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Why do you need storage volumes

The Docker image is made up of multiple read-only layers. When the container is started, Docker loads the read-only mirror layer and adds a read-write layer at the top of the image stack.

If the running container modifies an existing file, the file will be copied from the read-only layer below the read-write layer to the read-write tier. The read-only version of the file still exists, but has been hidden by the copy of the file in the read-write tier. This is the COW mechanism.

Problems with data storage

The following problems exist in the storage of Docker container data:

Stored in the federated file system, it is not easy for hosts to access data sharing between containers, it is not convenient to delete containers, and their data will be lost.

Close and restart the container so that its data is not affected. However, deleting the container will cause all previous changes to be lost.

Solution: volume (volume), or storage volume for ease of distinction.

A volume is one or more directories on a container that can be bound (associated) to a directory on the host, bypassing the federated file system.

Data volumes

The Volume is created when the container is initialized, and the data in the volume provided by base image is replicated during that time.

The original intention of Volumn is to persist data independent of the lifecycle of the container, so the volume will not be deleted when the container is deleted, nor will it garbage collect volumes that are not referenced.

The volume provides a container-independent data management mechanism for docker:

Think of the image as a static file, such as a program. Compare volume classes to dynamic content, such as data. Therefore, the mirror can be reused, and the volume can share the volume to achieve the separation of the program (mirror) and the data (volume), as well as the separation of the program (mirror) and the host that makes the mirror. When creating an image, users no longer need to consider the environment of the host where the container in which the image is running is located. Using volumes in a container

Now, when you create the container, add the relevant options for volumes.

Volumes types

Docker has two types of volumes, each of which has a mount point in the container, but has a different location on the host:

Bind-mount volume: both the directory in the container and the directory on the host need to be specified Docker-managed volume: the directory in the container needs to be specified, and the directory on the host is maintained by docker

Use the-v option for the docker run command to use Volume.

Docker-managed volume

Open a session to create a container and bind volume:

$docker run-- name v1-- rm-it-v / data busybox

Operate the directory on the host machine

Query the inspect of this container in another session, but there is a lot of content. You can see the volume information in Volumes:

"Volumes": {"/ data": {}}

There is also the mount point information. The-f parameter is used here to only look at the contents of the Mounts:

$docker inspect-f'{{json .Mounts}} v1 [{"Type": "volume", "Name": "bdd48fb729e802b7d3a067da74b748037e83ff770a84f7215c657f6cc2af2c9d", "Source": "/ var/lib/docker/volumes/bdd48fb729e802b7d3a067da74b748037e83ff770a84f7215c657f6cc2af2c9d/_data", "Destination": "/ data", "Driver": "local", "Mode": "", "RW": true, "Propagation": ""}] $

The path of the volume automatically assigned by the host is in the Source field. The following method directly acquires the path and calls the ls command:

$docker inspect-f'{{range .Mounts}} {{.Source}} {{end}} v1 | xargs ls$ docker inspect-f'{{with index .Mounts 0}} {{.Source}} {{end}}'v1 | xargs ls

Mounts is an array that may have multiple mount points. The command on the first line is to traverse all directories, and the command on the second line is to output only the first directory.

In the directory of the mount point, the container and the host can share data. Changes to the directory in the container can be seen in the host. On the contrary, the container can also see the changes to this directory in the host.

Delete Mirror

The above command to create a container uses the-- rm parameter, so that once the container stops, the container is automatically deleted. After the container is deleted, the Docker-managed volume will also be deleted.

So this is a temporary volume, and the data stored in the volume will still be lost as the container is deleted. There is no solution to the purpose of persistent storage of data in the container, but now the data of the container can be shared with the host, and the Ibig O of the data storage is directly determined by the file system of the host. The advantage is that the user does not have to manage the volume in addition, and the temporary data stored in the volume will be deleted as the container is deleted. Suitable for storing temporary data that needs to be shared outside the container.

Bind-mount Volme

The parameters are the same as before, but the contents of the-v parameter provide two parts, the directory of the host and the directory of the volume, separated by a colon:-v HOSTDIR:VOLUMEDIR.

Bind Bind-mount Volme

Open a session to create a container and bind Bind-mount Volme:

$docker run-- name v2-- rm-it-v / data/volumes/v2:/data busybox

You can still use the docker inspect command to view the details, so I won't demonstrate it here.

If the path to the host specified here does not exist, the docker will be created automatically.

Delete Container

After the container is deleted, the files on the host will still exist. In this way, the data can still be used after the container is deleted.

Shared Volum

As mentioned earlier, docker has two types of volumes, both of which have been mentioned. The shared volume here is essentially one of the above two types, but it just implements two ways to share the same volume between containers.

Multiple container volumes use the same host directory

This is not a problem, the same directory on the host is allowed to be used by multiple containers at the same time.

Copy volumes that use other containers

Use the-- volumes-from option:

$docker run-- name v3.0-- rm-itd-v / data busyboxe24397fffb44717a7f140010c829ec88f540ed7d95f7c8fb328a35a819becfb0 $docker run-- name v3.1-- rm-itd-- volumes-from v3.0 busybox28543eb6fc13e972a000ab955b9623630ebb9d90c8d3dc37a8b7608f24188926 $docker container attach v3.0 / # touch / data/test_v3_0/ # exit$ docker container attach v3.1 / # ls / datatest_v3_0/ # exit$

The example here replicates Docker-managed volume, and the same is true for Bind-mount Volme.

Shared network

Networks can also be shared, that is, federated networks. Here, volumes can also be shared. In some scenarios, two or more containers may need to share the same network and the same volume.

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