Linux VPS Out of Memory? How to Add a Swap File

Your application runs fine for days, then the database disappears from the process list and the logs say Out of memory: Killed process. That is the Linux OOM killer doing its job: when RAM runs out and there is nowhere else to put memory pages, the kernel kills the largest process. A swap file gives the kernel somewhere else to go. It does not make a small server fast, but it turns a crash into a slowdown you can see and fix.

What swap is, and what it is not

Swap is disk space the kernel uses as overflow for memory pages that are not being used right now. On NVMe storage it is far slower than RAM but much faster than older disks. Use it as a safety net for short spikes, such as a build, a backup job or a traffic burst. If your server uses swap heavily all day, the real fix is more RAM, not more swap.

Step 1: Check what you have

free -h
swapon --show

If swapon --show prints nothing, the server has no swap. Also check free disk space with df -h / before you continue.

Step 2: Choose a size

As a rule of thumb:

• 1 to 2 GB RAM: a swap file equal to your RAM.

• 4 to 8 GB RAM: 2 to 4 GB of swap is usually enough.

• 16 GB RAM and above: 4 GB as a safety margin, unless your software documents a different need.

Step 3: Create and enable the swap file

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

The chmod 600 step matters: swap can contain fragments of memory, including secrets, so only root should be able to read it. If fallocate is not supported on your filesystem, use sudo dd if=/dev/zero of=/swapfile bs=1M count=2048 instead.

Step 4: Make it permanent

Without this step the swap file is gone after the next reboot.

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

Run sudo swapon --show again to confirm it is active.

Step 5: Tune swappiness

The vm.swappiness setting controls how eagerly the kernel moves pages to swap. Many distributions default to 60. For a server where you want RAM used first and swap kept for emergencies, a lower value is a common choice:

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

How to tell whether swap is hiding a bigger problem

• Watch it over time: vmstat 5 shows the si and so columns. Occasional activity is fine. Constant swapping in and out means the server is short of memory.

• Find the memory user: ps aux --sort=-%mem | head lists the largest processes.

• Cap the heavy services: database caches, PHP-FPM worker counts and Node.js heap size can usually be set lower so they fit the server.

When to upgrade instead

If swap is busy most of the day, response times will suffer no matter how you tune it. On an IM HOST Linux VPS you get full root to set up swap exactly as above, with NVMe storage, a dedicated IPv4 plus an IPv6 /64, and DDoS protection, from $5/month. When the workload outgrows that, Cloud VPS plans start at 4 dedicated vCPU and 8 GB RAM and can be upgraded anytime.