Not enough memory? Configure swap space on Linux using a swapfile

Foreword

(This article was originally written in 2017)

A few months ago, I was still happily writing an article about upgrading my laptop's memory configuration, jumping from a beggarly 4G to a 16G "high, rich, and handsome" setup. However, over the past few months, I've been tormented by frequent computer freezes. Now I have reason to suspect I might have bought counterfeit memory modules. So, with no other choice, I had to switch back to the original 4G memory stick, (and then plan to save up for a desktop), no big deal.
After switching to the 4G memory stick, the effect was significant. With just one Google Chrome browser open, memory usage had already reached 80%. I dared not use any of JB's IDEs anymore. Yet, this still could barely satisfy the system's thirst for memory space. At this point, configuring swap space was the only option. * (i.e., virtual memory)*

Swap Concept

Swap partition (also known as exchange partition) is an area on the hard drive designated for the operating system to temporarily store data that can no longer be held in RAM. Essentially, this allows you to increase the amount of information your server can retain in working "memory," with the caveat that when there is insufficient space in RAM for application data in use, the swap space on the hard drive will be utilized.

Information written to the disk will be much slower than data stored in RAM, but the operating system prefers to keep application data in memory and uses swap for older data. Overall, when a system's RAM is exhausted, using swap space as a fallback can serve as an effective safety net to prevent out-of-memory issues in non-SSD storage systems.

Check System Information

Before starting, we can check if the system already has some available swap space. There might be multiple swap files or swap partitions, but usually, it should be sufficient. We can use the following command to see if the system has a swap partition:

sudo swapon --show

If there is no result or nothing is displayed, it means the system currently has no available swap space. The free command is used to check free memory space, including the swap partition space.

free -h

Check Available Space on Hard Drive Partition

The most common way to allocate space for swap is to use a dedicated partition for that specific task. However, changing the partition scheme is not always feasible, and we can simply create a swap file that resides on an existing partition.

Before starting, we should check the current disk usage by entering the following command:

df -h

Important Note

(Updated 2024-02-18)

The method used in this article for creating swap files is suitable for the ext4 filesystem. If you are using journaling filesystems like ZFS/btrfs, please exercise caution with the method described here. I tested that this approach does not work on ZFS—it will show the error swapfile has holes and refuse to function.

Switching to the fallocate command to create the swapfile also does not work.

References

• https://askubuntu.com/questions/1198903/can-not-use-swap-file-on-zfs-files-with-holes
• https://askubuntu.com/questions/1327500/swap-file-hard-disk-it-appears-to-have-holes

Creating a swap file

Since I've been using this system for a long time and there are no free partitions available on the disk, I've chosen to manually create a swapfile to act as swap space.

It's often recommended that the swap space should be about twice the size of the memory. Since I currently have only 4GB of memory, I'll naturally create an 8GB swap space.
Use the following command to create the swapfile:

sudo fallocate -l 8G /swapfile

After testing, it appears that on OpenSuSE systems, the following command must be used to successfully create a swapfile.

(Added on 2024-02-18) This is likely because the Btrfs file system was in use at the time, requiring the dd command for creation. In theory, the fallocate command should also work, but remember to set the swapfile to be untracked, as it may otherwise affect performance.

sudo dd if=/dev/zero of=/swapfile count=4096 bs=1MiB

Use the following command to verify that it was created correctly.

ls -lh /swapfile

The result should look something like this:

-rw-r--r-- 1 root root 8.0G Apr 26 17:04 /swapfile

Modify swapfile permissions

sudo chmod 600 /swapfile

Check the effect

ls -lh /swapfile

The result should look something like this:

-rw------- 1 root root 8.0G Apr 26 17:04 /swapfile

Enable Swap Space

sudo mkswap /swapfile
sudo swapon /swapfile

After that, use the following command to verify that the swap space has been successfully enabled:

sudo swapon --show

The result should look similar to the following:

NAME      TYPE SIZE USED PRIO
/swapfile file   8G   0B   -1

Add to fstab

This way, the system will automatically mount the swapfile as swap space every time it boots. First, please back up the fstab file yourself. Then add the following configuration to the end of the fstab file.

/swapfile none swap sw 0 0

Or directly use the following command:

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