Can we mount sub-directories of a named volume in docker?

Question

The docker-compose file https://docs.docker.com/compose/compose-file/#/volumes-volume-driver shows various ways to mount host sub-directories relative to the compose file.

For example:

volumes:    # Just specify a path and let the Engine create a volume    - /var/lib/mysql      # Specify an absolute path mapping    - /opt/data:/var/lib/mysql      # Path on the host, relative to the Compose file    - ./cache:/tmp/cache      # User-relative path    - ~/configs:/etc/configs/:ro      # Named volume    - datavolume:/var/lib/mysql 

Is is possible to mount a sub-directory of a named volume at a specific location? For example something like below, which I tried, but does not seem to work.

# Named volume   - datavolume/sql_data:/var/lib/mysql 

I am assuming I might be able to manage this by mounting the data volume to a location like /data and then in the Dockerfiles for each container, create symbolic links from the sub-directories to the locations.

for example in a docker-compose.yml file

volumes:   - datavolume:/data 

and then in the container Dockerfile

RUN ln -s /data/sql_data /var/lib/mysql 

I started going down this path but it was getting messy and was not working. Before I either abandon that approach or invest the time debugging it, I wanted to make sure there was no way to just specify sub-directories of a named vollume.

Answer 1

No because compose/config/config.py#load(config_details) check if datavolume/sql_data matches a named volume (in compose/config/validation.py#match_named_volumes())

datavolume would, datavolume/sql_data would not.

As memetech points out in the comments, the is an issue tracking this since April 2017:
moby/moby issue 32582: "[feature] Allow mounting sub-directories of named volumes".

In that issue, Joohansson adds (see comment)

In the meantime, I use this workaround to mount the whole volume on a separate path and then symlink it to the sub path.

# In the Dockerfile: RUN mkdir -p /data/subdir RUN ln -s /data/subdir /var/www/subdir 

Then mount the volume as normal.
The /subdir must exist in the volume.

docker run -d -v myvol:/data mycontainer 

Now anything read or written by the webserver will be stored in the volume subdir and can't access the other data.