Getting Started with Kubernetes | Further Analysis of Linux Containers

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By Tang Huamin (Huamin), Container Platform Technical Expert at Alibaba Cloud

The Linux container is a lightweight virtualization technology, which isolates and restricts process resources in kernel sharing scenarios based on namespace and cgroup technology. This article takes Docker as an example to provide a basic description of container images and container engines.

Containers

A container is a lightweight virtualization technology. In contrast with virtual machines (VMs), it does not contain the hypervisor layer. The following figure shows the startup process of a container.

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At the bottom layer, the disk stores container images. The container engine at the upper layer can be Docker or another container engine. The container engine sends a request, such as a container creation request, to run the container image on the disk as a process on the host.

For containers, the resources used by the process must be isolated and restricted. This is implemented by the cgroup and namespace technologies in the Linux kernel. This article uses Docker as an example to describe resource isolation and container images.

1. Resource Isolation and Restrictions

Namespace technology is used for resource isolation. Seven namespaces are available in the Linux kernel, and the first six are used in Docker. The cgroup namespace is not used in Docker but is implemented in runC.

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The following describes the namespaces in sequence:

  • The mount namespace is the view of the file system that is visible to a container. It is a file system provided by the container image, which means that other files on the host are invisible to it. You need to run -v parameter bound to make some directories and files on the host visible in the container.
  • The uts namespace isolates the host name and domain.
  • The pid namespace ensures that the container’s init process is started by process 1.
  • The network namespace is available for all network modes except the host network mode used by containers.
  • The user namespace maps the user UID and GID in the container and on the host. This namespace is seldom used.
  • The IPC namespace controls processes and communication, such as semaphores.
  • The cgroup namespace can be enabled or disabled, as shown in the right part of the preceding figure. When the cgroup namespace is used, the cgroup view is presented as a root for a container, just like that for the processes on the host. The cgroup namespace also makes the use of the cgroup in the container more secure.

The following describes how to create a namespace in a container by using unshare.

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The upper part of the figure is an example of using unshare, while the lower part is a pid namespace that is created by the unshare command. As shown in the figure, the bash process is in a new pid namespace, and the ps result indicates that the PID of the bash process is 1, indicating that it is a new pid namespace.

cgroup

Cgroup technology is used for resource restriction. Both systemd drivers and cgroupfs drivers are available for Docker containers.

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  • cgroupfs is easier to understand. For example, if you want to know the memory limit and CPU share, you can directly write the PID to a corresponding cgroup file and then write the resources to be restricted to the corresponding memory cgroup file and CPU cgroup file.
  • systemd is a cgroup driver, which can manage cgroups. Therefore, if you use systemd as the cgroup driver, you need to complete all cgroup write operations through the systemd interface but cannot manually modify the cgroup file.

The following describes the common cgroups for containers. The Linux kernel provides many cgroups. Only the following six types are used for Docker containers:

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  • The CPU cgroup controls the CPU utilization by setting the CPU share and CPU set.
  • The memory cgroup controls the memory usage of the process.
  • The device cgroup controls the devices that are visible in the container.
  • The freezer cgroup improves security, just like the device cgroup. When you stop a container, the freezer cgroup writes all the current processes into the cgroup and freezes them to prevent the fork operation of any processes. In this way, it prevents the process from escaping to the host and ensures security.
  • The blkio cgroup limits the input/output operations per second (IOPS) and bytes per second (BPS) of disks used by containers. If the cgroup is not unique, the blkio cgroup only restricts the synchronization I/O but not the Docker I/O.
  • The pid cgroup limits the maximum number of processes in a container.

Some cgroups are not used for Docker containers. cgroups are divided into common and uncommon cgroups. This distinction only applies to Docker because all cgroups except for rdma are supported by runC. However, they are not enabled for Docker. Therefore, Docker does not support the cgroups in the following figure.

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2. Container Images

This section uses a Docker image as an example to describe the container image structure.

Docker images are based on the union file system. The union file system allows files to be stored at different layers. However, all these files are visible on a unified view.

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In the preceding figure, the right part is a container storage structure obtained from the official Docker website.

This figure shows that the Docker storage is a hierarchical structure based on the union file system. Each layer consists of different files and can be reused by other images. When an image is run as a container, the top layer is the writable layer of the container. The writable layer of the container can be committed as a new layer of the image.

The bottom layer of the Docker image storage is based on different file systems. Therefore, its storage driver is customized for different file systems, such as AUFS, Btrfs, devicemapper, and overlay. Docker drives these file systems with graph drivers, which store images on disks.

Overlay

This section uses the overlay file system as an example to describe how Docker images are stored on disks.

The following figure shows how the overlay file system works.

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  • The lower layer is a read-only image layer.
  • The upper layer is the container read and write layer, which adopts a copy-on-write mechanism. That is, a file is copied from the lower layer only when the file needs to be modified, and all the modify operations are performed on the replicas of the upper layer.
  • The workdir layer works as an intermediate layer. The to-be-modified replica at the upper layer is modified at the workdir layer and then moved to the upper layer. This is how the overlay file system works.
  • The mergedir layer is a unified view layer. You can see all the data of the upper and lower layers at the mergedir layer. Then, you can run the docker exec command to view a file system in the container, which is the mergedir layer.

