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Guest assets

Kata Containers creates a VM in which to run one or more containers. It does this by launching a hypervisor to create the VM. The hypervisor needs two assets for this task: a Linux kernel and a small root filesystem image to boot the VM.

Guest kernel

The guest kernel is passed to the hypervisor and used to boot the VM. The default kernel provided in Kata Containers is highly optimized for kernel boot time and minimal memory footprint, providing only those services required by a container workload. It is based on the latest Linux LTS (Long Term Support) kernel.

Guest image

The hypervisor uses an image file which provides a minimal root filesystem used by the guest kernel to boot the VM and host the Kata Container. Kata Containers supports both initrd and rootfs based minimal guest images. The default packages provide both an image and an initrd, both of which are created using the osbuilder tool.

Notes:

  • Although initrd and rootfs based images are supported, not all hypervisors support both types of image.

  • The guest image is unrelated to the image used in a container workload.

    For example, if a user creates a container that runs a shell in a BusyBox image, they will run that shell in a BusyBox environment. However, the guest image running inside the VM that is used to host that BusyBox image could be running Clear Linux, Ubuntu, Fedora or any other distribution potentially.

    The osbuilder tool provides configurations for various common Linux distributions which can be built into either initrd or rootfs guest images.

  • If you are using a packaged version of Kata Containers, you can see image details by running the kata-collect-data.sh script as root and looking at the "Image details" section of the output.

Root filesystem image

The default packaged rootfs image, sometimes referred to as the mini O/S, is a highly optimized container bootstrap system.

If this image type is configured, when the user runs the example command:

  • The runtime will launch the configured hypervisor.
  • The hypervisor will boot the mini-OS image using the guest kernel.
  • The kernel will start the init daemon as PID 1 (systemd) inside the VM root environment.
  • systemd, running inside the mini-OS context, will launch the agent in the root context of the VM.
  • The agent will create a new container environment, setting its root filesystem to that requested by the user (Ubuntu in the example).
  • The agent will then execute the command (sh(1) in the example) inside the new container.

The table below summarises the default mini O/S showing the environments that are created, the services running in those environments (for all platforms) and the root filesystem used by each service:

Process Environment systemd service? rootfs User accessible Notes
systemd VM root n/a VM guest image debug console The init daemon, running as PID 1
Agent VM root yes VM guest image debug console Runs as a systemd service
chronyd VM root yes VM guest image debug console Used to synchronise the time with the host
container workload (sh(1) in the example) VM container no User specified (Ubuntu in the example) exec command Managed by the agent

See also the process overview.

Notes:

  • The "User accessible" column shows how an administrator can access the environment.

  • The container workload is running inside a full container environment which itself is running within a VM environment.

  • See the configuration files for the osbuilder tool for details of the default distribution for platforms other than Intel x86_64.

Initrd image

The initrd image is 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 mount that becomes the initial root filesystem.

If this image type is configured, when the user runs the example command:

  • The runtime will launch the configured hypervisor.
  • The hypervisor will boot the mini-OS image using the guest kernel.
  • The kernel will start the init daemon as PID 1 (the agent) inside the VM root environment.
  • The agent will create a new container environment, setting its root filesystem to that requested by the user (ubuntu in the example).
  • The agent will then execute the command (sh(1) in the example) inside the new container.

The table below summarises the default mini O/S showing the environments that are created, the processes running in those environments (for all platforms) and the root filesystem used by each service:

Process Environment rootfs User accessible Notes
Agent VM root VM guest image debug console Runs as the init daemon (PID 1)
container workload VM container User specified (Ubuntu in this example) exec command Managed by the agent

Notes:

  • The "User accessible" column shows how an administrator can access the environment.

  • It is possible to use a standard init daemon such as systemd with an initrd image if this is desirable.

See also the process overview.

Image summary

Image type Default distro Init daemon Reason Notes
image Clear Linux (for x86_64 systems) systemd Minimal and highly optimized systemd offers flexibility
initrd Alpine Linux Kata agent (as no systemd support) Security hardened and tiny C library

See also:

  • The osbuilder tool

    This is used to build all default image types.

  • The versions database

    The default-image-name and default-initrd-name options specify the default distributions for each image type.