Docker container data volume named mount and anonymous mount issues

What is a container data volume

The container data volume is the mount of the directory. The directory of our container is mounted on the host machine, so as to realize the file sharing function between the host machine and the container.

Why do we need container data volumes?

The idea of ​​Docker is to package the application and environment into an image; but what about the data? Let alone the database, a project will definitely generate a lot of logs during operation. These logs are very important to developers, because with these logs, we can know what problems occurred during the operation and then troubleshoot.

However, in the container, each time the project is updated and iterated, the container will be deleted and replaced with a new image. In this case, if you want to save these log files, if you copy them to the host machine every time, the workload will be a bit large, and if the log files are too large, the copying work will also be very time-consuming and labor-intensive. So at this time, you need to use the container data volume function. To put it simply, this function is very simple, which is to open up the file sharing function between the host and the container. The data files generated in the docker container will be synchronized to the host machine in real time. On the contrary, the files generated by the host machine will also be synchronized to the container. In this way, a two-way transmission pipeline is opened up.

After data sharing is implemented between containers, there is no distinction between main containers and sub-containers, because there is only one copy of the shared data, which is stored on the host machine. Deleting any container will not affect the data synchronization of other containers.

use

Using container data volumes is very simple. Just add the -v option when running.

docker run -d -v host directory: container directory tomcat

After running the above command, the corresponding directories will be automatically created on the container and the host, and any files created or modified in the directories will be automatically synchronized.

How to Check Whether a Data Volume is Used

To check whether a container uses the container data volume function, you can use the inspect function to view detailed information about the container.

docker inspect container id/container name

After executing the above command, a lot of formatted json strings will be printed. At this time, we find that the item with key Mounts is the relevant configuration of the container data volume.

"Mounts": [
    {
        "Type": "bind",  
        "Source": "/root/dockerContainer", # Directory of the host machine "Destination": "/text", # Directory of the container "Mode": "",
        "RW": true, # RW is readable and writable; ro is read-only and can only modify files on the host machine; 
        "Propagation": "rprivate"
    }
],

Named and anonymous mounts

Mount by specifying a path

In the above examples, we use the specified path mount, that is, configure the host path and the container path;

docker run -d -v host directory: container directory tomcat

Named mount

Mount to a directory with the specified name;

# /xxx is a directory, xxx is a volume name, the one without a slash is the volume name docker run -d -v volume name: container directory tomcat 

# Find the directory where the volume name is located docker volume inspect volume name

Let's test it. First, create a container and mount the directory.

docker run -d --name my_tomcat -v my_folder:/data/my_folder tomcat

Use the inspect command to view container information

docker inspect my_tomcat

Find Mounts item in the printed information, where the Source directory is the host directory; the Destination directory is the mounted container directory;

"Mounts": [
    {
        "Type": "volume",
        "Name": "my_folder",
        "Source": "/var/lib/docker/volumes/my_folder/_data",
        "Destination": "/data/my_folder",
        "Driver": "local",
        "Mode": "z",
        "RW": true,
        "Propagation": ""
    }
]

Then enter the container

docker exec -it my_tomcat /bin/bash

# This directory has also been created in the container root@ef94ff8928a1:/data/my_folder# pwd
/data/my_folder

Anonymous Mount

Anonymous mounting means that there is only a container directory, but no host directory, so the generated directory is a long encrypted string. Generally, anonymous mounting is not recommended; encrypted strings make it difficult to find.

docker run -d -v \container directory tomcat

Come, test it, first create a container and mount the directory

docker run -d --name my_tomcat_2 -v /my_folder_2 tomcat

Use the inspect command to view container information

docker inspect my_tomcat_2

Find Mounts item in the printed information, where the Source directory is the host directory; the Destination directory is the mounted container directory

"Mounts": [
    {
        "Type": "volume",
        "Name": "df4c649772a5ae65716de8ede0607d0776f8c1e2eda1d87b3ec9eaf011b43616",
        "Source": "/var/lib/docker/volumes/df4c649772a5ae65716de8ede0607d0776f8c1e2eda1d87b3ec9eaf011b43616/_data",
        "Destination": "/my_folder_2",
        "Driver": "local",
        "Mode": "",
        "RW": true,
        "Propagation": ""
    }
]

Data sharing between containers--volumes-from

There is a scenario where we need container A and container B to share data. That is, I want to see the content modified in container A on container B. So how should this function be achieved? Then you can use the data volume container function, which can also synchronize data between multiple containers, not just two containers.

1. Create the first container centos_1 and mount the /data/centos directory on the host. The directories of the host and the container are both /data/centos ;

docker run -it --name centos_1 -v /data/centos:/data/centos centos /bin/bash

2. Create a second container and bind it to the first container; --volumes-from centos_1 option binds the mount directory of the current container to the centos_1 container, thereby achieving data synchronization between the containers;

docker run -it --name centos_2 --volumes-from centos_1 centos /bin/bash

3. Now we create a third container and bind it to the second container centos_2

docker run -it --name centos_3 --volumes-from centos_2 centos /bin/bash

Next, we create a file in the /data/centos directory in each container

• Create the main.java file in the /data/centos directory of the host machine
• Create the centos_1.java file in the /data/centos directory of the centos_1 container
• Create the centos_2.java file in the /data/centos directory of the centos_2 container
• Create the centos_3.java file in the /data/centos directory of the centos_3 container

Finally, execute the ls command in the /data/centos directory in the four environments, and you can see the files created by all containers. In this way, we can achieve data synchronization between containers.

[root@259efdc362b4 centos]# ls
centos_1.java centos_2.java centos_3.java main.java

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