[quote="kurbads"]With an 16Gb movie it takes about 3 hours to download. If the movie is over 3 hours long with sequential download I can start watching straight away. It is brilliant feature.[/quote]Sequential downloading:
1.Hammers seeds and peers for "common" pieces, especially the 1st and last pieces in a torrent...when everyone else in the torrent swarm may be desperate for "rare" pieces near the end of the torrent. Seeds will be forced to upload the same starting pieces in a torrent to each new sequential downloader that arrives, leaving less time+upload for these seeds to send rare pieces to other peers.
2.Penalizes other peers trying to play fair (those peers prioritize randomly downloading rarest pieces) -- because sequential downloaders will usually not be interested in rare pieces and will not upload as much to other peers playing fair *because they gave it little*!
3.Do not even "play nice" with each other! 2 "pure" sequential downloaders (that only download the next piece in sequence) will NEVER tit-for-tat (give something to get something) with each other since at no time will both peers have a piece the other wants. So the one that gets further ahead will be pure leeched by ALL the other sequential downloaders as they try to catch up. That means the one in the lead will have to download even more from seeds that should be giving out only rarest pieces!
4.Seeds will be unable to use super seeding/initial seeding mode because sequential downloaders may refuse to download or will be very slow about sharing their rarest piece with other sequential downloaders.
5.Sequential downloading can slow down a torrent swarm to the point that it dies early if/when the last seeds leave after reaching their upload ratio. The remaining peers on the torrent will be very likely to not have enough pieces between them to complete the torrent, even if there's more than 10 of them. This becomes an extremely bad problem on torrents over 1 GB in size. I've seen the last few pieces in a torrent become exceedingly rare even when only about 2 out of 10 peers were sequential downloaders. Fortunately, I was already a seed by that time.
Torrent Streaming is even worse than sequential downloading!
Being able to play a video torrent as it downloads requires a download speed faster than the playback speed (unless the download has a long head-start) and MUCH higher than most seeds and peers can upload, so it's unsustainable if everyone does it.
The likelihood of someone stopping the torrent very shortly after downloading+playing it is much greater, not giving the last few pieces of the torrent enough time to be uploaded to others. The already-overloaded (or missing) seeds will be the only ones with the last few pieces in the torrent...so other peers will take much longer to completely download the torrent.
This only works ok where the torrent swarm is overwhelmingly seeds with very few peers...such as on private trackers.
It is bad if someone does this often on low traffic public torrents (with fewer than 20 seeds+peers, even if most are seeds), terrible if this is done by default for every new torrent they start, and beyond horrible if done for more than 1 downloading torrent at the same time!
(You can only effectively watch 1 video at a time, so doing this for 2+ downloads at once is just plain mean.)
A complete disaster if a common BitTorrent client supports this feature as the default behavior. Leechclient indeed!