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docs: Update architecture document
Refresh the content and formatting of the architecture document. Out of scope of these changes: - Diagram updates. - Updates to the Networking section. Fixes: #3190. Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>
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## Overview
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This is an architectural overview of Kata Containers, based on the 2.0 release.
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Kata Containers is an open source community working to build a secure
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container [runtime](#runtime) with lightweight virtual machines (VM's)
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that feel and perform like standard Linux containers, but provide
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stronger [workload](#workload) isolation using hardware
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[virtualization](#virtualization) technology as a second layer of
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defence.
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The primary deliverable of the Kata Containers project is a CRI friendly shim. There is also a CRI friendly library API behind them.
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Kata Containers runs on [multiple architectures](../../src/runtime/README.md#platform-support)
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and supports [multiple hypervisors](../hypervisors.md).
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The [Kata Containers runtime](../../src/runtime)
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is compatible with the [OCI](https://github.com/opencontainers) [runtime specification](https://github.com/opencontainers/runtime-spec)
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and therefore works seamlessly with the [Kubernetes\* Container Runtime Interface (CRI)](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-node/container-runtime-interface.md)
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through the [CRI-O\*](https://github.com/kubernetes-incubator/cri-o) and
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[Containerd\*](https://github.com/containerd/containerd) implementation.
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Kata Containers creates a QEMU\*/KVM virtual machine for pod that `kubelet` (Kubernetes) creates respectively.
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The [`containerd-shim-kata-v2` (shown as `shimv2` from this point onwards)](../../src/runtime/cmd/containerd-shim-kata-v2/)
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is the Kata Containers entrypoint, which
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implements the [Containerd Runtime V2 (Shim API)](https://github.com/containerd/containerd/tree/master/runtime/v2) for Kata.
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Before `shimv2` (as done in [Kata Containers 1.x releases](https://github.com/kata-containers/runtime/releases)), we need to create a `containerd-shim` and a [`kata-shim`](https://github.com/kata-containers/shim) for each container and the Pod sandbox itself, plus an optional [`kata-proxy`](https://github.com/kata-containers/proxy) when VSOCK is not available. With `shimv2`, Kubernetes can launch Pod and OCI compatible containers with one shim (the `shimv2`) per Pod instead of `2N+1` shims, and no standalone `kata-proxy` process even if no VSOCK is available.
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The container process is then spawned by
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[`kata-agent`](../../src/agent), an agent process running
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as a daemon inside the virtual machine. `kata-agent` runs a [`ttRPC`](https://github.com/containerd/ttrpc-rust) server in
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the guest using a VIRTIO serial or VSOCK interface which QEMU exposes as a socket
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file on the host. `shimv2` uses a `ttRPC` protocol to communicate with
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the agent. This protocol allows the runtime to send container management
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commands to the agent. The protocol is also used to carry the I/O streams (stdout,
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stderr, stdin) between the containers and the manage engines (e.g. CRI-O or containerd).
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For any given container, both the init process and all potentially executed
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commands within that container, together with their related I/O streams, need
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to go through the VSOCK interface exported by QEMU.
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The container workload, that is, the actual OCI bundle rootfs, is exported from the
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host to the virtual machine. In the case where a block-based graph driver is
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configured, `virtio-scsi` will be used. In all other cases a `virtio-fs` VIRTIO mount point
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will be used. `kata-agent` uses this mount point as the root filesystem for the
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container processes.
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This document is a summary of the Kata Containers architecture.
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## Virtualization
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How Kata Containers maps container concepts to virtual machine technologies, and how this is realized in the multiple
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hypervisors and VMMs that Kata supports is described within the [virtualization documentation](./virtualization.md)
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For details on how Kata Containers maps container concepts to VM
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technologies, and how this is realized in the multiple hypervisors and
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VMMs that Kata supports see the
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[virtualization documentation](./virtualization.md).
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## Compatibility
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The [Kata Containers runtime](../../src/runtime) is compatible with
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the [OCI](https://github.com/opencontainers)
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[runtime specification](https://github.com/opencontainers/runtime-spec)
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and therefore works seamlessly with the
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[Kubernetes Container Runtime Interface (CRI)](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-node/container-runtime-interface.md)
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through the [CRI-O](https://github.com/kubernetes-incubator/cri-o)
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and [containerd](https://github.com/containerd/containerd)
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implementations.
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Kata Containers provides a ["shimv2"](#shim-v2-architecture) compatible runtime.
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## Shim v2 architecture
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The Kata Containers runtime is shim v2 ("shimv2") compatible. This
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section explains what this means.
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### History
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In the old [Kata 1.x architecture](https://github.com/kata-containers/documentation/blob/master/design/architecture.md),
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the Kata [runtime](#runtime) was an executable called `kata-runtime`.
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The container manager called this executable multiple times when
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creating each container. Each time the runtime was called a different
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OCI command-line verb was provided. This architecture was simple, but
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not well suited to creating VM based containers due to the issue of
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handling state between calls. Additionally, the architecture suffered
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from performance issues related to continually having to spawn new
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instances of the runtime binary, and
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[Kata shim](https://github.com/kata-containers/shim) and
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[Kata proxy](https://github.com/kata-containers/proxy) processes for systems
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that did not provide VSOCK.
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### An improved architecture
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The
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[containerd runtime shimv2 architecture](https://github.com/containerd/containerd/tree/main/runtime/v2)
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or _shim API_ architecture resolves the issues with the old
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architecture by defining a set of shimv2 APIs that a compatible
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runtime implementation must supply. Rather than calling the runtime
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binary multiple times for each new container, the shimv2 architecture
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runs a single instance of the runtime binary (for any number of
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containers). This improves performance and resolves the state handling
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issue.
