Docker from entry to application (5): Docker container data volume

what is

As the name implies, the container data volume is the volume that Docker uses to store data; Docker packages the application and running environment into a container to run, and the running can be accompanied by the container, but our requirement for data is that it can be stored persistently, and we hope that different containers will able to share data

If the data generated by the Docker container is not converted into a new image through docker commit, so that the data is saved as part of the image, then when the container is deleted, the data will naturally disappear. In order to save the data, we use volumes in the container

Features of Container Data Volumes

A volume is a directory or a file that exists in one or more containers and is mounted to the container by Docker, but it does not belong to the union file system, so it can bypass the Union File System to provide some features for persistent storage or shared data. The volume is designed for persistence and is completely independent of the life cycle of the container. Therefore, Docker will not delete the mounted data volume when the container is deleted.

features

Data volumes can share or reuse data between containers
Changes in the volume can take effect directly
Changes in the data volume will not be included in the update of the mirror
The lifecycle of a data volume lasts until there are no containers using it

container add data volume

Container data volumes can be added through commands or through DockerFile. The introduction of DockerFile will be explained in the next chapter

command add

command :docker run -it -v /宿主机目录:/容器内目录镜像ID或名称:tag

Create a default centos container, docker run -it centoscreate and start the centos container, check the directory after the container is running, it is the default file structure

Configure the container data volume by adding -vparameters to the command,docker run -it --name reel_centos -v ~/reelHostTest:/reelDokcerTest centos

The two newly created folders in the host directory and the Docker container directory respectively indicate that the container will automatically create a data volume folder when it starts. At this time, you can docker inspect 容器IDcheck whether the data volume is mounted successfully by viewing the internal details of the container.

The data volume is successfully mounted, RW is true, that is, it has read and write permissions

Create a new a.log file in the reelDockerTest directory in the container, test data sharing, check whether the file exists in the corresponding directory of the host, and create a file in the container

The same file exists in the host machine after creation

Modify the content of the file in the host and check whether it is modified in the container

The content of the file in the container has been modified synchronously, indicating that there is data sharing between the host and the container

After the container stops and exits, the host modifies the data to check whether the data is synchronized

First stop the container to exit, the host modifies the file

Restart the container, the content of the file in the data volume has been modified synchronously

File read-only permission

-vThe default container of the data volume added through the parameter has read and write commands, and RO can be added to limit read-only

As shown in the figure, under the ReadOnly permission, the container cannot add files in this folder and docker inspectview the internal information of the container. At this time,"RW":false

Add the file readOnly.txt in the directory of the host, switch to the container to view

The file exists in the container and can be read normally but cannot be modified. When modifying the file, it prompts E45: 'readonly' option is set (add ! to override)

Add data volume through DockerFile

What is DockerFile

Dockerfile is a build file used to build a Docker image, and is a script composed of a series of commands and parameters. DockerFile defines everything that a process needs. DockerFile involves executing code or files, environment variables, dependent packages, runtime environments, dynamic link libraries, operating system releases, service processes, and kernel processes (when the application process requires When dealing with system services and system kernel processes, it is necessary to consider how to design namespace permission control), etc. In simple terms, DockerFile can build the image we need for locks. The detailed introduction of DokcerFile will be explained in the next chapter

Add a DockerFile file to the host and build a mirror image

Create a DockerFile file, add the following test content to the file, where the VOLUME instruction is used to add one or more data volumes to the image

# volume test
FROM centos
VOLUME ["/dockerFile1","/dockerFile2"]
CMD echo "finished,--------success1"
CMD /bin/bash

**Description:** For the sake of portability and sharing, this -v 主机目录：容器目录method cannot be directly implemented in Dockerfile . Since the host directory is dependent on a specific host, it cannot be guaranteed that such a specific directory exists on all hosts.

docker buildCommand to build a mirror ** Note that the ** number docker build -f DockerFile文件地址 -t 镜像名称：tag .at the end cannot be omitted (guess it should be the directory where the image is stored in the Docker engine).

Create and start a container, view the container directory, both dockerFile1 and dockerFile2 are created, and the data volume is bound successfully

When adding a container data volume through the command, you need to specify the data volume file directory in the host and the data volume file directory in the container. The files in the container are finally stored in the host disk. Then, the data volume created through DockerFile corresponds to the host’s Where is the file directory? Docker inspectCommand to view the internal details of the container and confirm the location of the data volume on the host

Switch to the corresponding directory and check whether dockerFile1 and dockerFile2 are created

When switching to the docker directory, it prompts that there is no permission, you can join it when running the container --privileged=true, or grant the user access permission

1. sudo chmod -R 777 /var/lib/docker/

2. docker run -it --privileged=true  --name privilegedtest -d ac9c738661da

Create a file after entering the folder, and check the synchronization in the container

File data sync successfully

data volume container

what is

The named container mounts the data volume, and other containers realize data sharing by mounting this (parent container). The container that mounts the data volume is called a data volume container.

Pass sharing between containers (–volumes-from)

Taking the image we just built as an example, first start the parent container df1, and create the file aa.txt in the dockerFile1 directory

Start the df2 and df3 containers inherited from the main commands of the df1 container --volumes-from, and add aa2.txt and aa3.txt files to the df2 and df3 containers respectively

Enter the df1 container, aa2.txt and aa3.txt files have been synchronized. Delete the df1 container, enter the df2 container, the file aa.txt created by df1 exists and can be modified

Then delete the df2 container, access df3, and the file can be accessed normally

Delete df3 after creating df4 container based on df3, the file can be accessed normally

**Conclusion: The container data volume is used to transfer configuration information between containers, and the life cycle of the data volume lasts until no container uses it**