linux_wiki:use_proc_sys_and_sysctl_to_modify_and_set_kernel_runtime_parameters [Owlbear Consulting

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====== Use Proc Sys And Sysctl To Modify And Set Kernel Runtime Parameters ======

**General Information**

Modifying runtime or persistent kernel parameters. 

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====== Lab Setup ======

The following virtual machines will be used:
  * server1.example.com (192.168.1.150) -> Configure the kernel parameters here

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====== Help ======

Finding help in this section.
  * Sysctl man page<code bash>man sysctl</code>

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====== Runtime Kernel Settings ======

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

\\
View all tunable kernel parameters
<code bash>
sysctl -a
</code>

\\
View the configuration of a specific setting
<code bash>
sysctl vm.swappiness
</code>

\\
Write a runtime only configuration change and verify
<code bash>
sysctl -w vm.swappiness=10
sysctl vm.swappiness

# Or
sysctl -a | grep swappiness
</code>
  * -w -> write

\\
Alternative: Echo the value into the correct location in /proc
<code bash>
echo "15" > /proc/sys/vm/swappiness
</code>

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====== Persistent Kernel Settings ======

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
<code bash>
vim /etc/sysctl.conf

net.ipv4.ip_forward = 1
</code>

\\
Load settings from a file
<code bash>
sysctl -p /etc/sysctl.d/mychanges.conf
</code>
  * If no file name is specified, defaults to: /etc/sysctl.conf

\\
Load all settings
<code bash>
sysctl --system
</code>

===== Tuned =====

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)<code bash>....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....</code>
  * Either way; be aware of tuned and its potential to conflict with sysctl.

\\
View the active tuned profile
<code bash>
tuned-adm active
</code>

\\
  * Location of built in system tuned profiles: /usr/lib/tuned/
  * Location of user defined profiles: /etc/tuned
  * **Example**: Create a virtual-guest override file<code bash>mkdir /etc/tuned/virtual-guest
cp /usr/lib/tuned/virtual-guest/tuned.conf /etc/tuned/virtual-guest/</code>
    * Edit the over ride file (/etc/tuned/virtual-guest/tuned.conf), make any changes.
    * Restart the tuned service<code bash>systemctl restart tuned</code>

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