[bittorrent] IO bound

Carsten Otto c-otto at gmx.de
Thu Apr 12 12:51:00 EDT 2007

Hi there!

My university has a 10 GBit/sec connection where I have access to (at
least) 1 GBit/sec. My experiments with various PCs and Bittorrent
clients indicate that the speed is dominated by the power of the
underlying disk system. Our FTP server has the same problem, 12 disks in
a RAID 5 (old 3Ware controller) only delivers about 25-30 MByte/sec
via FTP and HTTP (many different users, higher peeks possible).
With a newer but smaller RAID system I can get 11 MByte/sec with
bittorrent, older systems top at around 6 MByte/sec. I'd be happy to
push Debian with more than (aggregated) ~15 MByte/sec.

Long story short, delivering torrents from the disk is no good solution
for me. Buying lots of RAM unfortunately is no solution, neither.
Is there a client or at least some well-thought algorithm that solves
this problem?

One idea is to fake the message telling other clients what my client
has. This way I could offer as many chunks as my RAM can take, after a
while (when speed drops? constant time? other criteria?) the RAM is
refilled with other chunks. Of course every kind of fake is no good idea
in general, but for torrents where additional speed can help the "real"
seeders this should be OK.

As a modification one could try to find out which chunks are needed
most, but according to a discussion from 2004 and several other reasons
this seems to be impractical.

Feel free to write your opinion, even if you dislike this idea :)

PS: Of course it is possible to use the disk with a second client on the
same machine, so that one client offers "little" (< 2 GByte) very fast
(out of RAM), the other client has everything from disk, but is slower.

Carsten Otto
c-otto at gmx.de
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