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osbuilder

Introduction

The Kata Containers runtime creates a virtual machine (VM) to isolate a set of container workloads. The VM requires a guest kernel and a guest operating system ("guest OS") to boot and create containers inside the guest environment.

This repository contains tools to create a guest OS disk image.

Terms

This section describes the terms used for all documentation in this repository.

  • rootfs

    The root filesystem or "rootfs" is a slight misnomer as it is not a true filesystem. It is a tree of files contained in a particular directory, which represents the root disk layout. A rootfs can be turned into either an image or an initrd.

    See the rootfs creation section.

  • "Guest OS" (or "Guest Image")

    A "virtual disk" or "disk image" built from a rootfs. It contains a filesystem that is used by the VM, in conjunction with a guest kernel, to create an environment to host the container. Neither the guest OS nor the guest kernel need to be the same as the host operating system.

    See the image creation section.

  • initrd (or "initramfs")

    A compressed cpio(1) archive, created from a rootfs which is loaded into memory and used as part of the Linux startup process. During startup, the kernel unpacks it into a special instance of a tmpfs that becomes the initial root filesystem.

    See the initrd creation section.

  • "Base OS"

    A particular version of a Linux distribution used to create a rootfs from.

  • dracut

    A guest OS build method where the building host is used as the Base OS. For more information refer to the dracut homepage.

Building

The top-level Makefile contains an example of how to use the available components. Set DEBUG=true to execute build scripts in debug mode.

Two build methods are available, distro and dracut. By default, the distro build method is used, and this creates a rootfs using distro specific commands (e.g.: debootstrap for Debian or yum for CentOS). The dracut build method uses the distro-agnostic tool dracut to obtain the same goal.

By default components are run on the host system. However, some components offer the ability to run from within a container (for ease of setup) by setting the USE_DOCKER=true or USE_PODMAN=true variable. If both are set, USE_DOCKER=true takes precedence over USE_PODMAN=true.

For more detailed information, consult the documentation for a particular component.

When invoking the appropriate make target as showed below, a single command is used to generate an initrd or an image. This is what happens in details:

  1. A rootfs is generated based on the specified target distribution.
  2. The rootfs is provisioned with Kata-specific components and configuration files.
  3. The rootfs is used as a base to generate an initrd or an image.

When using the dracut build method however, the build sequence is different:

  1. An overlay directory is populated with Kata-specific components.
  2. dracut is instructed to merge the overlay directory with the required host-side filesystem components to generate an initrd.
  3. When generating an image, the initrd is extracted to obtain the base rootfs for the image.

Rootfs creation

This section shows how to build a basic rootfs using the default distribution. For further details, see the rootfs builder documentation.

Rootfs with systemd as init

$ sudo -E PATH=$PATH make USE_DOCKER=true rootfs

Rootfs with the agent as init

$ sudo -E PATH=$PATH make USE_DOCKER=true AGENT_INIT=yes rootfs

dracut based rootfs

Note: the dracut build method does not need a rootfs as a base for an image or initrd. However, a rootfs can be generated by extracting the generated initrd.

$ sudo -E PATH=$PATH make BUILD_METHOD=dracut rootfs

Image creation

This section shows how to create an image from the already-created rootfs. For further details, see the image builder documentation.

Image with systemd as init

$ sudo -E PATH=$PATH make USE_DOCKER=true image

Image with the agent as init

$ sudo -E PATH=$PATH make USE_DOCKER=true AGENT_INIT=yes image

dracut based image

Note: the dracut build method generates an image by first building an initrd, and then using the rootfs extracted from it.

$ sudo -E PATH=$PATH make BUILD_METHOD=dracut image

Initrd creation

Rootfs based initrd

Create an initrd from the already-created rootfs and with the agent acting as the init daemon using:

$ sudo -E PATH=$PATH make AGENT_INIT=yes initrd

dracut based initrd

Create an initrd using the dracut build method with:

$ sudo -E PATH=$PATH make BUILD_METHOD=dracut AGENT_INIT=yes initrd

For further details, see the initrd builder documentation.

dracut options

Add kernel modules

If the initrd or image needs to contain kernel modules, this can be done by:

  1. Specify the name of the modules (as reported by modinfo MODULE-NAME) in dracut/dracut.conf.d/10-drivers.conf. For example this file can contain:
drivers="9p 9pnet 9pnet_virtio"
  1. Set the DRACUT_KVERSION make variable to the release name of the kernel that is paired with the built image or initrd, using the uname -r format. For example:
$ make BUILD_METHOD=dracut DRACUT_KVERSION=5.2.1-23-kata AGENT_INIT=yes initrd

Testing

$ make test

For further details, see the tests documentation.

Platform-Distro Compatibility Matrix

The following table illustrates what target architecture is supported for each of the the osbuilder distributions.

Note: this table is not relevant for the dracut build method, since it supports any Linux distribution and architecture where dracut is available.

Alpine CentOS Clear Linux Debian/Ubuntu Fedora openSUSE
ARM64 ✔️ ✔️ ✔️ ✔️
PPC64le ✔️ ✔️ ✔️ ✔️ ✔️
s390x ✔️ ✔️ ✔️
x86_64 ✔️ ✔️ ✔️ ✔️ ✔️ ✔️