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The shimv2 API is similar to the
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[OCI runtime](https://github.com/opencontainers/runtime-spec)
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API in terms of the way the container lifecycle is split into
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different verbs. Rather than calling the runtime multiple times, the
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container manager creates a socket and passes it to the shimv2
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runtime. The socket is a bi-directional communication channel that
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uses a gRPC based protocol to allow the container manager to send API
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calls to the runtime, which returns the result to the container
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manager using the same channel.
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The shimv2 architecture allows running several containers per VM to
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support container engines that require multiple containers running
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inside a pod.
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With the new architecture [Kubernetes](#kubernetes-support) can
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launch both Pod and OCI compatible containers with a single
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[runtime](#runtime) shim per Pod, rather than `2N+1` shims. No stand
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alone `kata-proxy` process is required, even if VSOCK is not
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available.
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### Architectural comparison
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| Kata version | Kata Runtime process calls | Kata shim processes | Kata proxy processes (if no VSOCK) |
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|-|-|-|-|
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| 1.x | multiple per container | 1 per container connection | 1 |
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| 2.x | 1 per VM (hosting any number of containers) | 0 | 0 |
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> **Notes:**
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>
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> - A single VM can host one or more containers.
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>
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> - The "Kata shim processes" column refers to the old
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> [Kata shim](https://github.com/kata-containers/shim) (`kata-shim` binary),
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> *not* the new shimv2 runtime instance (`containerd-shim-kata-v2` binary).
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The diagram below shows how the original architecture was simplified
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with the advent of shimv2.
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## Root filesystem
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This document uses the term _rootfs_ to refer to a root filesystem
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which is mounted as the top-level directory ("`/`") and often referred
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to as _slash_.
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It is important to understand this term since the overall system uses
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multiple different rootfs's (as explained in the
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[Environments](#environments) section.
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## Example command
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The following containerd command creates a container. It is referred
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to throughout this document to help explain various points:
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```bash
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$ sudo ctr run --runtime "io.containerd.kata.v2" --rm -t "quay.io/libpod/ubuntu:latest" foo sh
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```
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This command requests that containerd:
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- Create a container (`ctr run`).
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- Use the Kata [shimv2](#shim-v2-architecture) runtime (`--runtime "io.containerd.kata.v2"`).
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- Delete the container when it [exits](#workload-exit) (`--rm`).
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- Attach the container to the user's terminal (`-t`).
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- Use the Ubuntu Linux [container image](#container-image)
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to create the container [rootfs](#root-filesystem) that will become
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the [container environment](#environments)
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(`quay.io/libpod/ubuntu:latest`).
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- Create the container with the name "`foo`".
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- Run the `sh(1)` command in the Ubuntu rootfs based container
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environment.
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The command specified here is referred to as the [workload](#workload).
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> **Note:**
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>
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> For the purposes of this document and to keep explanations
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> simpler, we assume the user is running this command in the
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> [host environment](#environments).
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## Container image
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In the [example command](#example-command) the user has specified the
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type of container they wish to run via the container image name:
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`ubuntu`. This image name corresponds to a _container image_ that can
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be used to create a container with an Ubuntu Linux environment. Hence,
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in our [example](#example-command), the `sh(1)` command will be run
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inside a container which has an Ubuntu rootfs.
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> **Note:**
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>
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> The term _container image_ is confusing since the image in question
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> is **not** a container: it is simply a set of files (_an image_)
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> that can be used to _create_ a container. The term _container
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> template_ would be more accurate but the term _container image_ is
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> commonly used so this document uses the standard term.
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For the purposes of this document, the most important part of the
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[example command line](#example-command) is the container image the
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user has requested. Normally, the container manager will _pull_
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(download) a container image from a remote site and store a copy
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locally. This local container image is used by the container manager
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to create an [OCI bundle](#oci-bundle) which will form the environment
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the container will run in. After creating the OCI bundle, the
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container manager launches a [runtime](#runtime) which will create the
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container using the provided OCI bundle.
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## OCI bundle
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To understand what follows, it is important to know at a high level
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how an OCI ([Open Containers Initiative](https://opencontainers.org)) compatible container is created.
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An OCI compatible container is created by taking a
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[container image](#container-image) and converting the embedded rootfs
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into an
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[OCI rootfs bundle](https://github.com/opencontainers/runtime-spec/blob/main/bundle.md),
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or more simply, an _OCI bundle_.
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An OCI bundle is a `tar(1)` archive normally created by a container
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manager which is passed to an OCI [runtime](#runtime) which converts
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it into a full container rootfs. The bundle contains two assets:
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- A container image [rootfs](#root-filesystem)
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This is simply a directory of files that will be used to represent
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the rootfs for the container.
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For the [example command](#example-command), the directory will
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contain the files necessary to create a minimal Ubuntu root
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filesystem.
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- An [OCI configuration file](https://github.com/opencontainers/runtime-spec/blob/main/config.md)
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This is a JSON file called `config.json`.
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The container manager will create this file so that:
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- The `root.path` value is set to the full path of the specified
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container rootfs.
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In [the example](#example-command) this value will be `ubuntu`.
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- The `process.args` array specifies the list of commands the user
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wishes to run. This is known as the [workload](#workload).
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In [the example](#example-command) the workload is `sh(1)`.
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## Workload
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The workload is the command the user requested to run in the
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container and is specified in the [OCI bundle](#oci-bundle)'s
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configuration file.
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In our [example](#example-command), the workload is the `sh(1)` command.
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### Workload root filesystem
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For details of how the [runtime](#runtime) makes the
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[container image](#container-image) chosen by the user available to
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the workload process, see the
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[Container creation](#container-creation) and [storage](#storage) sections.
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Note that the workload is isolated from the [guest VM](#environments) environment by its
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surrounding [container environment](#environments). The guest VM
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environment where the container runs in is also isolated from the _outer_
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[host environment](#environments) where the container manager runs.
