What Is Docker Volume?

Docker containers enable apps to execute in an isolated environment. All modifications made inside the container are lost by default when it ends. Docker volumes and bind mounts can be useful for storing data in between runs. One way to store data outside of containers is with volumes. All volumes are kept in a specific directory on your host, typically /var/lib/docker/volumes for Linux systems, and are controlled by Docker.

What are Docker Volumes?

Docker Volumes are a popular and effective method for assuring data permanence while working in containers. Docker volumes are file systems that are mounted on Docker containers to preserve the data generated by the container.

What is the Docker File System?

A Docker container executes the software stack specified in a Docker image. Images are built up of read-only layers that operate on the Union File System. When we start a new container, Docker adds a read-write layer on top of the image layers, allowing the container to function like a conventional Linux file system. So, each file modification within the container generates a functioning copy in the read-write layer. However, when the container is stopped or removed, the read-write layer disappears.

Types Of Mounts in Docker

The data appears the same from within the container in all mount modes. In the filesystem of the container, it is shown as a directory or a single file.

• Volumes: Docker manages volumes kept in a section of the host filesystem (/var/lib/docker/volumes on Linux). This portion of the filesystem shouldn't be altered by non-Docker processes. In Docker, volumes are the most effective way to store data. Using the docker volume create command, we may directly create a volume, or Docker can do it for us when it creates a container or service.
• Named Pipes: To facilitate communication between a container and the Docker host, a named pipe mount can be employed. Using a named pipe to connect to the Docker Engine API while running a third-party program inside a container is the typical use case.
• Bind Mounts: On the host system, bind mounts can be kept anywhere. These might be crucial system folders or files. They are always modifiable by non-Docker processes running on a Docker host or in a Docker container. Comparatively speaking, bind mounts are less useful than volumes.
• Tmpfs Mounts: These mounts are never written to the host system's filesystem; instead, they are kept solely in the memory of the host system. Neither on the Docker host nor in a container is it stored on a disc. Sensitive or non-persistent state data can be stored on the tmpfs mount for the duration of the container.

Docker Volume Plugins

Docker Engine volume plugins link Engine installations with external storage systems such as Amazon EBS, allowing data volumes to survive beyond the lifespan of a single Docker host. For further details, please refer to the plugin documentation.

Command-Line Changes

Use the --volume and --volume-driver options on the docker container run command to grant a container access to a volume. The host's volume name and path are accepted by the --volume (or -v) flag, whereas the driver type is accepted by the --volume-driver flag.

 $ docker volume create --driver=flocker volumename

 $ docker container run -it --volume volumename:/data busybox sh

Volume Plugin Protocol

If a plugin registers itself as a VolumeDriver when activated, it must provide the Docker Daemon with writeable paths on the host filesystem. The Docker daemon provides these paths to containers to consume. The Docker daemon makes the volumes available by bind-mounting the provided paths into the containers.

{
    "Name": "volume_name",
    "Opts": {}
}

Using Docker Volumes

Manually Creating and Linking Volumes with Proper Naming And Labeling Conventions

• Make sure you use appropriate name and labelling practices when establishing Docker volumes.

docker volume create \
--label description="my_vol" \
--label version="1.0.1" \
my_vol

Using Volumes in Dockerfiles with Controlling Permissions For Volumes

• In order to preserve data security and integrity, make sure the appropriate permissions are specified for Docker volumes.

FROM baseimage
RUN mkdir /app/data
RUN chown -R 1000:1000 /app/data
RUN chmod 647 /app/data
VOLUME /app/data

Mounting Volumes as Read-Only

Mounting volumes as read-only in Docker allows for the protection of sensitive or critical data from unintended modifications. By setting the volume option to read-only, you ensure that any changes made within the container are not persisted to the underlying volume, preserving data integrity and security.

docker run -d \
  -v /path/on/host:/path/in/container:ro \
  --name my_container \
  my_image

• -v /path/on/host:/path/in/container:ro mounts the directory /path/on/host on the host machine to /path/in/container in the container as read-only (ro).
• --name my_container assigns the name my_container to the Docker container.
• my_image is the name of the Docker image used to create the container.

Tracking And Controlling Volume Consumption

• To maximise resource consumption, track and adjust Docker volume usage on a regular basis.

$ docker system df -v

Populating Volume Content

When mounting volumes to container paths with existing data, Docker ensures data integrity by copying the existing container data into the new volume. Consequently, neighboring mount points and other containers using the volume will also access the populated content, preventing inadvertent data loss.

Reusing Volumes When Containers Start

Instead of manually specifying each volume with the -v flag, you can use --volumes-from to inherit volumes from an existing container when starting a new container:

# Create the first container
$ docker run -d --name test -v my_vol:/data image:latest

# Create the second container
$ docker run -d --name backup --volumes-from test image:latest

This command automatically mounts all volumes from the "test" container into the "backup" container, simplifying the setup process. It's handy for tasks like backing up data from one container to another.