This section describes how to perform file operations in a container based on overlay storage.

File Operations

  • Read: If the upper layer has no replicas, all data is read from the lower layer.
  • Write: When a container is created, the upper layer is empty. A file is copied from the lower layer only when the file needs to be written.
  • Delete: The delete operation does not affect the lower layer. Deleting a file actually means adding a mark to the file so that the file is not displayed. A file can be deleted through whiteout or by setting xattr trusted.overlay.opaque = y.

When a container is created, the upper layer is empty. If you try to read data at this time, all the data is read from the lower layer.

As mentioned above, the overlay upper layer has a copy-on-write mechanism. When some files need to be modified, the overlay file system copies the files from the lower layer and modifies them.

There is no real delete operation in the overlay file system. Deleting a file actually means adding a mark to the file at the unified view layer so that the file is not displayed. Files can be deleted in two ways:

  • whiteout
  • directory deletion, which can be done by setting extended permissions for the directories and setting extended parameters.

This section describes how to run the docker run command to start a busybox container and what the overlay mount point is.

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The second figure shows the mount command used to view the mount point. The container rootfs mount point is of the overlay type, and includes the upper, lower, and workdir layers.

Next, let’s learn how to write new files into a container. Run the docker exec command to create a file. As shown in the preceding figure, diff is an upperdir of the new file. The content in the file in upperdir is also written by the docker exec command.

The mergedir directory contains the content in upperdir and lowerdir and the written data.

3. Container Engine

This section describes the general architecture of containerd on a container engine based on Cloud Native Computing Foundation (CNCF). The following figure shows the containerd architecture.

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As shown in the preceding figure, containerd provides two main functions.

One is runtime, which is container lifecycle management. The other is storage, which is image storage management. containerd pulls and stores images.

Horizontally, the containerd structure is divided into the following layers:

  • The first layer includes gRPC and metrics. containerd provides services for the upper layer through the gRPC server. Metrics provides some cgroup metrics.
  • At the lower layer, the left part is storage for container images. The metadata of images and containers, which is stored on a disk through bootfs. Tasks in the right part manages the container structure. Events send an event to the upper layer for certain operations on the container and the upper layer can subscribe to the event to monitor the container status changes.
  • The underlying layer is Runtimes and can be divided by type, such as runC or security container.

This section describes the general structure of containerd at the Runtimes layer. The following figure is taken from the official kata website. The upper part is the source image, while some extended examples are added to the lower part. Let’s look at the architecture of containerd at the Runtimes layer.

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The preceding figure shows a process from the upper layer to the Runtime layer from left to right.

A CRI Client is shown to the leftmost. Generally, Kubelet sends a CRI request to containerd. After receiving the request, containerd passes it through a containerd-shim that manages the container lifecycle and performs the following operations:

  • Forwards I/O.
  • Transmits signals.

The upper part of the figure shows the security container, which is a kata process. The lower part of the figure shows various shims. The following describes the architecture of a containerd-shim.

Initially, there is only one shim in containerd, which is enclosed in the blue box. The shims in all containers, such as kata, runC, and gVisor containers, are containerd-shims.

Containerd is extended for different types of runtimes through the shim-v2 interface. In other words, different shims can be customized for different runtimes through the shim-v2 interface. For example, the runC container can create a shim named shim-runc, the gVisor container can create a shim named shim-gvisor, and the kata container can create a shim named shim-kata. These shims can replace the containerd-shims in the blue boxes.

This has many advantages. For example, when shim-v1 is used, there are three components due to the limits of kata. However, when shim-v2 is used, the three components can be made into one shim-kata component.

This section uses two examples to describe how a container process works. The following two figures show the workflow of a container based on the containerd architecture.

Start Process

The following figure shows the start process.

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The process consists of three parts:

  • The container engine can be a Docker or another engine.
  • containerd and containerd-shim are parts of the containerd architecture.
  • The container is pulled by a runtime, or a container is created by a shim by running the runC command.

The numbers marked in the figure show the process by which containerd creates a container.

It first creates metadata and then sends a request to the task service to create a container. The request is sent to a shim through a series of components. containerd interacts with container-shim through gRPC. After containerd sends the creation request to container-shim, container-shim calls the runtime to create a container.

Exec Process

The following figure shows how to execute a container.

The exec process is similar to the start process. The numbers marked in the figure shows the steps by which containerd performs exec.

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As shown in the preceding figure, the exec operation is also sent to containerd-shim. There is no essential difference between starting a container and executing a container.

The only difference is whether a namespace is created for the process running in the container.

  • During exec, the process must be added to an existing namespace.
  • During start, the namespace of the container process must be created.

Summary

I hope this article helped you better understand Linux containers. Let’s summarize what we have learned in this article:

  1. How to use namespaces for resource isolation and cgroups for resource restriction in containers.
  2. The container image storage based on the overlay file system.
  3. How the container engine works based on Docker and containerd.

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