Added swap notes to the knowledge base

knowledge base/swap.md

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+# Swap
+
+## TL;DR
+
+```shell
+# show the swap usage
+swapon --show
+free -h
+
+# enable or disable a swap partition or file
+sudo swapon /root/swapfile
+sudo swapoff LABEL=swap
+sudo swapoff /dev/sda2
+
+# enable or disable *all* swap partition or file
+sudo swapon -a
+sudo swapoff --all
+
+# chech what processes are swapping
+# see the "si" (swap in) and "so" (swap out) columns
+vmstat
+vmstat --wide 1
+```
+
+## Swappiness
+
+```shell
+# change the current value
+sudo sysctl vm.swappiness=10
+sudo sysctl -w vm/swappiness=5
+
+# persistent configuration
+echo 'vm.swappiness=10'  | sudo tee -a /etc/sysctl.conf
+echo 'vm.swappiness = 5' | sudo tee -a /etc/sysctl.d/99-swappiness.conf
+```
+
+## Swapfile
+
+```shell
+# add a swapfile
+sudo fallocate -l 1G /swapfile   # or sudo dd if=/dev/zero of=/swapfile bs=1024 count=1048576
+sudo chmod 600 /swapfile
+sudo mkswap /swapfile
+sudo swapon /swapfile
+echo '/swapfile swap swap defaults 0 0' | sudo tee -a /etc/fstab
+
+# remove a swapfile
+sudo swapoff -v /swapfile
+sudo sed -i.bak '/\/swapfile/d' /etc/fstab
+sudo rm /swapfile
+```
+
+## Further readings
+
+- [create a linux swap file]
+- [How to reload sysctl.conf variables on Linux]
+- [How to empty swap if there is free RAM]
+
+[create a linux swap file]: https://linuxize.com/post/create-a-linux-swap-file/
+[how to reload sysctl.conf variables on linux]: https://www.cyberciti.biz/faq/reload-sysctl-conf-on-linux-using-sysctl/
+[how to empty swap if there is free ram]: https://askubuntu.com/questions/1357/how-to-empty-swap-if-there-is-free-ram#1379