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## System overview
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### Environments
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The following terminology is used to describe the different or
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environments (or contexts) various processes run in. It is necessary
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to study this table closely to make sense of what follows:
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| Type | Name | Virtualized | Containerized | rootfs | Rootfs device type | Mount type | Description |
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|-|-|-|-|-|-|-|-|
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| Host | Host | no `[1]` | no | Host specific | Host specific | Host specific | The environment provided by a standard, physical non virtualized system. |
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| VM root | Guest VM | yes | no | rootfs inside the [guest image](#guest-image) | Hypervisor specific `[2]` | `ext4` | The first (or top) level VM environment created on a host system. |
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| VM container root | Container | yes | yes | rootfs type requested by user ([`ubuntu` in the example](#example-command)) | `kataShared` | [virtio FS](#virtio-fs) | The first (or top) level container environment created inside the VM. Based on the [OCI bundle](#oci-bundle). |
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**Key:**
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- `[1]`: For simplicity, this document assumes the host environment
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runs on physical hardware.
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- `[2]`: See the [DAX](#dax) section.
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> **Notes:**
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>
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> - The word "root" is used to mean _top level_ here in a similar
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> manner to the term [rootfs](#root-filesystem).
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>
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> - The term "first level" prefix used above is important since it implies
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> that it is possible to create multi level systems. However, they do
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> not form part of a standard Kata Containers environment so will not
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> be considered in this document.
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The reasons for containerizing the [workload](#workload) inside the VM
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are:
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- Isolates the workload entirely from the VM environment.
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- Provides better isolation between containers in a [pod](#kubernetes-support).
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- Allows the workload to be managed and monitored through its cgroup
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confinement.
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### Container creation
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The steps below show at a high level how a Kata Containers container is
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created using the containerd container manager:
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1. The user requests the creation of a container by running a command
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like the [example command](#example-command).
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1. The container manager daemon runs a single instance of the Kata
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[runtime](#runtime).
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1. The Kata runtime loads its [configuration file](#configuration).
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1. The container manager calls a set of shimv2 API functions on the runtime.
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1. The Kata runtime launches the configured [hypervisor](#hypervisor).
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1. The hypervisor creates and starts (_boots_) a VM using the
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[guest assets](#guest-assets):
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- The hypervisor [DAX](#dax) shares the [guest image](#guest-image)
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into the VM to become the VM [rootfs](#root-filesystem) (mounted on a `/dev/pmem*` device),
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which is known as the [VM root environment](#environments).
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- The hypervisor mounts the [OCI bundle](#oci-bundle), using [virtio FS](#virtio-fs),
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into a container specific directory inside the VM's rootfs.
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|
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This container specific directory will become the
|
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[container rootfs](#environments), known as the
|
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[container environment](#environments).
|
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|
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1. The [agent](#agent) is started as part of the VM boot.
|
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|
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1. The runtime calls the agent's `CreateSandbox` API to request the
|
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agent create a container:
|
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|
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1. The agent creates a [container environment](#environments)
|
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in the container specific directory that contains the [container rootfs](#environments).
|
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|
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The container environment hosts the [workload](#workload) in the
|
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[container rootfs](#environments) directory.
|
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|
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1. The agent spawns the workload inside the container environment.
|
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|
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> **Notes:**
|
||||
>
|
||||
> - The container environment created by the agent is equivalent to
|
||||
> a container environment created by the
|
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> [`runc`](https://github.com/opencontainers/runc) OCI runtime;
|
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> Linux cgroups and namespaces are created inside the VM by the
|
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> [guest kernel](#guest-kernel) to isolate the workload from the
|
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> VM environment the container is created in. See the
|
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> [Environments](#environments) section for an explanation of why
|
||||
> this is done.
|
||||
>
|
||||
> - See the [guest image](#guest-image) section for details of
|
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> exactly how the agent is started.
|
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|
||||
1. The container manager returns control of the container to the
|
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user running the `ctr` command.
|
||||
|
||||
> **Note:**
|
||||
>
|
||||
> At this point, the container is running and:
|
||||
>
|
||||
> - The [workload](#workload) process ([`sh(1)` in the example](#example-command))
|
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> is running in the [container environment](#environments).
|
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> - The user is now able to interact with the workload
|
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> (using the [`ctr` command in the example](#example-command)).
|
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> - The [agent](#agent), running inside the VM is monitoring the
|
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> [workload](#workload) process.
|
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> - The [runtime](#runtime) is waiting for the agent's `WaitProcess` API
|
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> call to complete.
|
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|
||||
Further details of these steps are provided in the sections below.
|
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|
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### Container shutdown
|
||||
|
||||
There are two possible ways for the container environment to be
|
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terminated:
|
||||
|
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- When the [workload](#workload) exits.
|
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|
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This is the standard, or _graceful_ shutdown method.
|
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|
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- When the container manager forces the container to be deleted.
|
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|
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#### Workload exit
|
||||
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The [agent](#agent) will detect when the [workload](#workload) process
|
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exits, capture its exit status (see `wait(2)`) and return that value
|
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to the [runtime](#runtime) by specifying it as the response to the
|
||||
`WaitProcess` agent API call made by the [runtime](#runtime).
|
||||
|
||||
The runtime then passes the value back to the container manager by the
|
||||
`Wait` [shimv2 API](#shim-v2-architecture) call.
|
||||
|
||||
Once the workload has fully exited, the VM is no longer needed and the
|
||||
runtime cleans up the environment (which includes terminating the
|
||||
[hypervisor](#hypervisor) process).
|
||||
|
||||
> **Note:**
|
||||
>
|
||||
> When [agent tracing is enabled](../tracing.md#agent-shutdown-behaviour),
|
||||
> the shutdown behaviour is different.
