Proposal: Add device_config to Device

[mariash]

What is the problem?

The Device Object in the Volume Mount Object currently only supports existing shared devices. It allows to pass mount_config, which is used by the volume driver to mount volume.

There is no mechanism for a service broker to provide configuration that driver can use to create a device on demand.

Who does this affect?

Volume service broker authors who need to create devices on-demand rather than using existing volumes.

Do you have any proposed solutions?

Add a new optional field device_config of type object to the Device Object. This field contains configuration used to define the device, separate from mount_config which is used to mount it.

Spec changes

Device Object:

Response Field	Type	Description
volume_id*	string	ID of the shared volume to mount on every app instance.
mount_config	object	Configuration object to be passed to the driver when the volume is mounted.
device_config	object	**Configuration object to be passed to the driver when the device is created. **

Updated JSON example:

{
  "volume_mounts": [{
    "driver": "cephdriver",
    "container_dir": "/data/images",
    "mode": "r",
    "device_type": "shared",
    "device": {
      "volume_id": "bc2c1eab-05b9-482d-b0cf-750ee07de311",
      "device_config": {
        "size_mb": 10240,
        "fs_type": "ext4"
      },
      "mount_config": {
        "key": "value"
      }
    }
  }]
}

Semantics

• device_config is an object, its schema is driver-specific, same as mount_config.
• The platform passes device_config to the volume driver at device creation, and passes mount_config to the driver at mount.
• The change is backwards compatible. The field is optional.

Additional context

Volume services that create and mount devices on demand need a way to pass configuration separately from mount configuration.