Two ways to manage volumes in Docker

In the previous article, I introduced the basic knowledge of Docker: how to mount a local directory. Today, I will introduce two ways to manage data volumes in Docker. The specific contents are as follows:

What is a data volume

Data volume: A volume is a specific file or folder that exists in one or more containers. This directory exists in the host machine in a form independent of the union file system and facilitates data sharing and persistence.

Why use data volumes?

Problems with Docker layered file system:
Docker images are composed of a series of read-only layers. When starting a container, Docker loads all read-only layers of the image and adds a read-write layer on the top. This design enables Docker to improve the efficiency of image building, storage, and distribution, saving time and storage space. However, it also has the following problems:
1. Poor performance.
2. Data between multiple containers cannot be shared.
3. The life cycle is the same as that of the container. When you delete a container, the data generated by the container will be lost. Benefits of the data volume mechanism:
1. Mount to the host, bypassing the layered file system.
2. Volumes can be shared and reused between different containers.
3. Same performance as the host disk.
4. Operations on the data in the volume will not affect the image itself.
5. The life cycle of a volume is independent of the life cycle of a container. Even if the container is deleted, the volume will still exist. A volume that is not used by any container will not be deleted by Docker.
Docker provides a volumedriver interface. By implementing this interface, we can provide different volume storage supports for Docker containers. Currently, the local volumedriver is implemented by default, which uses the host's file system to provide volume for Docker containers.

Two ways to manage data volumes

The essence of Docker volume is a special directory in the container. During the container creation process, Docker will mount the specified directory on the host (a directory named with the volume ID, or a specified host directory) to the specified directory in the container (using the bing mount method), so the host directory after the mount is completed and the target directory in the container is consistent.

1. Bind mount

Bind mount is to mount the directory or file on the host into the container. Intuitive and efficient to use, easy to understand.
Use the -v option to specify the mount path in the format <host path>:<container path>
#The first part is the path that actually exists on the host machine: the second part is the path in the container

Run a container using the nginx image in the background and mount the host's /data directory to the container's directory /usr/share/nginx/html [root@server1 ~]# docker run -d --name demo -v /data:/usr/share/nginx/html nginx 

Switch to the foreground to run, and check the contents of the specified directories of the host and container respectively. They are the same. This is because this method of mounting is the same as the mount method we usually use. The original data is hidden and replaced with the data of the host machine.
##There are so many things in the /data directory because the default data directory of Docker is /data

[root@server1 ~]# docker exec -it demo bash 

The default permissions for bind mount are read-write (rw), and you can specify read-only (ro) when mounting.

The path specified by the -v option will be automatically created when mounting if it does not exist.
docker run -it --name vm1 \ /etc/yum.repos.d/dvd.repo:/etc/yum.repos.d/dvd.repo:ro rhel7 bash

2. Docker managed volume

Bind mount must specify the host file system path, which limits portability.
Docker managed volume does not need to specify the mount source, Docker automatically creates a data volume directory for the container. The default data volume directories created are all in /var/lib/docker/volumes.
If the mount points to an existing directory in the container, the original data in the container will be copied to the volume.
How to create a volume:

[root@server1 ~]# docker volume create webdata #Create a volume named webdata[root@server1 ~]# docker rm -f demo #Delete the volume created above[root@server1 ~]# docker run -d --name demo -v webdata:/usr/share/nginx/html nginx		
#Mount the webdata volume to the /usr/share/nginx/html directory in the container and run a container

Mount the created webdata volume to the /usr... directory of the container

Enter the data volume directory created by Docker by default to view the contents of the volume. We can see that only the volume is created above, and nothing is written. However, the content in the volume is because there was something in the specified directory in the container when it was mounted, so it was copied over.

We use the nginx image to run the container. There is no problem accessing it.

If the mount source is not specified when mounting, Docker will automatically create a random volume with a long file name.

[root@server1 ~]# docker rm -f demo
[root@server1 ~]# docker run -d --name demo -v /usr/share/nginx/html nginx
67ab13a7b24c19c53f4ce117136b9d0e4dec93c615a0192ead919d10e6c2acae 

We use the docker inspect demo command to view the source directory of the volume

After getting the path, you can view the contents of the directory

ls /var/lib/docker/volumes/2ca22fd769e4b7b6f5a02dd96fe8d47a6df5578074c0d340ced3ab33b25456ca/_data 

Comparison between bind mount and Docker managed volumes

Similarities: Both are paths in the host file system.
The differences are shown in the figure:

This concludes this article on two ways to manage volumes in Docker. For more information about Docker volumes, please search for previous articles on 123WORDPRESS.COM or continue browsing the following related articles. I hope you will support 123WORDPRESS.COM in the future!