In-depth analysis of the failure to copy files within the docker container

In recent years, Docker has become one of the most commonly used containerization technologies in the field of cloud computing. Docker containers can greatly optimize application running efficiency and simplify the deployment process. However, when using Docker containers to deploy applications, we sometimes encounter failures in copying files within the container. Next, we will analyze this problem and its solution in depth.

Problem Analysis

In a Docker container, if we need to copy files on the host to the container, we usually use the docker cp command. For example, if we want to copy the test.txt file on the host to the /opt directory in the container, we can execute the following command:

docker cp test.txt container_id:/opt/

However, after executing the above command, we may encounter the following Error message:

Error response from daemon: Get "***": EOF

This error message indicates that the connection was closed or the connection timed out, causing the file copy to fail and the folder in the Docker container has not changed. The most common cause of this problem is that the Docker container is out of memory, causing the connection to not be maintained or closed.

Solution

Below, I will introduce several solutions.

Method 1: Increase the memory of the Docker container

We can bypass this problem by increasing the memory of the Docker container. In Docker containers, the memory of the container is limited. When we need to copy a large number of files into a container, the memory in the container may be exhausted, causing the file copy to fail. We can solve this problem by increasing the memory inside the container.

For example, we can use the following command to add 2GB of memory to the container named test_container:

docker update --memory=2g test_container

Method 2: Use the copy command inside the Docker daemon process

Except Using the docker cp command, there is also a copy command inside the Docker daemon, which does not use the container's network connection. This way, we can ensure that there are no problems with network connectivity and that as long as the container is running, the internal copy command will work successfully even if the host is shut down.

For example, we can use the following command to copy the test.txt file on the host to a container named test_container:

docker exec test_container sh -c 'cat > /opt/test.txt' < test.txt

Method 3: Use Docker Volume

Docker Volume provides a persistent data storage area for Docker containers. Using Docker Volume, we can mount a directory on the host inside the container, so that we can directly access the directory inside the container without having to copy files between the host and the container. In this way, even if the container does not have enough memory, it will not affect the copy process.

The following is an example command to use Docker Volume:

docker run -it -v /path/to/host/dir:/path/to/container/dir my_image

This command will create a container and mount the /path/to/host/dir directory on the host to the container /path/to/container/dir directory. We can access them directly within the container without worrying about errors caused by the container running out of memory.

In short, when using Docker containers, failure to copy files is a common problem, but it can also be solved. By increasing memory, using the copy command inside the Docker daemon, or using Docker Volume, we can bypass this problem and easily deploy applications using Docker containers.

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