General Information

Modifying runtime or persistent kernel parameters.

The following virtual machines will be used:

• server1.example.com (192.168.1.150) → Configure the kernel parameters here

Finding help in this section.

• Sysctl man page

  man sysctl

Everything in this section are runtime ONLY changes; the values will go back to their original setting upon reboot.

View all tunable kernel parameters

sysctl -a

View the configuration of a specific setting

sysctl vm.swappiness

Write a runtime only configuration change and verify

sysctl -w vm.swappiness=10
sysctl vm.swappiness
 
# Or
sysctl -a | grep swappiness

• -w → write

Alternative: Echo the value into the correct location in /proc

echo "15" > /proc/sys/vm/swappiness

To make changes that are persistent across reboots…

• Default system values: /usr/lib/sysctl.d/
• Persistent configuration over rides: /etc/sysctl.d/filename.conf

Create entries in /etc/sysctl.d/mychanges.conf

vim /etc/sysctl.conf
 
net.ipv4.ip_forward = 1

Load settings from a file

sysctl -p /etc/sysctl.d/mychanges.conf

• If no file name is specified, defaults to: /etc/sysctl.conf

Load all settings

sysctl --system

Tuned is a “dynamic adaptive system tuning daemon”.

• Notes
  • Starting in EL 7.2, some parameters (such as vm.swappiness) are set AFTER systemd-sysctl.service by the tuned service.
  • In EL 7.4, it appears that tuned defaults to honor sysctl (/etc/tuned/tuned-main.conf)

    ....SNIP....
    # Whether to reapply sysctl from the e.g /etc/sysctl.conf, /etc/sysctl.d, ...
    # If enabled these sysctls will be re-appliead after Tuned sysctls are
    # applied, i.e. Tuned sysctls will not override system sysctls.
    reapply_sysctl = 1
    ....SNIP....

• Either way; be aware of tuned and its potential to conflict with sysctl.

View the active tuned profile

tuned-adm active

• Location of built in system tuned profiles: /usr/lib/tuned/
• Location of user defined profiles: /etc/tuned
• Example: Create a virtual-guest override file

  mkdir /etc/tuned/virtual-guest
  cp /usr/lib/tuned/virtual-guest/tuned.conf /etc/tuned/virtual-guest/

  • Edit the over ride file (/etc/tuned/virtual-guest/tuned.conf), make any changes.
  • Restart the tuned service

    systemctl restart tuned