Linux Swap Files Explained

Swap space is what the kernel falls back to when RAM fills up: inactive memory pages get moved out to disk instead of the system triggering an out-of-memory kill. It's slower than RAM, but it's the difference between a heavy workload merely slowing down versus crashing outright. This walks through creating, securing, and enabling a 16GB swap file.

Disable Any Existing Swap

Start from a clean slate, especially if replacing or resizing an existing swap file:

sudo swapoff -a

-a disables every swap device listed in /proc/swaps.

Allocate the Swap File

sudo fallocate -l 16G /swapfile

fallocate pre-allocates the space directly — much faster than the older dd if=/dev/zero approach.

Secure It

Swap can contain data moved straight out of RAM, including things like passwords held in memory — so the file must only be readable by root:

sudo chmod 600 /swapfile

600 means read/write for the owner (root) only; no permissions for group or others.

Format it as Swap

sudo mkswap /swapfile

This sets up the file as a Linux swap area so the kernel recognizes it as valid swap storage.

Enable it for the Current Session

sudo swapon /swapfile

Make it Persistent Across Reboots

The steps above only take effect until the next reboot. To have it load automatically at boot, add it to /etc/fstab:

sudo cp /etc/fstab /etc/fstab.bak
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

Verify

sudo swapon --show    # should list /swapfile with SIZE 16G
free -h                # check the Swap: row for total/used

Removing it Later

sudo swapoff -v /swapfile
sudo nano /etc/fstab     # delete the /swapfile line
sudo rm /swapfile