Container volume masks host volume in the compose file

[mekanixdk]

I have a little snippet in docker-compose.yml like:

volumes:
  - ${HOST_PATH}:/container/path

If I change the value of HOST_PATH any subsequent up or build, the container will still mount the volume with the old value of HOST_PATH.

Is this because volumes is not a run time option like docker -v or is this a bug?

[dnephin]

I can't reproduce this issue with master .What version of docker-compose are you running?

Here's what I ran:

web:                                                                                                                                                                                                                                                                              
  image: 'alpine:edge'                                                                                                                                                                                                                                                            
  volumes:                                                                                                                                                                                                                                                                        
    - '${HOST_PATH}:/contents'                                                                                                                                                                                                                                                    
  command: 'ls /contents'   

HOST_PATH=`pwd`/examples docker-compose up
# Prints the list of files in examples

HOST_PATH=. docker-compose up
# Prints a different list of files in the current directory

[mekanixdk]

I was using 1.5.0 RC1 and RC2. I can see that 1.5.0 have just been released. I will try that with your example above.

My container looks like this:

mysql:
  container_name:
    dungbeetle_mysql
  image:
    mysql:5.6
  environment:
    MYSQL_ROOT_PASSWORD: password
  expose:
    - "3306"
  ports:
    - "3306:3306"
  volumes:
    - ${DUNGBEETLE_MYSQL_DATA}:/var/lib/mysql
  hostname: database

[mekanixdk]

@dnephin you example worked like you said.

I am a noob regarding docker, but here is a theory of what I am seeing.

In the mysql Dockerfile VOLUME is defined (VOLUME /var/lib/mysql), and as I read the docs the container creates that "volume" on the host /somewhere/where/docker/containers/are/stored/too

In docker-compose.yml I define:

volumes:
  - ${HOST_PATH}:/var/lib/mysql

Ie. I try to define where that "volume" should be located on the host.

It appears like this overwrite the VOLUME in Dockerfile, and consequently at build time I can define my own path.

At runtime the /var/lib/mysql is already defined/mounted so the ${HOST_PATH}:/var/lib/mysql is ignored (I don't see any errors or warnings that this should be the case though.).

[mekanixdk]

So the solution for me seems to change the default location of the DB from /var/lib/mysql to something I control.

Question though... is this expected behaviour? It isn't clear from reading the docs (well, I expected something else :-))

[dnephin]

It is by design. I recently added this paragraph to the docs, to try and call attention to it: http://docs.docker.com/compose/#preserve-volume-data-when-containers-are-created

But I think it could use more detail

[dnephin]

Maybe we can add a warning when there's a host volume that is going to be ignored for the container volume. I think that would be better than adding documentation that users will have to find.