Increase file descriptors for Transmission on Linux

Have you run out of file descriptors for Transmission? The torrent will be stopped (for no apparent reason), and when you examine it, you’ll see an error similar to:

$ transmission-remote -t 1 -i | grep -i 'open files'
Unable to save resume file. Too many open files.

Time to increase the number of file descriptors available. This article is tailored towards Debian and Ubuntu.

It’s unlikely you’ll need to raise your system’s global limit. Check with:

$ cat /proc/sys/fs/file-max
397460

The OS needs a couple thousand file descriptors for itself. Make sure to make space for them with whatever numbers to choose below. In my case, I’ve more than enough.

If you still need to raise your system’s limit, you can easily; to set it to a million (which will be remembered across reboots):

sudo sh -c "echo fs.file-max=$(dc -e '2 20 ^ p') > /etc/sysctl.d/file-descriptors-max.conf
sudo service procps restart

While you may not need to change your system’s global limit, you probably will need to change the limit for your users. Check that limit with:

$ ulimit -Sn
1024
$ ulimit -Hn
1024

If you’re working with hundreds of torrents (each with dozens to hundreds of files) with Transmission, this isn’t enough. To let a user have a few thousand (in the below example, 16,384, with 128 more for the hard limit), create a new file /etc/security/limits.d/debian-transmission.conf:

sudo sh -c "echo debian-transmission soft nofile $(dc -e '2 14 ^ p')" > /etc/security/limits.d/debian-transmission.conf
sudo sh -c "echo debian-transmission hard nofile  $(dc -e '2 14 ^ 2 7 ^ + p')" >> /etc/security/limits.d/debian-transmission.conf

Replace “debian-transmission” with the user that is running Transmission.

For the changes to go into effect, you need to logout completely (e.g. close multiplexed SSH connections, etc), and log back in again. Or to be sure, just reboot to make sure changes kick in. You’ll see you have many more file descriptors available:

$ ulimit -Sn
16384
$ ulimit -Hn
16512

Now, we need to configure Transmission to use this many. In /etc/transmission-daemon/settings.json, find the open-file-limit option and set to a larger number (e.g. 16000 or so). When done, restart transmission-daemon:

sudo service transmission-daemon restart

If you’re not running Transmission as a system user, edit the right settings.json and restart the daemon appropriately.

That’s it. Have fun!