Volumes and Dockerfiles

By February 17, 2018Docker Volumes

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Volumes and Dockerfiles

Docker provides a number of ways to work with volume for containers and services. You can specify the volume for a service using the --mount flag. For containers, you can use either the --volume flag or the recommended --mount flag which is supported for docker run as of Docker 17.06.

The --mount and --volume flags are the recommended way to work with volumes. But what if you need to work with a Dockerfile for an application on Docker Hub? The Dockerfile itself may include a VOLUME instruction.

VOLUME Instruction

The VOLUME instruction is used to specify one or more directories that bypass the Union File System. This instruction is how the author of the Dockerfile defines where stateful information should be written.

The benefits of using the VOLUME instruction are higher performance I/Os as changes are made directly to the host volume. Data written to the host volume persist even if the container is deleted. Also the volume can be shared and reused with other containers.

The VOLUME instruction tells Docker to create a mount point with the specified path inside the container. This mount point is marked as holding an external volume from the host. The value for the VOLUME instruction can be a JSON array, VOLUME ["/data/db/"] or a string with one or more arguments, e.g. VOLUME /var/log or VOLUME /var/log /data.

MongoDB Example

We will use MongoDB on Docker Hub as an example for this guide. Most application vendors post their Dockerfile on the main page for the image. Generally multiple versions with links to the Dockerfile are available. In our example with MongoDB, we’ll use their latest version – Dockerfile version 3.7.

Looking at the VOLUME instruction in the Dockerfile for MongoDB, we can see that two container mount points and therefore two host volumes will be created for this image.

VOLUME /data/db /data/configdb

We’ll use the CIO volume plugin, --volume-driver=cio and the --mount flag of the docker service create command to create the volumes for both paths.

The volume properties for the first path (/data/db) will be specified using --volume-opt option and the MONGODB profile. Source specifies the name of the volume, mongodb.

--mount source=mongodb,target=/data/db,volume-driver=cio,volume-opt=profile=MONGODB

The volume for the second path (/data/configdb) is created without setting volume options, i.e. it will be provisioned using default parameters of 20 GB, thin provisioned volume. The name of the volume is mongoconfig.

--mount source=mongoconfig,target=/data/configdb,volume-driver=cio

We could have decided not to define the second volume for /data/configdb. Docker would then create a local volume without a name. The result will be a long unique id string for the volume that contains no reference to the container it’s attached to. Unless Docker is told to to remove volumes when containers are removed, these ‘anonymous’ volumes will remain, unlikely to be used again.

Defining the second volume keeps the named volume visible in docker volume ls.

Finally to start the service, we run

$ docker service create \
--mount source=mongodb,target=/data/db,volume-driver=cio,volume-opt=profile=MONGODB \
--mount source=mongoconfig,target=/data/configdb,volume-driver=cio \
--replicas 1 \
--name mongo \
mongo

Inspect

We use docker service inspect mongo to verify that the volumes above were created and mounted correctly. Looking in the Mounts section we see both volumes.

"Mounts": [
    {
        "Type": "volume",
        "Source": "mongodb",
        "Target": "/data/db",
        "VolumeOptions": {
            "DriverConfig": {
                "Name": "cio",
                "Options": {
                    "profile": "MONGODB"
                }
            }
        }
    },
    {
        "Type": "volume",
        "Source": "mongoconfig",
        "Target": "/data/configdb",
        "VolumeOptions": {
            "DriverConfig": {
                "Name": "cio"
            }
        }
    }
],

Explore Further

Volumes
Volumes for Containers
Volumes for Services
Volumes for Docker Compose