What exactly is a docker image?

Docker image is a special file system. In addition to providing the programs, libraries, resources, configuration and other files required for container runtime, it also contains some configuration parameters prepared for runtime (such as anonymous volumes). , environment variables, users, etc.); the image does not contain any dynamic data, and its content will not be changed after it is built.

The operating environment of this article: ubuntu 18.04 system, Docker version 20.10.11, Dell G3 computer.

What exactly is a docker image?

First understand what an image is:

We all know that the operating system is divided into kernel and user space. For Linux, after the kernel is started, the root file system will be mounted to provide user space support. The Docker image (Image) is equivalent to a root file system. For example, the official image ubuntu:18.04 contains a complete set of root file systems for Ubuntu 18.04 minimal system.

Docker imageis a special file system. In addition to providing the programs, libraries, resources, configuration and other files required for container runtime, it also contains some files prepared for runtime. Configuration parameters (such as anonymous volumes, environment variables, users, etc.). The image does not contain any dynamic data, and its content will not be changed after it is built.

Hiered Storage

Because the image contains the complete root file system of the operating system, its volume is often huge, so when designing Docker, we make full use of the technology , designed as a hierarchical storage architecture. So strictly speaking, an image is not a packaged file like an ISO. An image is just a virtual concept. Its actual manifestation is not composed of a file, but a set of file systems, or in other words, a combination of multi-layer file systems. composition.

When the image is built, it will be built layer by layer, with the previous layer being the basis of the next layer. After each layer is constructed, it will not change again. Any changes on the subsequent layer only occur on its own layer. For example, the operation of deleting a file at the previous level does not actually delete the file at the previous level, but only marks the file as deleted at the current level. When the final container is run, although this file will not be seen, in fact, the file will always follow the image. Therefore, when building an image, you need to be extra careful. Each layer should only contain what needs to be added to the layer. Any extra things should be cleaned up before the construction of the layer is completed.

The characteristics of tiered storage also make it easier to reuse and customize images. You can even use the previously built image as the base layer, and then further add new layers to customize what you need and build a new image.

About mirror construction, we will further explain it in subsequent related articles.

Recommended learning: "Docker Video Tutorial"

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