• Create a Swap File on Ubuntu Safely

    Create a Swap File on Ubuntu Safely

    Swap is disk space that Linux can use when RAM pressure gets high. It is slower than real memory, but it can keep a small VPS or a memory-hungry maintenance job from falling over immediately.

    This is the short version I use for Ubuntu servers when I need a swap file, not a separate swap partition.

    Check Current Swap

    swapon --show
free -h

    If swapon --show already lists a swap device or file, do not add another one blindly. Decide whether you want more swap, a different priority, or no change at all.

    Create the File

    Pick a size. For a small VPS, 2-4 GB is often enough. Here I will create a 4 GB file at /swapfile.

    sudo fallocate -l 4G /swapfile
sudo chmod 600 /swapfile

    The permissions matter. swapon rejects world-readable swap files because swap may contain private process memory.

    If fallocate is not supported by the filesystem, or if swapon later complains about holes in the file, use dd instead:

    sudo dd if=/dev/zero of=/swapfile bs=1M count=4096 status=progress
sudo chmod 600 /swapfile

    The swapon(8) manual notes that swap files with holes can be rejected, and that dd is the most portable way to create a populated swap file.

    Format and Enable It

    sudo mkswap /swapfile
sudo swapon /swapfile

    Verify:

    swapon --show
free -h

    You should see /swapfile in the swapon --show output and non-zero swap in free -h.

    Enable It After Reboot

    Back up /etc/fstab first:

    sudo cp /etc/fstab /etc/fstab.bak

    Add the swap entry:

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

    The none target is conventional for swap entries. swapon --all reads swap entries from /etc/fstab during boot.

    If you want to test the entry without rebooting:

    sudo swapoff /swapfile
sudo swapon -a
swapon --show

    Only do this when the machine is not under heavy memory pressure.

    Optional: Tune Swappiness

    vm.swappiness controls how eagerly the kernel tends to use swap. The default may be fine. On many small servers, I prefer a lower value so swap is a safety net rather than a place the system uses eagerly.

    Check the current value:

    cat /proc/sys/vm/swappiness

    Set it until reboot:

    sudo sysctl vm.swappiness=10

    Persist it:

    echo 'vm.swappiness=10' | sudo tee /etc/sysctl.d/99-swappiness.conf

    Do not tune this by folklore. If the machine is swapping constantly, it probably needs less memory pressure or more RAM, not a prettier swappiness value.

    Rollback

    To remove the swap file:

    sudo swapoff /swapfile
sudo sed -i.bak '\|/swapfile none swap|d' /etc/fstab
sudo rm /swapfile

    Then verify:

    swapon --show
free -h

    Caveats

    • Do not put a swap file on NFS.
    • Be careful on Btrfs and other copy-on-write filesystems; swap files have extra constraints there.
    • If hibernation/resume matters, a swap file is not just “add swap and forget it”. Resume configuration is a separate topic.
    • Swap is not a replacement for memory planning. It is a pressure valve.