|
||||
|
||||
#### Container manager requested shutdown
|
||||
|
||||
If the container manager requests the container be deleted, the
|
||||
[runtime](#runtime) will signal the agent by sending it a
|
||||
`DestroySandbox` [ttRPC API](../../src/agent/protocols/protos/agent.proto) request.
|
||||
|
||||
## Guest assets
|
||||
|
||||
The hypervisor will launch a virtual machine which includes a minimal guest kernel
|
||||
and a guest image.
|
||||
Kata Containers creates a VM in which to run one or more containers. It
|
||||
does this by launching a [hypervisor](#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 virtual
|
||||
machine. 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. This is based on a very current upstream Linux
|
||||
kernel.
|
||||
The [guest kernel](../../tools/packaging/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](https://www.kernel.org).
|
||||
|
||||
### Guest image
|
||||
|
||||
Kata Containers supports both an `initrd` and `rootfs` based minimal 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](../install/) provide both
|
||||
an image and an initrd, both of which are created using the
|
||||
[`osbuilder`](../../tools/osbuilder) tool.
|
||||
|
||||
> **Notes:**
|
||||
>
|
||||
> - Although initrd and rootfs based images are supported, not all
|
||||
> [hypervisors](#hypervisor) 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](../../tools/osbuilder/rootfs-builder)
|
||||
> which can be built into either initrd or rootfs guest images.
|
||||
>
|
||||
> - If you are using a [packaged version of Kata
|
||||
> Containers](../install), you can see image details by running the
|
||||
> [`kata-collect-data.sh`](../../src/runtime/data/kata-collect-data.sh.in)
|
||||
> script as `root` and looking at the "Image details" section of the
|
||||
> output.
|
||||
|
||||
#### Root filesystem image
|
||||
|
||||
The default packaged root filesystem image, sometimes referred to as the "mini O/S", is a
|
||||
highly optimized container bootstrap system based on [Clear Linux](https://clearlinux.org/). It provides an extremely minimal environment and
|
||||
has a highly optimized boot path.
|
||||
The default packaged rootfs image, sometimes referred to as the _mini
|
||||
O/S_, is a highly optimized container bootstrap system.
|
||||
|
||||
The only services running in the context of the mini O/S are the init daemon
|
||||
(`systemd`) and the [Agent](#agent). The real workload the user wishes to run
|
||||
is created using libcontainer, creating a container in the same manner that is done
|
||||
by `runc`.
|
||||
If this image type is [configured](#configuration), when the user runs
|
||||
the [example command](#example-command):
|
||||
|
||||
For example, when `ctr run -ti ubuntu date` is run:
|
||||
- The [runtime](#runtime) will launch the configured [hypervisor](#hypervisor).
|
||||
- The hypervisor will boot the mini-OS image using the [guest kernel](#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](#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](#example-command)).
|
||||
- The agent will then execute the command (`sh(1)` in [the example](#example-command))
|
||||
inside the new container.
|
||||
|
||||
- The hypervisor will boot the mini-OS image using the guest kernel.
|
||||
- `systemd`, running inside the mini-OS context, will launch the `kata-agent` in
|
||||
the same context.
|
||||
- The agent will create a new confined context to run the specified command in
|
||||
(`date` in this example).
|
||||
- The agent will then execute the command (`date` in this example) inside this
|
||||
new context, first setting the root filesystem to the expected Ubuntu\* root
|
||||
filesystem.
|
||||
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](#guest-image)| [debug console][debug-console] | The init daemon, running as PID 1 |
|
||||
| [Agent](#agent) | VM root | yes | [VM guest image](#guest-image)| [debug console][debug-console] | Runs as a systemd service |
|
||||
| `chronyd` | VM root | yes | [VM guest image](#guest-image)| [debug console][debug-console] | Used to synchronise the time with the host |
|
||||
| container workload (`sh(1)` in [the example](#example-command)) | VM container | no | User specified (Ubuntu in [the example](#example-command)) | [exec command](#exec-command) | Managed by the agent |
|
||||
|
||||
See also the [process overview](#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](../../tools/osbuilder/rootfs-builder)
|
||||
> for details of the default distribution for platforms other than
|
||||
> Intel x86_64.
|
||||
|
||||
#### Initrd image
|
||||
|
||||
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.
|
||||
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.
|
||||
|
||||
The only service running in the context of the initrd is the [Agent](#agent) as the init daemon. The real workload the user wishes to run is created using libcontainer, creating a container in the same manner that is done by `runc`.
|
||||
If this image type is [configured](#configuration), when the user runs
|
||||
the [example command](#example-command):
|
||||
|
||||
- The [runtime](#runtime) will launch the configured [hypervisor](#hypervisor).
|
||||
- The hypervisor will boot the mini-OS image using the [guest kernel](#guest-kernel).
|
||||
- The kernel will start the init daemon as PID 1 (the [agent](#agent))
|
||||
inside the VM root environment.
|
||||
- The [agent](#agent) will create a new container environment, setting its root
|
||||
filesystem to that requested by the user (`ubuntu` in
|
||||
[the example](#example-command)).
|
||||
- The agent will then execute the command (`sh(1)` in [the example](#example-command))
|
||||
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](#agent) | VM root | [VM guest image](#guest-image) | [debug console][debug-console] | Runs as the init daemon (PID 1) |
|
||||
| container workload | VM container | User specified (Ubuntu in this example) | [exec command](#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](#process-overview).
|
||||
|
||||
#### Image summary
|
||||
|
||||
| Image type | Default distro | Init daemon | Reason | Notes |
|
||||
|-|-|-|-|-|
|
||||
| [image](#root-filesystem-image) | [Clear Linux](https://clearlinux.org) (for x86_64 systems)| systemd | Minimal and highly optimized | systemd offers flexibility |
|
||||
| [initrd](#initrd-image) | [Alpine Linux](https://alpinelinux.org) | Kata [agent](#agent) (as no systemd support) | Security hardened and tiny C library |
|
||||
|
||||
See also:
|
||||
|
||||
- The [osbuilder](../../tools/osbuilder) tool
|
||||
|
||||
This is used to build all default image types.
