Why is docker-compose creating volume in random path instead of the path I specified in docker-compose.yml?

Question

I am trying to store nexus data in persistent volume. To do that I am using this compose yaml:

version: '3.5'

services:
  nexus:
    image: sonatype/nexus3
    volumes:
       - ./nexus-data:/nexus-data sonatype/nexus3
    ports:
      - "8081:8081"
    networks:
      - devops
    extra_hosts:
      - "my-proxy:my-proxy-address"
    restart: on-failure

networks:
  devops:
    name: devops
    driver: bridge

Before running docker-compose up I have created the nexus-data folder and gave required permissions to the uid/guid 200 as suggested here:

https://github.com/sonatype/docker-nexus3/blob/master/README.md#persistent-data.

root@master-node:~/docker# ll
total 16
drwxr-xr-x  3 root root 4096 Jan  8 13:37 ./
drwx------ 22 root root 4096 Jan  8 13:40 ../
-rw-r--r--  1 root root  319 Jan  8 13:36 docker-compose.yml
drwxr-xr-x  2  200  200 4096 Jan  8 13:37 nexus-data/

And here docker's volumes list before running compose file (it's empty):

root@master-node:~/docker# docker volume ls
DRIVER              VOLUME NAME

After docker-compose up command, docker created the volume as shown below:

root@master-node:~/docker# docker volume ls
DRIVER              VOLUME NAME
local               7b7b6517e5ed0e286a8fc7caef756141b5bbdb6e074ef93a657850da3dd78b2b
root@master-node:~/docker# docker volume inspect  7b7b6517e5ed0e286a8fc7caef756141b5bbdb6e074ef93a657850da3dd78b2b 
[
    {
        "CreatedAt": "2020-01-08T13:42:34+03:00",
        "Driver": "local",
        "Labels": null,
        "Mountpoint": "/var/lib/docker/volumes/7b7b6517e5ed0e286a8fc7caef756141b5bbdb6e074ef93a657850da3dd78b2b/_data",
        "Name": "7b7b6517e5ed0e286a8fc7caef756141b5bbdb6e074ef93a657850da3dd78b2b",
        "Options": null,
        "Scope": "local"
    }
]
root@master-node:~/docker# ls /var/lib/docker/volumes/7b7b6517e5ed0e286a8fc7caef756141b5bbdb6e074ef93a657850da3dd78b2b/_data
admin.password  cache  db  elasticsearch  etc  generated-bundles  instances  javaprefs  karaf.pid  keystores  lock  log  orient  port  restore-from-backup  tmp

but the folder I specified in compose file (nexus-data) is still empty:

root@master-node:~/docker# ls nexus-data/
root@master-node:~/docker# 

So, what am I doing wrong here? Why isnexus-data empty and docker creating volume in another path?

Answer 1

You defined a volume instead of a bind mount, which is what you want. Read the documentation about it.

Basically, your configuration makes docker create a volume, which maps to a randomly created directory somewhere under /var/lib/docker/volumes.

If you want a specific directory that you control, you have to create a bind. That's the reason you don't have data on your chosen directory, as docker ignores it because it is not useful for a volume.

For it to work, set it like:

    volumes:
      - type: bind
        source: ./nexus-data
        target: /nexus-data

as explained in the compose documentation. (Also removed the image name from that configuration)

Answer 2

You've created a host volume, aka bind mount (which doesn't show in docker volume ls since it's not a named volume)B from ./nexus-data on the host to /nexus-data sonatype/nexus3 inside the container. This looks like a copy and paste error from a docker run command since you are adding the image name to the path being mounted inside the container. You should be able to exec into the container and see your files with:

docker exec ... ls "/nexus-data sonatype/nexus3"

You should remove the image name from the volume path to mount the typical location inside the container:

version: '3.5'

services:
  nexus:
    image: sonatype/nexus3
    volumes:
       - ./nexus-data:/nexus-data
    ports:
      - "8081:8081"
    networks:
      - devops
    extra_hosts:
      - "my-proxy:my-proxy-address"
    restart: on-failure

networks:
  devops:
    name: devops
    driver: bridge

The volume you did see was an anonymous volume. This would be from the image itself defining a volume that you did not include in your container spec. Inspecting the container using that volume would show where it's mounted, most likely at /nexus-data.

Answer 3

The below docker-compose.yaml works as expected:

version: '3.5'

services:
  nexus:
    image: sonatype/nexus3
    volumes:
       - ./nexus-data:/nexus-data
    ports:
      - "8081:8081"
    networks:
      - devops
    extra_hosts:
      - "my-proxy:my-proxy-address"
    restart: on-failure

networks:
  devops:
    name: devops
    driver: bridge

The trailing sonatype/nexus3 in your original docker-compose.yaml spec:

    ...
    volumes:
       - ./nexus-data:/nexus-data sonatype/nexus3
    ...

... throws docker off - the randomly named volume is created given the VOLUME instruction in the sonatype/nexus3 Dockerfile and is mounted to the running container hence why the intended locally mounted directory is empty.