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

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Add how-to-setup-swap-devices-in-guest-kernel.md to how-to to introduce
how to setup swap device in guest kernel.

Fixes: #2326

Signed-off-by: Hui Zhu <teawater@antfin.com>
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Hui Zhu 2021-07-29 11:12:33 +08:00 committed by Hui Zhu
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docs/how-to/README.md
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- [How to set sandbox Kata Containers configurations with pod annotations](how-to-set-sandbox-config-kata.md)
- [How to monitor Kata Containers in K8s](how-to-set-prometheus-in-k8s.md)
- [How to use hotplug memory on arm64 in Kata Containers](how-to-hotplug-memory-arm64.md)
- [How to setup swap devices in guest kernel](how-to-setup-swap-devices-in-guest-kernel.md)

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# 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](how-to-set-sandbox-config-kata.md#container-options). So [extra configuration is needed for containerd](how-to-set-sandbox-config-kata.md#containerd-configuration).
Kata Containers just supports setup swap device in guest kernel with QEMU.
Install and setup Kata Containers as shown [here](../install/README.md).
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 Container utilizing swap device
Use following command to start a Kata Container 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
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 $pod_yaml)
$ cid=$(sudo crictl create $podid $container_yaml $pod_yaml)
$ sudo crictl start $cid
```
Kata Container 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|