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By default, data reading and writing for containers occurs at the storage level of the container, and data on containers is lost when they are deleted. Therefore, we should try our best to ensure that no write operations occur in the container storage layer. In order to achieve persistent storage of data, we need to choose a scheme to save data. Currently, there are several ways:

VolumesBind mountstmpfs mounts

The following diagram illustrates these three technologies:

Volumes

Volumes is a special directory on the host that can be used by one or more containers and has the following characteristics:

Volumes can be shared and reused across containers Writes to volumes have no impact on mirroring Volumes persist by default, even if containers are deleted

The purpose of using data volumes is to persist the data in containers to share between containers or to prevent data loss (data written to the container storage layer will be lost).

The steps for working with volumes are generally divided into two steps:

Create a volume Mount the volume in the specified directory using the-v or--mount parameter so that all writes from the container to the specified directory are saved in Volume on the host.

Volume Management

Creating a Volume:

$ docker volume create my-vol

View Volumes:

$ docker volume lslocal my-vol$ docker volume inspect my-vol[ { "Driver": "local", "Labels": {}, "Mountpoint": "/var/lib/docker/volumes/my-vol/_data", "Name": "my-vol", "Options": {}, "Scope": "local" }]

We can see that the created Volume my-vol is saved in the directory/var/lib/docker/volumes/, and all future write data for this Volume will be saved in the directory/var/lib/docker/volumes/my-vol/_data.

Delete a Volume:

$ docker volume rm my-vol

Or delete all unused Volumes:

docker volume prune

Mount data volumes to container directories

After creating a Volume, we can use it by specifying the-v or--mount parameter when running the container:

Use the--mount parameter:

$ docker run -d \ --name=nginxtest \ --mount source=nginx-vol,destination=/usr/share/nginx/html \ nginx:latest

Source specifies volume, destination specifies a file or folder within the container.

Or use the-v parameter:

$ docker run -d \ --name=nginxtest \ -v nginx-vol:/usr/share/nginx/html \ nginx:latest

After successful mounting, the container reads or writes data from/usr/share/nginx/html, which is actually reading or writing data from the nginx-vol data volume of the host machine. So Volumes or Bind mounts can also be seen as a way for containers and hosts to share files.

The-v argument uses a colon to separate source and destination, the first half of which is source and the second half is destination.

If you mount a volume that doesn't exist yet, Docker will automatically create it. (Therefore, the step of creating the data volume is not required)

If the directory to be mounted in the container is not an empty directory, the files in that directory are copied to the data volume. (Under Bind mounts, directories on the host always overwrite directories to be mounted in containers)

The-v parameter and the--mount parameter generally have almost the same function, the only difference being that the--mount parameter can only be used to mount the data volume when running a service.

Use read-only volumes

In some cases, we want a volume to be read-only for a container, which can be achieved by adding the readonly option:

$ docker run -d \ --name=nginxtest \ --mount source=nginx-vol,destination=/usr/share/nginx/html,readonly \ nginx:latest

Or use the-v parameter:

$ docker run -d \ --name=nginxtest \ -v nginx-vol:/usr/share/nginx/html:ro \ nginx:latest

Volume usage scenarios

Please refer to this article: Docker Data Storage Summary

refer to the article

https://docs.docker.com/storage/volumes/#share-data-among-machines

summary

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