How to Add a Swap File on Linux and Adjust Swappiness

If your server constantly reports out‑of‑memory errors and services are being killed, enabling a swap area is a practical way to add virtual memory without purchasing additional RAM.

In this example, a 1‑CPU 1‑GB Alibaba Cloud ECS instance runs memory‑intensive software such as the ELK stack, making swap useful.

Typically, setting swap size to twice the physical memory is sufficient; larger sizes only waste disk space.

2. Add a Swap File

Because the disk is already partitioned, we use a file as the swap area. All commands must be run as the root user.

Check current memory usage with free -h ; you will see that physical memory is nearly exhausted and swap is currently a file.

Create a file for swap (e.g., 2 GB):

dd if=/dev/zero of=/root/swapfile2 bs=1M count=2048

Set proper permissions so the swap file is secure:

chmod 0600 /root/swapfile2

Initialize the file as swap space:

mkswap /root/swapfile2

Enable the swap file immediately:

swapon /root/swapfile2

After enabling, you can see that the virtual memory increases by the swap size.

To make the swap file activate on boot, edit /etc/fstab and add the line:

/root/swapfile2 swap swap defaults 0 0

3. Adjust Swap Usage Priority

If you have enough RAM, you may want Linux to use swap less aggressively by changing the swappiness value.

A swappiness of 0 tells the kernel to prefer physical memory as much as possible, while 100 forces aggressive swapping.

Check the current value:

cat /proc/sys/vm/swappiness

Temporarily set it to 60 (effective until reboot):

sudo sysctl vm.swappiness=60

To make the change permanent, edit /etc/sysctl.conf , add or modify the line:

vm.swappiness = 60

Save the file and the new setting will persist after a reboot.