Linux – How to increase max FD limit for a daemon process running under a headless user

Your operating system set limits on how many files can be opened by any running application on your host. You can extend the basic values usually 1024 easily by modifying 2 configuration files:

# vi /etc/sysctl.conf

fs.file-max = 32000

# vi /etc/security/limits.conf

youruser       soft    nofile   10000
youruser       hard    nofile   30000

The hard and soft limits:

man 5 limits.conf

hard
for enforcing hard resource limits. These limits are set by the superuser and 
enforced by the Kernel. The user cannot raise his requirement of system resources
above such values.

soft
for enforcing soft resource limits. These limits are ones that the user
can move up or down within the permitted range by any pre-exisiting hard 
limits. The values specified with this token can be thought of as default values,
for normal system usage.

HTH

So it seems MongoDB 1.6.5 doesn't work with the limit nofile line. You need to put ulimit -n X (where X is the file descriptors you want) just after the script keyword.

    script
        ulimit -n 60000
        PORT=27027
...

I guess the method using the pre-start block is for later versions of MongoDB.

Also for note, MongoDB has a hard limit of 20,000 connections, so it doesn't matter if you set file descriptors higher than that.

Another thing learned: starting and stopping using upstart scripts is not the same as REstarting. Starting and stopping threw errors on the limit nofile command, whereas restarting did not.