how-to-setup-swap-devices-in-guest-kernel.md

Setup swap device in guest kernel

Introduction

Setup swap device in guest kernel can help to increase memory capacity, handle some memory issues and increase file access speed sometimes. Kata Containers can insert a raw file to the guest as the swap device.

Requisites

The swap config of the containers should be set by annotations. So extra configuration is needed for containerd.

Kata Containers just supports setup swap device in guest kernel with QEMU. Install and setup Kata Containers as shown here.

Enable setup swap device in guest kernel as follows:

$ sudo sed -i -e 's/^#enable_guest_swap.*$/enable_guest_swap = true/g' /etc/kata-containers/configuration.toml

Run a Kata Containers utilizing swap device

Use following command to start a Kata Containers with swappiness 60 and 1GB swap device (swap_in_bytes - memory_limit_in_bytes).

$ pod_yaml=pod.yaml
$ container_yaml=container.yaml
$ image="quay.io/prometheus/busybox:latest"
$ cat << EOF > "${pod_yaml}"
metadata:
  name: busybox-sandbox1
  uid: $(uuidgen)
  namespace: default
EOF
$ cat << EOF > "${container_yaml}"
metadata:
  name: busybox-test-swap
annotations:
  io.katacontainers.container.resource.swappiness: "60"
  io.katacontainers.container.resource.swap_in_bytes: "2147483648"
linux:
  resources:
    memory_limit_in_bytes: 1073741824
image:
  image: "$image"
command:
- top
EOF
$ sudo crictl pull $image
$ podid=$(sudo crictl runp --runtime kata $pod_yaml)
$ cid=$(sudo crictl create $podid $container_yaml $pod_yaml)
$ sudo crictl start $cid

Kata Containers setups swap device for this container only when io.katacontainers.container.resource.swappiness is set.

The following table shows the swap size how to decide if io.katacontainers.container.resource.swappiness is set.

io.katacontainers.container.resource.swap_in_bytes	memory_limit_in_bytes	swap size
set	set	io.katacontainers.container.resource.swap_in_bytes - memory_limit_in_bytes
not set	set	memory_limit_in_bytes
not set	not set	io.katacontainers.config.hypervisor.default_memory
set	not set	cgroup doesn't support this usage