Configuring swap memory on Kubernetes nodes

This page provides an example of how to provision and configure swap memory on a Kubernetes node using kubeadm.

Objectives

• Provision swap memory on a Kubernetes node using kubeadm.
• Learn to configure both encrypted and unencrypted swap.
• Learn to enable swap on boot.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

• Killercoda
• KodeKloud
• Play with Kubernetes

To check the version, enter kubectl version.

You need at least one worker node in your cluster which needs to run a Linux operating system. It is required for this demo that the kubeadm tool be installed, following the steps outlined in the kubeadm installation guide.

On each worker node where you will configure swap use, you need:

• fallocate

• mkswap

• swapon

• For encrypted swap space (recommended), you also need:

• cryptsetup

Install a swap-enabled cluster with kubeadm

Create a swap file and turn swap on

If swap is not enabled, there's a need to provision swap on the node. The following sections demonstrate creating 4GiB of swap, both in the encrypted and unencrypted case.

• Setting up encrypted swap
• Setting up unencrypted swap

# Allocate storage and restrict access
fallocate --length 4GiB /swapfile
chmod 600 /swapfile

# Create an encrypted device backed by the allocated storage
cryptsetup --type plain --cipher aes-xts-plain64 --key-size 256 -d /dev/urandom open /swapfile cryptswap

# Format the swap space
mkswap /dev/mapper/cryptswap

# Activate the swap space for paging
swapon /dev/mapper/cryptswap

# Allocate storage and restrict access
fallocate --length 4GiB /swapfile
chmod 600 /swapfile

# Format the swap space
mkswap /swapfile

# Activate the swap space for paging
swapon /swapfile

Verify that swap is enabled

Swap can be verified to be enabled with both swapon -s command or the free command.

Using swapon -s:

Filename       Type		Size		Used		Priority
/dev/dm-0      partition 	4194300		0		-2

Using free -h:

               total        used        free      shared  buff/cache   available
Mem:           3.8Gi       1.3Gi       249Mi        25Mi       2.5Gi       2.5Gi
Swap:          4.0Gi          0B       4.0Gi

Enable swap on boot

After setting up swap, to start the swap file at boot time, you typically either set up a systemd unit to activate (encrypted) swap, or you add a line similar to /swapfile swap swap defaults 0 0 into /etc/fstab.

Using systemd for swap activation allows the system to delay kubelet start until swap is available, if that is something you want to ensure. In a similar way, using systemd allows your server to leave swap active until kubelet (and, typically, your container runtime) have shut down.

Set up kubelet configuration

After enabling swap on the node, kubelet needs to be configured in the following way:

 # this fragment goes into the kubelet's configuration file
 failSwapOn: false
 memorySwap:
     swapBehavior: LimitedSwap

In order for these configurations to take effect, kubelet needs to be restarted.