CODEWITHSUNDEEP
dockerdocker-compose
Egee
Egee

Reputation: 77

How can I share data between host and container using mounts

I've been attempting to share data between my host and my container. I've been reading a lot about volumes and I believe I have misunderstood some of the fundamentals around sharing data.

Here's how I've been doing it (with Docker Compose)

version: "2"
services:
  my-server:
    volumes:
      - type: bind
        source: ./test/
        target: /var/logs

The problem with this approach is that the initial creation of the mount destroys any data in the target folder. So for example if my image was built from another image that had some logs in that folder (for whatever reason), the logs would be destroyed.

This is a major problem with my use case. I need to mount a volume (a folder, basically) so that I can share data between my host and guest, similar to how a shared folder with a VM would work.

I've looked into named volumes but from what I understand, named and anonymous volumes are designed to share data between containers, and not to share data with the host (which is what I need for my use case).

So besides bind mounts, is it possible share data between the host and container?

Upvotes: 1

Views: 351

Answers (1)

dpwr
dpwr

Reputation: 2812

This is not really a Docker problem. I think you'll run into this with any mount. Basically you are already using the correct mechanism for sharing data between the host and your container.

When you mount something in linux, the mount target (i.e. the path at which you mount something) is always replaced with the root of whatever you mount. It does not merge the contents of the mount target with the contents of the (in this case) bind mounted directory. I'm surprised that works with VM shared folders because you run a high risk of a collision. e.g. same file in both locations. How would it resolve that? File system mounts are not the same as a dropbox like synchronisation of files between two locations.

I suggest that you do your bind mount to somewhere else in your container which has no contents and then modify your in-container workflow to handle this. In your example it sounds like you are attempting to collect logs. It also sounds like the containers configured log directory might have some contents which you want to be copied to the host. You could achieve this by having your container init itself by configuring a new log directory before starting your services/running anything, and copying any existing logs to that location. This new location would be the bind mount. Your init script could also detect if the bind mount was already used in this fashion and not sync over the data. This is really an application specific problem.

Upvotes: 1

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