What to do if the docker parent image cannot be deleted (two methods)

Docker is a very popular open source tool for creating, deploying and running applications. Compared with virtual machines, Docker provides more lightweight virtualization, allowing applications to be quickly deployed and run on any system that supports Docker.

In Docker, an image is an executable Docker container that contains all the dependencies and configuration required for the application to run. Docker containers can be built based on existing images. This construction method is called layered construction. Each layer represents a specific configuration or dependency.

The working principle of the Docker container is similar to the tree structure in the data structure. Each image is a node, and a parent-child relationship is formed between layers. This raises a question. If we delete a parent image, will its child images become unusable?

The answer is yes. Because the child mirror depends on certain configurations or dependencies in the parent image, if the parent image is deleted, the child mirror will not work properly.

The file system at each layer in the Docker container is read-only, so when an image is run in Docker, it creates a read-write layer that allows the container to modify the file system inside the container. When modifications are made inside the container, Docker uses a joint file system to merge the read-write layer and the image layer, so that the modified file is updated in the read-write layer without affecting the original image layer.

When a container is stopped and deleted, its read-write layer will also be deleted, but the mirror layer will not be deleted. Therefore, if an image serves as the parent image of another image, its image layer must always exist, otherwise the child image will not be usable.

So, what should we do if we want to delete a mirror but do not want to affect the use of its sub-mirrors? There are two solutions here:

1. Modify the dependencies and configuration in the Dockerfile

If an image is built dependent on other images, then we can modify the Dockerfile Dependencies and configurations in the file to solve the problem that the parent image cannot be deleted. You can use the COPY or ADD instructions in the Dockerfile to copy the required files or directories to the image instead of getting them from other images. In this way, there is no need to rely on other image builds, and it will not be affected by deleting the parent image.

2. Use mirror export and import

If a mirror has been used as the parent mirror of other mirrors, but we don’t want to affect the use of its child mirrors, then we can use Mirror export and import to solve the problem. First, we need to export the image:

docker save <image-name>:<tag> > /path/to/save/image.tar

Then, we can use the docker load command to import the image on other systems so that the image can continue to be used.

docker load < /path/to/save/image.tar

In this way, we can archive the image for use when needed.

Summary:

In Docker, the dependency of the image is very important, and the existence of the parent image is crucial to the use of the child image. If we want to delete a parent image, we must consider its impact on other images. When building a Docker image, we can use the COPY or ADD instructions to copy the required files or directories to the image to avoid relying on the parent image. When we need to migrate the image, we can use the image export and import methods to facilitate use in other systems.

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