Volumes

Docker volumes allow you to persist data between container executions. They are docker’s preferred way of working with data.

Importantly! docker volumes are entirely managed by docker. This means that they are not directly accessible or reachable on your host machines file system and are instead sequestered in a linux VM. This can be useful, as it means the data persisted by docker is entirely separate from the file system that the container is running on.

Multiple containers can access a single volume at once while they are running.

Volume Management

You can create a volume using the volume create command.

docker volume create <VOLUME_NAME>

You can list existing volumes with the ls command.

docker volume ls

You can view data about a volume using the inspect command.

docker volume inspect <VOLUME_NAME>

Container Volumes

You can link a container to a volume when it starts with the run command by using the -v / --volume flags or the --mount flag.

If using the -v or –volume flag you **submit a list of three :` colon separated fields**. These are

1. The name of the volume If you want to specify a hosted path on your OS then you want bind mounts which can also be specified by submitting the path in place here
2. The path where the volume is mounted on the container
3. Optional comma-separated configuration settings

docker run --volume <VOLUME_NAME>:<PATH_ON_CONTAINER> <IMAGE_ID>

If using the --mount flag you can submit options as key value pairs after the flag. There are

• type: can be bind, volume or tmpfs This is also the default type across other invocations of the --mount command
• source or src: the name of the volume
• destination, dst or target: the path on the container to be mounted to
• readonly: allows you to control whether the container has write permissions to the volume
• volume-opt: allows you to define other volume specific options as further key pairs

docker run –mount type=volume,src=,destination=

You can access data in a volume by spinning up a docker container with that volume mounted in the file system of the container. In the example below, if I had already added data to the my-vol volume, then when the container starts it will have access to any data in that volume in the /data folder at the root of the container. The mounted folder is always the same as the root of the volume.

docker run -it --mount src=my-vol,destination=/data ubuntu:16.04

If you specify a volume that has not been create it will be created automatically when starting the container.

You can get files out from a volume onto your local file system by using the docker cp command in conjunction with a container that has access to that volume.

1. Spin up a container with the desired volume mounted to its file system
2. Copy the files out of the container onto your local file system using the cp command.

The example below starts a container with my-vol mounted (which contains some previously created data in the form of .txt file) and then the cp command is used to copy that file out.

docker run --mount src=my-vol,destination=/data --name data-container ubuntu:16.04
docker cp data-container:/data/my_file.txt .

You can run a volume as read only simply by including the readonly flag with no value, or ro when using the --volume command.

# using mount
docker run --mount src=my-vol,destination=/data,readonly --name data-container ubuntu:16.04
# using volume
docker run --volume my-vol:/data:ro --name data-container ubuntu:16.04

You can see how a mount is working with a container from the Mounts section of the output from running docker inspect.

You can populate data from a container to a volume by mounting a new volume to a folder on a container that contains data. This only works if it is a brand new volume, otherwise the data will not populate and the existing volume will overwrite the container’s internal volume. For example, if I have a container with an internal directory with some data in and I run…

docker run --mount src=new-volume,destination=/internal ubuntu:16.04

Then the data that was in the internal folder on the container will be avialable on the volume.