What is the difference between docker volume and mount

The difference between "docker volume" and mount is: "docker volume" is declarative, "Docker Engine" itself will occupy a certain directory in the system, and Docker will allocate the volume under the occupied path by default. A path; mounting will not occupy a path as a mount point.

The operating environment of this tutorial: linux7.3 system, docker-1.13.1 version, Dell G3 computer.

What is the difference between docker volume and mount

For a long time, when multiple containers need to share access to the same data directory, or when data in the container needs to be persisted (such as a database), we They all use the form of mounting directories (bind mounts) to mount a directory on the host to a specified directory in the container. This method can solve the problem, but this method always has some shortcomings:

• Containers deployed on different servers need to modify the path according to the actual disk mounting directory
• The file and directory permissions of different operating systems will make you confused and angry?

These problems can be solved using Volume. Let's first compare bind mounts and docker volume, and then see how volume solves the problem of bind mounts. Let’s look at a picture first:

This picture shows that bind mount and volume actually use the host’s file system , the difference is that volume is a subdirectory in the directory managed by docker itself, so there is no mounting problem caused by permissions, and the directory path is managed by docker itself, so there is no need to specify different paths on different servers. , you don’t need to care about the path (actually not all of them, we will care about it below?). Next, let’s take a look at the different uses of bind mount and volume.

1. When deploying containers on different servers, you need to modify the path according to the actual disk mounting directory

For example:

In Linux systems, we often Use "/var/someDir" as the mounting directory;

However, on Mac, /var/ is not a real directory. Mac users will tell you that we are more advanced than Linux and we do not use /var/ , users don’t need it;

In Windows systems, Windows users will ask you: What is /var/? C drive and D drive are the most reasonable divisions~~

2. The file and directory permissions of different operating systems will make you confused and angry?

Originally, the test in the Linux system was quite good, but as a result, various problems with the mounting path on Windows were encountered. I will not go into details here. If you have not encountered these problems, you can try it and experience it. But it's better in Mac, after all, it's the same as Linux.

In addition to the mounting method, there is also a Volume that can persist data in Docker. It’s a bit embarrassing to mention this. After using Docker for so long, I have always regarded mounting as a Volume. But don’t blame me. In the Docker-compose file There is no problem in writing the mapping relationship between the container and the host mount path in the volume section, but this has always been a misunderstanding. . (Fancy blame?)

In fact, "mounting" and "Docker Volume" are not the same thing. There is a certain difference. Docker Volume is declarative. Docker Engine itself will occupy a certain directory in the system, generally "/var/lib/docker" in Linux, and can be adjusted in both Mac and Windows. When we declare a volume, Docker will assign a path to the volume under the occupied path by default, for example:

relative to the mount, volume It is Docker Engine that allocates a path as a mount point in its own "site", and the permissions of its own site must be clearly arranged. Therefore, the above problems of mounting the host path are solved. ?

When using it, just use the volume name instead of the host path name. Assume that we created a volume named "test_vol" above:

1. docker run -d -v "test_vol:/var/data" some_image, this will mount the /var/data directory in the container to the mount point of "test_vol";
2. It’s similar in docker-compose, but the volume must be declared in the docker-compose.yaml file. Let’s modify the above example:

##Attention !!

It should be noted that the volume will cause the docker directory to expand, because both the image and the volume need to be stored. It is best not to put it on the system disk. Configure the docker installation directory to another Larger mount disk.
1. The two have a different behavior: when the corresponding directory outside the container is empty, volume will first copy the contents in the container to the directory outside the container, while mount will overwrite the external directory in the container's internal directory! !
2. volume Another disadvantage of bind mount is that it cannot directly mount files, such as mounting the configuration file of the nginx container: nginx.conf.

It should be noted here that a single file method similar to the configuration file is not suitable for using volume. Although bind mount can also solve the problem, because the config file contains some sensitive information such as database passwords, So the best way is to use tmpfs.

The volume of kubernetes also reflects a similar design. Although subPath can solve the problem of configuration file mounting, the actual best way is to use configMap.

Recommended learning: "

docker video tutorial"

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