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Introduction

Guarding against out-of-memory errors is crucial for applications, and one effective method is to add swap space to your server. This guide walks you through the process of adding a swap file to an Ubuntu 22.04 server.

What is Swap?

Swap is a dedicated portion of hard drive storage reserved for the operating system to temporarily store data when it exceeds the capacity of RAM. Although writing to swap is slower than RAM, it serves as a safety net when RAM is depleted, preventing out-of-memory errors.

Step 1 – Checking the System for Swap Information

Before starting, check if your system already has swap space:

sudo swapon --show

If there's no output, your system currently lacks swap space.

Verify using the free utility:

free -h

No active swap is indicated in the output.

Step 2 – Checking Available Space on the Hard Drive Partition

Ensure sufficient disk space by checking current usage:

df -h

Look for available space on the device with "/" in the Mounted on column. A rule of thumb is to allocate swap equal to or double the RAM size.

Step 3 – Creating a Swap File

Create a swap file using the efficient fallocate program. For example, on a server with 1GB RAM:

sudo fallocate -l 1G /swapfile

Verify the reserved space:

ls -lh /swapfile

Step 4 – Enabling the Swap File

Secure the swap file permissions so that only root user can modify it:

sudo chmod 600 /swapfile

Verify permissions:

ls -lh /swapfile

Mark the file as swap space:

sudo mkswap /swapfile

Enable the swap file:

sudo swapon /swapfile

Verify availability:

sudo swapon --show

Use free utility for confirmation:

free -h

Step 5 – Making the Swap File Permanent

Ensure the swap file persists after a reboot by adding it to /etc/fstab:

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

Add the swap file information:

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

Next we’ll review some settings we can update to tune our swap space.

Step 6 – Tuning your Swap Settings

Adjust the swappiness parameter-

View Current parameters:

cat /proc/sys/vm/swappiness

Set swappiness (e.g., 10):

sudo sysctl vm.swappiness=10

Make the change persistent:

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

Adjust the vfs_cache_pressure-

View Current parameters:

cat /proc/sys/vm/vfs_cache_pressure

Set vfs_cache_pressure to 50:

sudo sysctl vm.vfs_cache_pressure=50

Persist the change:

echo 'vm.vfs_cache_pressure=50' | sudo tee -a /etc/sysctl.conf

At the bottom, add the line that specifies your new value:

Save and close the file when you are finished.

Conclusion

Adding swap space provides a safety net against out-of-memory issues. Optimize application configurations or consider a server upgrade if encountering frequent OOM errors.

Happy Coding!