|
||||
|
||||
- The [versions database](../../versions.yaml)
|
||||
|
||||
The `default-image-name` and `default-initrd-name` options specify
|
||||
the default distributions for each image type.
|
||||
|
||||
## Hypervisor
|
||||
|
||||
The [hypervisor](../hypervisors.md) specified in the
|
||||
[configuration file](#configuration) creates a VM to host the
|
||||
[agent](#agent) and the [workload](#workload) inside the
|
||||
[container environment](#environments).
|
||||
|
||||
> **Note:**
|
||||
>
|
||||
> The hypervisor process runs inside an environment slightly different
|
||||
> to the host environment:
|
||||
>
|
||||
> - It is run in a different cgroup environment to the host.
|
||||
> - It is given a separate network namespace from the host.
|
||||
> - If the [OCI configuration specifies a SELinux label](https://github.com/opencontainers/runtime-spec/blob/main/config.md#linux-process),
|
||||
> the hypervisor process will run with that label (*not* the workload running inside the hypervisor's VM).
|
||||
|
||||
## Agent
|
||||
|
||||
[`kata-agent`](../../src/agent) is a process running in the guest as a supervisor for managing containers and processes running within those containers.
|
||||
The Kata Containers agent ([`kata-agent`](../../src/agent)), written
|
||||
in the [Rust programming language](https://www.rust-lang.org), is a
|
||||
long running process that runs inside the VM. It acts as the
|
||||
supervisor for managing the containers and the [workload](#workload)
|
||||
running within those containers. Only a single agent process is run
|
||||
for each VM created.
|
||||
|
||||
For the 2.0 release, the `kata-agent` is rewritten in the [RUST programming language](https://www.rust-lang.org/) so that we can minimize its memory footprint while keeping the memory safety of the original GO version of [`kata-agent` used in Kata Container 1.x](https://github.com/kata-containers/agent). This memory footprint reduction is pretty impressive, from tens of megabytes down to less than 100 kilobytes, enabling Kata Containers in more use cases like functional computing and edge computing.
|
||||
### Agent communications protocol
|
||||
|
||||
The `kata-agent` execution unit is the sandbox. A `kata-agent` sandbox is a container sandbox defined by a set of namespaces (NS, UTS, IPC and PID). `shimv2` can
|
||||
run several containers per VM to support container engines that require multiple
|
||||
containers running inside a pod.
|
||||
The agent communicates with the other Kata components (primarily the
|
||||
[runtime](#runtime)) using a
|
||||
[`ttRPC`](https://github.com/containerd/ttrpc-rust) based
|
||||
[protocol](../../src/agent/protocols/protos).
|
||||
|
||||
`kata-agent` communicates with the other Kata components over `ttRPC`.
|
||||
> **Note:**
|
||||
>
|
||||
> If you wish to learn more about this protocol, a practical way to do
|
||||
> so is to experiment with the
|
||||
> [agent control tool](#agent-control-tool) on a test system.
|
||||
> This tool is for test and development purposes only and can send
|
||||
> arbitrary ttRPC agent API commands to the [agent](#agent).
|
||||
|
||||
## Runtime
|
||||
|
||||
`containerd-shim-kata-v2` is a [containerd runtime shimv2](https://github.com/containerd/containerd/blob/v1.4.1/runtime/v2/README.md) implementation and is responsible for handling the `runtime v2 shim APIs`, which is similar to [the OCI runtime specification](https://github.com/opencontainers/runtime-spec) but simplifies the architecture by loading the runtime once and making RPC calls to handle the various container lifecycle commands. This refinement is an improvement on the OCI specification which requires the container manager call the runtime binary multiple times, at least once for each lifecycle command.
|
||||
The Kata Containers runtime (the [`containerd-shim-kata-v2`](../../src/runtime/cmd/containerd-shim-kata-v2
|
||||
) binary) is a [shimv2](#shim-v2-architecture) compatible runtime.
|
||||
|
||||
`containerd-shim-kata-v2` heavily utilizes the
|
||||
[virtcontainers package](../../src/runtime/virtcontainers/), which provides a generic, runtime-specification agnostic, hardware-virtualized containers library.
|
||||
> **Note:**
|
||||
>
|
||||
> The Kata Containers runtime is sometimes referred to as the Kata
|
||||
> _shim_. Both terms are correct since the `containerd-shim-kata-v2`
|
||||
> is a container runtime, and that runtime implements the containerd
|
||||
> shim v2 API.
|
||||
|
||||
The runtime makes heavy use of the [`virtcontainers`
|
||||
package](../../src/runtime/virtcontainers), which provides a generic,
|
||||
runtime-specification agnostic, hardware-virtualized containers
|
||||
library.
|
||||
|
||||
The runtime is responsible for starting the [hypervisor](#hypervisor)
|
||||
and it's VM, and communicating with the [agent](#agent) using a
|
||||
[ttRPC based protocol](#agent-communications-protocol) over a VSOCK
|
||||
socket that provides a communications link between the VM and the
|
||||
host.
|
||||
|
||||
This protocol allows the runtime to send container management commands
|
||||
to the agent. The protocol is also used to carry the standard I/O
|
||||
streams (`stdout`, `stderr`, `stdin`) between the containers and
|
||||
container managers (such as CRI-O or containerd).
|
||||
|
||||
## Utility program
|
||||
|
||||
The `kata-runtime` binary is a utility program that provides
|
||||
administrative commands to manipulate and query a Kata Containers
|
||||
installation.
