Transmission's bandwidth overhead is enourmous.

Binary package hint: transmission-gtk

I tried to download a big file (5.6 GiB) using the Transmission BitTorrent client coming with Ubuntu 8.10. Available bandwidth is about 62 KiB/s, so I set Transmission to 50 KiB/s download, 9 KiB/s upload.

An hour into this download, Transmission reported a remaining time of 1 day, 12 hours. This is consistent with the reported 35 to 50 KiB/s download rate.

Some 20 hours later, Transmission still reported about 1 day, 10 hours left. The download was now about 230 MiB. I would have expected the download to be at least half done after this time, so I took manual measurements. Over 34 minutes I regularly watched the reported download speed and it was at the ceiling, at 50 KiB/s most of the time. This should result in a download size of about 40 KiB/s * 34 min * 60 sec/min = 81,000 KiB or short of 80 MiB. However, reported file size progressed by just 5 MiB. That's an overhead of 93.7% of the available bandwidth.

To test wether it's only a display issue, I've lifted Transmission's bandwidth cap very high and indeed, the reported download speed is limited by my provider's cap where I would expect it (59 - 60 KiB/s). So, the bandwidth waste is obviously real.

To solve the problem for me, I installed Deluge and Deluge behaves like I'd expect (file size grows more than 100 MiB/hour). Deluge continued to download into the very same file, so Deluge checked Transmissions download. It found just 7.6 MiB worth of completed chunks, so Transmission managed to assemble a pile of (7.6 MiB to 230 Mib) 96% unverified data. Chunk size is 2 MiB.

As I have quite a bunch of bugs open related to Ubuntu I can't invest into serious debugging, but as Transmission is Ubuntu's default BitTorrent I think it was important to get this very poor experience reported.

~$ apt-cache policy transmission-common transmission-gtk
transmission-common:
  Installiert: 1.34-0ubuntu2.2
  Kandidat: 1.34-0ubuntu2.2
  Versions-Tabelle:
 *** 1.34-0ubuntu2.2 0
        500 http://ubuntu.intergenia.de intrepid-updates/main Packages
        100 /var/lib/dpkg/status
     1.34-0ubuntu2 0
        500 http://ubuntu.intergenia.de intrepid/main Packages
transmission-gtk:
  Installiert: 1.34-0ubuntu2.2
  Kandidat: 1.34-0ubuntu2.2
  Versions-Tabelle:
 *** 1.34-0ubuntu2.2 0
        500 http://ubuntu.intergenia.de intrepid-updates/main Packages
        100 /var/lib/dpkg/status
     1.34-0ubuntu2 0
        500 http://ubuntu.intergenia.de intrepid/main Packages

~$ lsb_release -rd
Description: Ubuntu 8.10
Release: 8.10

~$ uname -a:
Linux piccard 2.6.27-13-generic #1 SMP Thu Feb 26 07:31:49 UTC 2009 x86_64 GNU/Linux