Docker for Windows: Accessing named volume mounts

Question

Please note that this is not a duplicate of Locating data volumes in Docker Desktop (Windows) as back in 2017 the inner workings of docker on windows were quite different - e.g. docker volume inspect output is quite different nowadays.

I have trouble accessing data mounted to containers in docker for windows via named volume mounts.

docker inspect [vol-id]

[
{
    "CreatedAt": "2019-04-02T11:58:14Z",
    "Driver": "local",
    "Labels": {
        "com.docker.compose.project": "foo",
        "com.docker.compose.version": "1.24.0",
        "com.docker.compose.volume": "mongodata-foo"
    },
    "Mountpoint": "/var/lib/docker/volumes/foo_mongodata-foo/_data",
    "Name": "foo_mongodata-payoff",
    "Options": null,
    "Scope": "local"
}
]

--> Mountpoint is inside the HyperV VM used in Docker. How to get access to that data? Is there an easily manageable way to to this?

Note: I don't have C:\ProgramData\Docker\Volumes as described here. Instead, what was created with Docker Desktop 2.0.0.3, Engine 18.03.3, was C:\ProgramData\DockerDesktop. That does not contain any Volumes as far as I can tell.

Background: I need named mounts with default location inside HyperV, as mounting it manually via docker run -v or specifying the driver device location looks to be unsupported by mongodb (I have exactly the same behavior as described there. It looks like mongodb is incompatible with NTFS-originating volume mounts.

Answer 1

method using built-in docker cp

Use docker cp [containername]:[path] [host-path] to e.g. copy data out - reverse the params to copy data in - it works just like scp. To get shell access to the data you can just attach to the running container.

pro: nothing additional needed in docker compose

con: no integration (that I know of) with a file explorer GUI like WinSCP. need to do a terminal based copy each time a file is to be updated between host and container.

method using a dockerized ssh server

pro: can integrate with any tool that can talk over ssh/sftp

con: needs additional setup

The following approach starts an ssh server within a service, setup with docker-compse such that it automatically starts up and uses public key encryption between host and container for authorization. This way, data can be uploaded/downloaded via scp or sftp.

The full docker-compose.yml for a node.js (keystone) + mongodb app is below, together with some documentation on how to use ssh service:

version: '3'
services:
  foo:
    build: .
    image: localhost.localdomain/${repository_name}:${tag}
    container_name: ${container_name}
    ports:
      - "3333:3333"
    links:
      - mongodb-foo
    depends_on:
      - mongodb-foo
      - sshd
    volumes:
      - "${host_log_directory}:/var/log/app"

  mongodb-foo:
    container_name: mongodb-${repository_name}
    image: "mongo:3.4-jessie"
    volumes:
      - mongodata-foo:/data/db
    expose:
      - '27017'

  #since mongo data on Windows only works within HyperV virtual disk (as of 2019-4-3), the following allows upload/download of mongo data
  #setup: you need to copy your ~/.ssh/id_rsa.pub into $DOCKER_DATA_DIR/.ssh/id_rsa.pub, then run this service again
  #download (all mongo data): scp -r -P 2222 user@localhost:/data/mongodb [target-dir within /c/]
  #upload (all mongo data): scp -r -P 2222 [source-dir within /c/] user@localhost:/data/mongodb
  sshd:
    image: maltyxx/sshd
    volumes:
        - mongodata-foo:/data/mongodb
        - $DOCKER_DATA_DIR/.ssh/id_rsa.pub:/home/user/.ssh/keys/id_rsa.pub:ro
    ports:
        - "2222:22"
    command: user::1001

#please note: using a named volume like this for mongo is necessary on Windows rather than mounting an NTFS directory.
#mongodb (and probably most other databases) are not compatible with windows native data directories due ot permissions issues.
#this means that there is no direct access to this data, it needs to be dumped elsewhere if you want to reimport something.
#it will however be persisted as long as you don't delete the HyperV virtual drive that docker host is using.
#on Linux and Docker for Mac it is not an issue, named volumes are directly accessible from host.
volumes:
  mongodata-foo:

note: for a fully working example, before any docker-compose call the following script needs to be run:

#!/usr/bin/env bash
set -o errexit
set -o pipefail
set -o nounset

working_directory="$(pwd)"
host_repo_dir="${working_directory}"
repository_name="$(basename ${working_directory})"
branch_name="$(git rev-parse --abbrev-ref HEAD)"
container_name="${repository_name}-${branch_name}"
host_log_directory="${DOCKER_DATA_DIR}/log/${repository_name}"
tag="${branch_name}"

export host_repo_dir
export repository_name
export container_name
export tag
export host_log_directory