Work in progress

Client:

• rTorrent: requires "d.down.sequential(.set)" commands, see jesec/rtorrent@bd904e3
• qBittorrent: supported
• Transmission: rejected, see https://trac.transmissionbt.com/ticket/452

Drawbacks:

• https://wiki.vuze.com/w/Sequential_downloading_is_bad

Benefits:

1. Sequential downloading helps with I/O performance and lowers resource consumption and cost of hardware. It enables predictable I/O patterns and efficient caching. My hard disk is really noisy while downloading a torrent. But with sequential downloading, it does not make a sound. Speed is much better as well. Sequential I/O patterns also eliminate disk fragmentation problem that damages performance/lifespan of hard disks in long-term, which is a headache for long seeders.
2. "It is bad for swarms." That's correct for unhealthy swarms. However, for private torrent users, in most cases, the seeder/leecher relationship is "many seeder, single/little leecher". Plus, with the incentives/punishments of tracker sites, there is little to no risk of "draining". As such, it makes more sense to protect hardware of everyone in the swarm. Predictable I/O patterns also allow seeders to seed many torrents more efficiently: if leechers use sequential download, the read patterns become predictably sequential, which allows better I/O performance and reduces the failure rate of hard disks.
3. For seedbox users: seedboxes are virtual machines. That means many users share the same physical machine. Random chunk downloading is extremely taxing on disks. As a result, usually the speeds are limited by I/O more than bandwidth. If the swarm pattern is usually "many seeder, single leecher", sequential downloading can help a lot.
4. Widely known "self" benefits: stream early, stream while downloading, organize episodes quick and unpack some files before finish, etc.