Docker volume of a directory that is already part of a volume-CodePudding

I'm running an application in a Docker container and expose its entire app-data directory to the host. The app-data directory inside the container contains many files and directories, also a logs subdirectory which I want to treat differently and map it to another disk partition on the host. My docker-compose.yml now contains this:

volumes:
  - /mnt/app:/var/app-data
  - /tmp/logs:/var/app-data/logs

While this seems to work, I encountered a side-effect that I don't understand. After starting the application, the /tmp/logs directory correctly contains the application logs, but the /mnt/app directory now contains an empty logs directory:

drwxr-xr-x.  2 root      root       4096 Dec  1 10:47 logs

Where does this empty directory come from? Is that just expected Docker behavior and totally safe to deploy this way?

CodePudding user response：

That empty directory is needed as the mount point for the nested mount.

The first important detail is that Docker internally sorts all mounts by their container path, and mounts them in order. So, the /var/app-data mount is completed before Docker starts to consider the /var/app-data/logs directory.

When Docker does the bind mount, it uses the Linux mount(2) system call. Docker needs to tell the kernel what to mount and where to mount it. If the mount point doesn't exist, Docker creates it first.

In your scenario, after /var/app-data is mounted, the directory /var/app-data/logs doesn't exist, so Docker creates it before doing the actual mount. But, when it does create the directory, the outer directory is already mounted, so when it creates the directory it creates it on the host system too.

This is a normal consequence of using nested mounts and it's nothing to worry about.