Ubuntu – What’s the difference between setting open file limits in /etc/sysctl.conf vs /etc/security/limits.conf

file-descriptorslinuxsysctlUbuntuulimit

I've read two separate ways of increasing the allowed open file count (I'm attempting to modify for root, if it matters).

One way is to update the settings in /etc/security/limits.conf with something like:

*                soft    nofile          500000

*                hard    nofile          500000

root             soft    nofile          500000

root             hard    nofile          500000

To make settings for the active shell, it looks like you can just do ulimit -n 500000, which wouldn't require a reboot or to logout/login, but may require restarting services (?).

The other option is to update /etc/sysctl.conf:

echo 'fs.file-max = 500000' >> /etc/sysctl.conf

To make settings for the active shell, we can do sysctl -p, and verify with sysctl fs.file-max.

So my question is, what's the difference? Is there one? I'm on Ubuntu 14.04.2 LTS

Best Answer

The difference is the scope, and how it's applied. Open file limits set via sysctls apply to the entire system, whereas limits set via /etc/security/limits.conf apply only to things that meet the criteria specified there. The other primary difference is that /etc/security/limits.conf limits are applied via ulimit, and thus can be changed more readily, while the sysctl limit is essentially setting up a memory allocation limit in the kernel itself.

As a general rule, you almost always want to use /etc/security/limits.conf, even if you're setting global limits with the wildcard match there, as it is a bit more reliable, and things usually fail more gracefully when hit with ulimit restrictions than hitting kernel memory allocation limits.

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