moby--moby/docs/extend/plugins_volume.md

Write a volume plugin

Docker volume plugins enable Docker deployments to be integrated with external storage systems, such as Amazon EBS, and enable data volumes to persist beyond the lifetime of a single Docker host. See the plugin documentation for more information.

Command-line changes

A volume plugin makes use of the -vand --volume-driver flag on the docker run command. The -v flag accepts a volume name and the --volume-driver flag a driver type, for example:

$ docker run -ti -v volumename:/data --volume-driver=flocker   busybox sh

This command passes the volumename through to the volume plugin as a user-given name for the volume. The volumename must not begin with a /.

By having the user specify a volumename, a plugin can associate the volume with an external volume beyond the lifetime of a single container or container host. This can be used, for example, to move a stateful container from one server to another.

By specifying a volumedriver in conjunction with a volumename, users can use plugins such as Flocker to manage volumes external to a single host, such as those on EBS.

Create a VolumeDriver

The container creation endpoint (/containers/create) accepts a VolumeDriver field of type string allowing to specify the name of the driver. It's default value of "local" (the default driver for local volumes).

Volume plugin protocol

If a plugin registers itself as a VolumeDriver when activated, then it is expected to provide writeable paths on the host filesystem for the Docker daemon to provide to containers to consume.

The Docker daemon handles bind-mounting the provided paths into user containers.

/VolumeDriver.Create

Request:

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

Instruct the plugin that the user wants to create a volume, given a user specified volume name. The plugin does not need to actually manifest the volume on the filesystem yet (until Mount is called). Opts is a map of driver specific options passed through from the user request.

Response:

{
    "Err": null
}

Respond with a string error if an error occurred.

/VolumeDriver.Remove

Request:

{
    "Name": "volume_name"
}

Delete the specified volume from disk. This request is issued when a user invokes docker rm -v to remove volumes associated with a container.

Response:

{
    "Err": null
}

Respond with a string error if an error occurred.

/VolumeDriver.Mount

Request:

{
    "Name": "volume_name"
}

Docker requires the plugin to provide a volume, given a user specified volume name. This is called once per container start. If the same volume_name is requested more than once, the plugin may need to keep track of each new mount request and provision at the first mount request and deprovision at the last corresponding unmount request.

Response:

{
    "Mountpoint": "/path/to/directory/on/host",
    "Err": null
}

Respond with the path on the host filesystem where the volume has been made available, and/or a string error if an error occurred.

/VolumeDriver.Path

Request:

{
    "Name": "volume_name"
}

Docker needs reminding of the path to the volume on the host.

Response:

{
    "Mountpoint": "/path/to/directory/on/host",
    "Err": null
}

Respond with the path on the host filesystem where the volume has been made available, and/or a string error if an error occurred.

/VolumeDriver.Unmount

Request:

{
    "Name": "volume_name"
}

Indication that Docker no longer is using the named volume. This is called once per container stop. Plugin may deduce that it is safe to deprovision it at this point.

Response:

{
    "Err": null
}

Respond with a string error if an error occurred.