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Docker image transmission layering; because the docker image is built in layers, the process of building the image will be dismantled, find the common points with other services, and formulate it into a basic image. Each item in the Dockerfile If specified, a new layer will be created. This saves disk space, so the transfer of docker images is layered.
The operating environment of this tutorial: linux7.3 system, docker version 19.03, Dell G3 computer.
An image is a lightweight, executable independent software package that contains everything needed to run a certain software. We package the application and configuration into a formed, deliverable, and deployable operating environment, including code, libraries required for runtime, environment variables, and configuration files, etc. This large packaged operating environment is an image image file.
Docker container instances can only be generated through image files.
docker image layering
Meaning: Disassemble the process of building an image, find commonalities with other services, and formulate it into a basic image .
Advantages: save disk space,
When uploading/downloading images, if the basic image already exists, it will not be uploaded and downloaded repeatedly, improving upload and download speeds
Disadvantages: changing the basis Mirror, all sub-mirrors will change.
Docker image layering mechanism
Docker images are built in layers. As we learn more about Docker, we will understand that each specification in the Dockerfile will create a new one. layer. Take the following Dockerfile instructions as an example:
FROM ubuntu：20.04#基础镜像 COPY . /app#复制文件 RUN make /app#编译文件 CMD python /app/app.py#入口文件
The above four instructions only record the changes made by this layer on each layer, and these layers are read-only layers. When starting a container, Docker commands to add a read-write layer at the top. All changes made in the container (writing logs, modifying, deleting files, etc., are saved in the read-write layer). This layer is generally called the container layer, as follows As shown in the figure:
In fact, the main difference between a container and an image is that the container adds a top-level read and write layer. All modifications to the container occur at this layer, and the image will not be modified, which is the COW (copy-on-write) technology mentioned earlier. When the container needs to read a file, it can read it directly from the bottom read-only layer. If a file needs to be modified, the file is copied to the top read-write layer for modification, and the read-only layer remains unchanged.
Each container has its own read-write layer, so multiple containers can use the same image. In addition, when the container is deleted, its corresponding read-write layer will also be deleted. If you want multiple containers To share or persist data, you can use Docker volumes.
Recommended learning: "docker video tutorial"
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