Add a swap file

Gain headroom when RAM is tight.

1. Check what exists:

   swapon --show
   free -h

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2. Create a 2 GB file and secure it:

   sudo fallocate -l 2G /swapfile
   sudo chmod 600 /swapfile

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3. Format it then enable it:

   sudo mkswap /swapfile
   sudo swapon /swapfile

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4. Make it permanent:

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

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5. On a Btrfs file system, creating a swap file needs special steps: see your distribution's documentation.