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

2023: As noted by Michael Bolli in the comments, that feature is now a work-in-progress:

PR 45687: "volumes: Implement subpath mount"

Make it possible to mount subdirectory of a named volume.

Q1 2024: this is merged! Commit 31ccdbb.
Possibly for Moby 26.0.

March 2024, as mentioned in issue 32582:

CLI already supports it (starting from v26.0.0-rc1): docker/cli PR #4331

(and you can already install it from the test channel)

Compose support is still WIP (it needs to move to v26 first).

March 2024: Paweł Gronowski confirms in the same issue:

moby v26.0.0 is released, so you can already try out this feature.

---

2016: 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.