|
||||
|
||||
> **Note:**
|
||||
>
|
||||
> In Kata 1.x, this program also acted as the main
|
||||
> [runtime](#runtime), but this is no longer required due to the
|
||||
> improved shimv2 architecture.
|
||||
|
||||
### exec command
|
||||
|
||||
The `exec` command allows an administrator or developer to enter the
|
||||
[VM root environment](#environments) which is not accessible by the container
|
||||
[workload](#workload).
|
||||
|
||||
See [the developer guide](../Developer-Guide.md#connect-to-debug-console) for further details.
|
||||
|
||||
### Configuration
|
||||
|
||||
The runtime uses a TOML format configuration file called `configuration.toml`. By default this file is installed in the `/usr/share/defaults/kata-containers` directory and contains various settings such as the paths to the hypervisor, the guest kernel and the mini-OS image.
|
||||
|
||||
The actual configuration file paths can be determined by running:
|
||||
```
|
||||
$ kata-runtime --show-default-config-paths
|
||||
```
|
||||
Most users will not need to modify the configuration file.
|
||||
|
||||
The file is well commented and provides a few "knobs" that can be used to modify the behavior of the runtime and your chosen hypervisor.
|
||||
See the [configuration file details](../../src/runtime/README.md#configuration).
|
||||
|
||||
The configuration file is also used to enable runtime [debug output](../Developer-Guide.md#enable-full-debug).
|
||||
|
||||
## Process overview
|
||||
|
||||
The table below shows an example of the main processes running in the
|
||||
different [environments](#environments) when a Kata Container is
|
||||
created with containerd using our [example command](#example-command):
|
||||
|
||||
| Description | Host | VM root environment | VM container environment |
|
||||
|-|-|-|-|
|
||||
| Container manager | `containerd` | |
|
||||
| Kata Containers | [runtime](#runtime), [`virtiofsd`](#virtio-fs), [hypervisor](#hypervisor) | [agent](#agent) |
|
||||
| User [workload](#workload) | | | [`ubuntu sh`](#example-command) |
|
||||
|
||||
## Networking
|
||||
|
||||
Containers will typically live in their own, possibly shared, networking namespace.
|
||||
@ -135,8 +647,10 @@ In order to do so, container engines will usually add one end of a virtual
|
||||
ethernet (`veth`) pair into the container networking namespace. The other end of
|
||||
the `veth` pair is added to the host networking namespace.
|
||||
|
||||
This is a very namespace-centric approach as many hypervisors/VMMs cannot handle `veth`
|
||||
interfaces. Typically, `TAP` interfaces are created for VM connectivity.
|
||||
This is a very namespace-centric approach as many hypervisors or VM
|
||||
Managers (VMMs) such as `virt-manager` cannot handle `veth`
|
||||
interfaces. Typically, `TAP` interfaces are created for VM
|
||||
connectivity.
|
||||
|
||||
To overcome incompatibility between typical container engines expectations
|
||||
and virtual machines, Kata Containers networking transparently connects `veth`
|
||||
@ -170,47 +684,91 @@ The following diagram illustrates the Kata Containers network hotplug workflow.
|
||||
|
||||
|
||||
## Storage
|
||||
Container workloads are shared with the virtualized environment through [virtio-fs](https://virtio-fs.gitlab.io/).
|
||||
|
||||
The [devicemapper `snapshotter`](https://github.com/containerd/containerd/tree/master/snapshots/devmapper) is a special case. The `snapshotter` uses dedicated block devices rather than formatted filesystems, and operates at the block level rather than the file level. This knowledge is used to directly use the underlying block device instead of the overlay file system for the container root file system. The block device maps to the top read-write layer for the overlay. This approach gives much better I/O performance compared to using `virtio-fs` to share the container file system.
|
||||
### virtio SCSI
|
||||
|
||||
Kata Containers has the ability to hotplug and remove block devices, which makes it possible to use block devices for containers started after the VM has been launched.
|
||||
If a block-based graph driver is [configured](#configuration),
|
||||
`virtio-scsi` is used to _share_ the workload image (such as
|
||||
`busybox:latest`) into the container's environment inside the VM.
|
||||
|
||||
Users can check to see if the container uses the devicemapper block device as its rootfs by calling `mount(8)` within the container. If the devicemapper block device
|
||||
is used, `/` will be mounted on `/dev/vda`. Users can disable direct mounting of the underlying block device through the runtime configuration.
|
||||
### virtio FS
|
||||
|
||||
If a block-based graph driver is _not_ [configured](#configuration), a
|
||||
[`virtio-fs`](https://virtio-fs.gitlab.io) (`VIRTIO`) overlay
|
||||
filesystem mount point is used to _share_ the workload image instead. The
|
||||
[agent](#agent) uses this mount point as the root filesystem for the
|
||||
container processes.
|
||||
|
||||
For virtio-fs, the [runtime](#runtime) starts one `virtiofsd` daemon
|
||||
(that runs in the host context) for each VM created.
|
||||
|
||||
### Devicemapper
|
||||
|
||||
The
|
||||
[devicemapper `snapshotter`](https://github.com/containerd/containerd/tree/master/snapshots/devmapper)
|
||||
is a special case. The `snapshotter` uses dedicated block devices
|
||||
rather than formatted filesystems, and operates at the block level
|
||||
rather than the file level. This knowledge is used to directly use the
|
||||
underlying block device instead of the overlay file system for the
|
||||
container root file system. The block device maps to the top
|
||||
read-write layer for the overlay. This approach gives much better I/O
|
||||
performance compared to using `virtio-fs` to share the container file
|
||||
system.
|
||||
|
||||
#### Hot plug and unplug
|
||||
|
||||
Kata Containers has the ability to hot plug add and hot plug remove
|
||||
block devices. This makes it possible to use block devices for
|
||||
containers started after the VM has been launched.
