How to solve the problem that docker cannot clear cache

With the development of cloud computing and containerization technology, containerization has become the mainstream method of modern application development and deployment. Docker, the most popular containerization engine, provides an environment for developing, testing, and deploying applications.

However, although Docker provides a convenient containerization solution, in practice, using Docker often encounters problems. One of them is the problem when clearing cache. In this article, we will explore why cache cannot be cleared in Docker and how to fix it.

Why the cache cannot be cleared in Docker

Docker uses images as a base when creating containers so that the containers can start the required services and applications. Docker images are composed of multiple layers, each containing files and commands defined in a Dockerfile. When building a new image, each layer is cached locally in the Docker engine.

When building a new image using an old cached image, the Docker engine may use the old version in the cache instead of using the latest version of the file. This can cause build errors or strange behavior in the application.

By default, the Docker engine will try to use caching to speed up image building. However, in some cases we need to clear the cache and use the latest files. However, when we try to clear the cache, we often find that we cannot clear the cache completely.

This is because the Docker engine has multiple components, including the Docker daemon, Docker images, Docker containers, and Docker networks. Although Docker provides some commands such as docker system prune to clean up unused object resources, it cannot clear unused images ("dangling" images) that have been created but are still cached locally.

Although these cache images are no longer used, they will not be cleared. This can take up a lot of local storage, especially if the image is built at runtime.

How to solve the problem of being unable to clear the cache

Although the Docker engine provides some cleanup commands, due to historical reasons and design limitations, it is impossible to completely clear all cached images. In this case, we need to use some additional tools and techniques.

The following are some methods to solve the problem of being unable to clear the Docker cache:

1. Use third-party cleaning tools

In addition to the cleaning commands provided by the Docker engine itself, there are many third-party cleaning tools. Third-party tools can help us clean the Docker cache. For example, you can use the Docker-cleanup tool to clean up unused Docker resources, including unused images, containers, and networks. This tool can be downloaded and used for free from GitHub and is a very convenient cleaning tool.

# 安装 Docker-cleanup 工具
wget https://gist.github.com/wdullaer/f1af16bd7e970389bad3/raw \
  -O /usr/local/bin/docker-cleanup && chmod +x /usr/local/bin/docker-cleanup

# 执行清理命令，清除所有未使用的 Docker 资源
docker-cleanup

2. Use Docker’s official cleanup script

Docker officially provides a cleanup script for clearing the Docker cache and other unused resources. This script can be downloaded from Docker's GitHub and is a very simple and convenient cleaning tool. Here's how to use Docker's official cleanup script:

# 下载 Docker 清理脚本
curl -fsSL https://raw.githubusercontent.com/docker/docker-ce/17.12.0-ce/components/cli/contrib/clean.sh \
  -o /usr/local/bin/docker-clean && chmod +x /usr/local/bin/docker-clean

# 执行清理命令，清除所有未使用的 Docker 资源
docker-clean

3. Manually clear the Docker cache

If you don't want to use third-party tools or official cleanup scripts, you can also manually clear the Docker cache. Although this process can be tedious, you can ensure that the cleanup is complete and that you know exactly which resources you want to delete.

Here are the steps to manually clear the Docker cache:

1. Stop all running containers.
2. Use the docker images command to list all local images.
3. Use the docker rmi IMAGE command to delete all unused images. Note that you need to replace IMAGE with the name or ID of the image you want to delete. If some images are in use, they cannot be deleted.
4. Execute the docker system prune command to clear all unused resources, including images, containers, and networks.

# 停止所有运行中的容器
docker stop $(docker ps -q)

# 列出所有本地镜像
docker images -a

# 删除未使用的镜像
docker rmi $(docker images -f "dangling=true" -q)

# 清理系统所有未使用的资源
docker system prune -a

In short, when using Docker cache, we need to be aware of this problem and use various available methods to clear the cache. Each method has its own advantages and disadvantages, and you can choose the method that works best for you based on your needs.

It is worth noting that we should operate with caution when cleaning the Docker cache. Accidentally deleting some resources that are in use can cause problems for your application. Therefore, it is best to back up your important data before doing anything.

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