Interacting With Docker Volumes

Each volume's name and the storage driver that supports it will be shown. Use docker volume inspect to obtain more in-depth details about a particular volume instead:

Inspecting Volumes

To inspect volumes in Docker, you can use the docker volume inspect command followed by the name or ID of the volume you want to inspect. For example:

docker volume inspect my_vol

Removing Volumes

To remove volumes in Docker, you can use the docker volume rm command followed by the name or ID of the volume you want to remove. For example:

docker volume rm my_vol

Pruning Volumes

To prune volumes in Docker, you can use the docker volume prune command. This command removes all volumes not used by at least one container. Here's how you can use it:

docker volume prune

Starting a Container with a Volume

On Using -v Option

• we may start a container with a bind mount using the -v option:

$ docker run -v $(pwd):/var/opt/project bash:latest \
  bash -c "ls /var/opt/project"

• This shows nothing from the mount position. However, if we write to the volume within a single execution of the container:

$ docker run -v data-volume:/var/opt/project bash:latest \
  bash -c "echo Baeldung > /var/opt/project/Baeldung.txt"

Using The –mount Option

• To indicate the volume we want to mount, we might find it easier to use the more obvious –mount option:

$ docker run --mount \
  'type=volume,src=data-volume,\
  dst=/var/opt/project,volume-driver=local,\
  readonly' \ 
  bash -c "ls /var/opt/project"

On Using Shared Volumes

Assume that we used the data-volume mount in a container to run our echo script. Afterwards, we could make a list of every container we've used:

$ docker ps -a
CONTAINER ID   IMAGE             COMMAND                  CREATED              STATUS                          PORTS      NAMES
5774502f857   bash              "docker-entrypoint.s…"   8 minutes ago        Exited (0) 8 minutes ago                   exciting_payne

How to use Docker Volumes?

The following command launches a fresh Ubuntu 22.04 container and connects your terminal to it (-it), enabling you to execute example commands in the ensuing stages. Within the container, a volume named demo_volume is mounted to /data. Use the following command right now:

$ docker run -it -v demo_volume:/data ubuntu:22.06

• Give a list of everything in the /data directory of your container:

$ ls /data

• Include a test file with any random content:

$ echo "foobar" > /data/foo
$ cat /data/foo
foobar

• Launch a fresh container with the same volume attached now:

$ docker run -it -v demo_volume:/app alpine:latest

• Add the --mount option to the docker run command in order to mount a data volume to a container.
• It stores the data created inside the virtual environment by adding volume to the designated container.
• To launch a container and mount a data drive to it, use the following syntax:

$ docker run --mount source=[volume_name],destination=[path_in_container] [docker_image]  

Using Volumes With Docker Compose

In Docker Compose, volumes may also be defined and utilised. Create a top-level volumes field in your docker-compose.yml file, identify the volumes you want to create, then mount your volumes into your containers in the services section:

services:
  app:
    image: app-image:latest
    volumes:
      - app_data:/data
volumes:
  app_data:

• To use an already-existing volume, include it in the docker-compose.yml file's volumes section and set the external flag to true:

volumes:
  demo_volume:
    external: true

So this is the volume of the docker. We saw that Docker typically starts a container with a blank filesystem, but that data may be stored for a longer period of time than the container's lifetime thanks to bind mounts and volumes.We learned how to use the command line to attach volumes to an active container as well as how to list and manage Docker volumes.

Troubleshooting Common Docker Volume Issues

Permission Denied When Mounting Volumes

• If you are mounting a local disc or host directory with the -v option while operating a Docker container, as follows:

docker run -it --rm \
    -p 8888:8888 \
    -v <my-vol>:<container-dir> \
    quay.io/jupyter/minimal-notebook:latest

Incompatible CPU detected

A processor (CPU) that supports virtualization—more especially, the Apple Hypervisor framework—is necessary for Docker Desktop to function. Only Mac computers with CPUs that support the Hypervisor framework may use Docker Desktop.

$ sysctl kern.hv_support

Path Conversion On Windows

When using Linux, mounting a route to another path is handled by the system. For instance, when executing the subsequent command on a Linux system:

 $ docker run --rm -ti -v /home/user/work:/work alpine

Permissions Errors On Data Directories For Shared Volumes

Docker Desktop defaults the read, write, and execute permissions for both users and groups on shared volumes to 0777 when sharing files from Windows.On shared discs, the default permissions are not customisable. You must either utilise non-host-mounted volumes or figure out a means to get the programmes to operate with the default file permissions if you are working with applications that need permissions different from the shared volume defaults during container runtime.

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