|
||||
|
||||
Users can check to see if the container uses the `devicemapper` block
|
||||
device as its rootfs by calling `mount(8)` within the container. If
|
||||
the `devicemapper` block device is used, the root filesystem (`/`)
|
||||
will be mounted from `/dev/vda`. Users can disable direct mounting of
|
||||
the underlying block device through the runtime
|
||||
[configuration](#configuration).
|
||||
|
||||
## Kubernetes support
|
||||
|
||||
[Kubernetes\*](https://github.com/kubernetes/kubernetes/) is a popular open source
|
||||
[Kubernetes](https://github.com/kubernetes/kubernetes/), or K8s, is a popular open source
|
||||
container orchestration engine. In Kubernetes, a set of containers sharing resources
|
||||
such as networking, storage, mount, PID, etc. is called a
|
||||
[Pod](https://kubernetes.io/docs/user-guide/pods/).
|
||||
[pod](https://kubernetes.io/docs/user-guide/pods/).
|
||||
|
||||
A node can have multiple pods, but at a minimum, a node within a Kubernetes cluster
|
||||
only needs to run a container runtime and a container agent (called a
|
||||
[Kubelet](https://kubernetes.io/docs/admin/kubelet/)).
|
||||
|
||||
A Kubernetes cluster runs a control plane where a scheduler (typically running on a
|
||||
dedicated master node) calls into a compute Kubelet. This Kubelet instance is
|
||||
responsible for managing the lifecycle of pods within the nodes and eventually relies
|
||||
on a container runtime to handle execution. The Kubelet architecture decouples
|
||||
lifecycle management from container execution through the dedicated
|
||||
`gRPC` based [Container Runtime Interface (CRI)](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/node/container-runtime-interface-v1.md).
|
||||
Kata Containers represents a Kubelet pod as a VM.
|
||||
|
||||
In other words, a Kubelet is a CRI client and expects a CRI implementation to
|
||||
handle the server side of the interface.
|
||||
[CRI-O\*](https://github.com/kubernetes-incubator/cri-o) and [Containerd\*](https://github.com/containerd/containerd/) are CRI implementations that rely on [OCI](https://github.com/opencontainers/runtime-spec)
|
||||
compatible runtimes for managing container instances.
|
||||
A Kubernetes cluster runs a control plane where a scheduler (typically
|
||||
running on a dedicated master node) calls into a compute Kubelet. This
|
||||
Kubelet instance is responsible for managing the lifecycle of pods
|
||||
within the nodes and eventually relies on a container runtime to
|
||||
handle execution. The Kubelet architecture decouples lifecycle
|
||||
management from container execution through a dedicated gRPC based
|
||||
[Container Runtime Interface (CRI)](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/node/container-runtime-interface-v1.md).
|
||||
|
||||
Kata Containers is an officially supported CRI-O and Containerd runtime. Refer to the following guides on how to set up Kata Containers with Kubernetes:
|
||||
In other words, a Kubelet is a CRI client and expects a CRI
|
||||
implementation to handle the server side of the interface.
|
||||
[CRI-O](https://github.com/kubernetes-incubator/cri-o) and
|
||||
[containerd](https://github.com/containerd/containerd/) are CRI
|
||||
implementations that rely on
|
||||
[OCI](https://github.com/opencontainers/runtime-spec) compatible
|
||||
runtimes for managing container instances.
|
||||
|
||||
- [How to use Kata Containers and Containerd](../how-to/containerd-kata.md)
|
||||
Kata Containers is an officially supported CRI-O and containerd
|
||||
runtime. Refer to the following guides on how to set up Kata
|
||||
Containers with Kubernetes:
|
||||
|
||||
- [How to use Kata Containers and containerd](../how-to/containerd-kata.md)
|
||||
- [Run Kata Containers with Kubernetes](../how-to/run-kata-with-k8s.md)
|
||||
|
||||
#### OCI annotations
|
||||
|
||||
In order for the Kata Containers runtime (or any virtual machine based OCI compatible
|
||||
runtime) to be able to understand if it needs to create a full virtual machine or if it
|
||||
has to create a new container inside an existing pod's virtual machine, CRI-O adds
|
||||
In order for the Kata Containers [runtime](#runtime) (or any VM based OCI compatible
|
||||
runtime) to be able to understand if it needs to create a full VM or if it
|
||||
has to create a new container inside an existing pod's VM, CRI-O adds
|
||||
specific annotations to the OCI configuration file (`config.json`) which is passed to
|
||||
the OCI compatible runtime.
|
||||
|
||||
@ -221,70 +779,86 @@ or `container`. Kata Containers will then use this annotation to decide if it ne
|
||||
respectively create a virtual machine or a container inside a virtual machine associated
|
||||
with a Kubernetes pod:
|
||||
|
||||
```Go
|
||||
containerType, err := ociSpec.ContainerType()
|
||||
if err != nil {
|
||||
return err
|
||||
}
|
||||
|
||||
handleFactory(ctx, runtimeConfig)
|
||||
|
||||
disableOutput := noNeedForOutput(detach, ociSpec.Process.Terminal)
|
||||
|
||||
var process vc.Process
|
||||
switch containerType {
|
||||
case vc.PodSandbox:
|
||||
process, err = createSandbox(ctx, ociSpec, runtimeConfig, containerID, bundlePath, console, disableOutput, systemdCgroup)
|
||||
if err != nil {
|
||||
return err
|
||||
}
|
||||
case vc.PodContainer:
|
||||
process, err = createContainer(ctx, ociSpec, containerID, bundlePath, console, disableOutput)
|
||||
if err != nil {
|
||||
return err
|
||||
}
|
||||
}
|
||||
|
||||
```
|
||||
| Annotation value | Kata VM created? | Kata container created? |
|
||||
|-|-|-|
|
||||
| `sandbox` | yes | yes (inside new VM) |
|
||||
| `container`| no | yes (in existing VM) |
|
||||
|
||||
#### Mixing VM based and namespace based runtimes
|
||||
|
||||
> **Note:** Since Kubernetes 1.12, the [`Kubernetes RuntimeClass`](https://kubernetes.io/docs/concepts/containers/runtime-class/)
|
||||
> has been supported and the user can specify runtime without the non-standardized annotations.
