” The feature was added to support finding sources from http/ftp/ed2k services on multi-file torrents.
For example, if BitComet users are downloading a set of .mp3 files, it tries to get some of those files from a non-BitTorrent source if possible. This is good both for the (BitComet) user and the swarm. However, the implementation of these padding files create problems for non-Bitcomet clients and the web sites that carry information from the *.torrent files.
Why is this a problem?
Unfortunately, BitComet’s development team sprung these padding files on to the rest of the community. If they had been more communicative, such as pre-publishing a specification, client makers and administrators of Torrent sites could then program their systems to mask them.
It impacts people who use uTorrent, Azureus or any other client than BitComet. The padding feature is enabled by default, so if a BitComet user created a .torrent, non BitComet users have to download these useless padding files. A padding file is created for every file in the .torrent, so if you download a collection of 100 MP3s you’ll be forced to download 100 (useless) padding files (see example). The average added overhead for an MP3 album will be around 3%, not too bad, but annoying because its junk data to most people. However, it is possible in rare circumstances that the amount of junk data caused by these files might exceed 10%.
The padding file feature might come to bite BitComet users. It has been reported that a malicious user could create a torrent with a fake padding file. This means that Bitcomet 0.85+ users will never be able to complete their downloads without switching to another client.
Some BitTorrent users are starting to get annoyed by these (for most people) useless padding files. “Fuzzier,” who has been leading the charge on several forums (including Wikipedia), sums it up:
“I and lots of my friends don’t use BitComet, and many others stick with older versions of BitComet. We see more and more useless padding files in torrents, and it gets really inconvenient â€” delete them then we cannot pass hash check and cannot seed; and no matter what, we get a bunch of wastes especially our precious upload bandwidth.”
Unless this practice becomes more widely adopted in the BitTorrent community, BitComet might consider disabling this feature by default, and suppling client makers and administrators of BitTorrent sites with the specs so they can decide how to deal with them.