|
||||
|
||||
With `RuntimeClass`, users can define Kata Containers as a `RuntimeClass` and then explicitly specify that a pod being created as a Kata Containers pod. For details, please refer to [How to use Kata Containers and Containerd](../../docs/how-to/containerd-kata.md).
|
||||
With `RuntimeClass`, users can define Kata Containers as a
|
||||
`RuntimeClass` and then explicitly specify that a pod must be created
|
||||
as a Kata Containers pod. For details, please refer to [How to use
|
||||
Kata Containers and containerd](../../docs/how-to/containerd-kata.md).
|
||||
|
||||
## Tracing
|
||||
|
||||
The [tracing document](../tracing.md) provides details on the tracing
|
||||
architecture.
|
||||
|
||||
# Appendices
|
||||
|
||||
## DAX
|
||||
|
||||
Kata Containers utilizes the Linux kernel DAX [(Direct Access filesystem)](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/dax.rst?h=v5.14)
|
||||
feature to efficiently map some host-side files into the guest VM space.
|
||||
In particular, Kata Containers uses the QEMU NVDIMM feature to provide a
|
||||
memory-mapped virtual device that can be used to DAX map the virtual machine's
|
||||
root filesystem into the guest memory address space.
|
||||
Kata Containers utilizes the Linux kernel DAX
|
||||
[(Direct Access filesystem)](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/dax.rst?h=v5.14)
|
||||
feature to efficiently map the [guest image](#guest-image) in the
|
||||
[host environment](#environments) into the
|
||||
[guest VM environment](#environments) to become the VM's
|
||||
[rootfs](#root-filesystem).
|
||||
|
||||
Mapping files using DAX provides a number of benefits over more traditional VM
|
||||
file and device mapping mechanisms:
|
||||
If the [configured](#configuration) [hypervisor](#hypervisor) is set
|
||||
to either QEMU or Cloud Hypervisor, DAX is used with the feature shown
|
||||
in the table below:
|
||||
|
||||
| Hypervisor | Feature used | rootfs device type |
|
||||
|-|-|-|
|
||||
| Cloud Hypervisor (CH) | `dax` `FsConfig` configuration option | PMEM (emulated Persistent Memory device) |
|
||||
| QEMU | NVDIMM memory device with a memory file backend | NVDIMM (emulated Non-Volatile Dual In-line Memory Module device) |
|
||||
|
||||
The features in the table above are equivalent in that they provide a memory-mapped
|
||||
virtual device which is used to DAX map the VM's
|
||||
[rootfs](#root-filesystem) into the [VM guest](#environments) memory
|
||||
address space.
|
||||
|
||||
The VM is then booted, specifying the `root=` kernel parameter to make
|
||||
the [guest kernel](#guest-kernel) use the appropriate emulated device
|
||||
as its rootfs.
|
||||
|
||||
### DAX advantages
|
||||
|
||||
Mapping files using [DAX](#dax) provides a number of benefits over
|
||||
more traditional VM file and device mapping mechanisms:
|
||||
|
||||
- Mapping as a direct access device allows the guest to directly
|
||||
access the host memory pages (such as via Execute In Place (XIP)),
|
||||
bypassing the [guest kernel](#guest-kernel)'s page cache. This
|
||||
zero copy provides both time and space optimizations.
|
||||
|
||||
- Mapping as a direct access devices allows the guest to directly access
|
||||
the host memory pages (such as via Execute In Place (XIP)), bypassing the guest
|
||||
page cache. This provides both time and space optimizations.
|
||||
- Mapping as a direct access device inside the VM allows pages from the
|
||||
host to be demand loaded using page faults, rather than having to make requests
|
||||
via a virtualized device (causing expensive VM exits/hypercalls), thus providing
|
||||
a speed optimization.
|
||||
- Utilizing `MAP_SHARED` shared memory on the host allows the host to efficiently
|
||||
share pages.
|
||||
|
||||
Kata Containers uses the following steps to set up the DAX mappings:
|
||||
1. QEMU is configured with an NVDIMM memory device, with a memory file
|
||||
backend to map in the host-side file into the virtual NVDIMM space.
|
||||
2. The guest kernel command line mounts this NVDIMM device with the DAX
|
||||
feature enabled, allowing direct page mapping and access, thus bypassing the
|
||||
guest page cache.
|
||||
- Utilizing `mmap(2)`'s `MAP_SHARED` shared memory option on the host
|
||||
allows the host to efficiently share pages.
|
||||
|
||||
|
||||
|
||||
Information on the use of NVDIMM via QEMU is available in the [QEMU source code](http://git.qemu-project.org/?p=qemu.git;a=blob;f=docs/nvdimm.txt;hb=HEAD)
|
||||
For further details of the use of NVDIMM with QEMU, see the [QEMU
|
||||
project documentation](https://www.qemu.org).
|
||||
|
||||
## Agent control tool
|
||||
|
||||
The [agent control tool](../../src/tools/agent-ctl) is a test and
|
||||
development tool that can be used to learn more about a Kata
|
||||
Containers system.
|
||||
|
||||
## Terminology
|
||||
|
||||
See the [project glossary](../../Glossary.md).
|
||||
|
||||
[debug-console]: ../Developer-Guide.md#connect-to-debug-console
|
||||
|
||||
Loading…
Reference in New Issue
Block a user