Docker Desktop - Filesharing notification about poor performance

Question

When my Docker containers start, I receive the following notification that reads:

Docker Desktop has detected that you shared a Windows file into a WSL 2 container, which may perform poorly. Click here for more details.

My questions are:

1. What does this mean?
2. What is the better practice / how should this be avoided?
3. If the message has been closed, or I've clicked "Don't show again", how can I get to the details of this warning?

I am happy to share the Dockerfile or Docker-Compose setup if needed, but I simply cannot find anything either here on SO or by a Google search that points me in any direction, so I'm not sure where to start. I'm assuming the issue lies in the Dockerfile since that is where we running COPY to move some files around.

Docker Version: Docker Desktop 2.4.0.0 (48506) Community

Operating System: Windows 10 Pro (version 10.0.19041)

Answer 1

1. This error means that accessing files on the Windows host file system from a Linux container will perform a little slower than accessing files that are already in a Linux filesystem. Accessing Windows files from the Linux container will perform like accessing files on a remote file share.

2. Docker and Microsoft recommend avoiding this by storing your source files in a WSL2 distro's file system (which you can bind mount to the container) or building your container image to include all the files needed rather than storing your files in the Windows file system.

3. If you've clicked "Don't show again", you can get to the details of this message by going to Develop with Docker and WSL 2.

For more information, Docker for Windows Best Practices says:

• Linux containers only receive file change events (“inotify events”) if the original files are stored in the Linux filesystem. For example, some web development workflows rely on inotify events for automatic reloading when files have changed.
• Performance is much higher when files are bind-mounted from the Linux filesystem, rather than remoted from the Windows host. Therefore avoid docker run -v /mnt/c/users:/users (where /mnt/c is mounted from Windows).
• Instead, from a Linux shell use a command like docker run -v ~/my-project:/sources <my-image> where ~ is expanded by the Linux shell to $HOME.

Microsoft's Comparing WSL 1 and WSL 2 article has a whole section on Performance across OS file systems, and its opening paragraph says:

We recommend against working across operating systems with your files, unless you have a specific reason for doing so. For the fastest performance speed, store your files in the WSL file system if you are working in a Linux command line (Ubuntu, OpenSUSE, etc). If you're working in a Windows command line (PowerShell, Command Prompt), store your files in the Windows file system.

Also, the Docker blog article Docker Desktop: WSL 2 Best practices has an "Awesome mounts performance" section that says:

Both your own WSL 2 distro and docker-desktop run on the same utility VM. They share the same Kernel, VFS cache etc. They just run in separate namespaces so that they have the illusion of running totally independently. Docker Desktop leverages that to handle bind mounts from a WSL 2 distro without involving any remote file sharing system. This means that when you mount your project files in a container (with docker run -v ~/my-project:/sources <...>), docker will propagate inotify events and share the same cache as your own distro to avoid reading file content from disk repeatedly.

A little warning though: if you mount files that live in the Windows file system (such as with docker run -v /mnt/c/Users/Simon/windows-project:/sources <...>), you won’t get those performance benefits, as /mnt/c is actually a mountpoint exposing Windows files through a Plan9 file share.

All of that advice is great if you want your primary development workflow to be in Linux. Docker wants you to go "all in" on Linux containers. But if you work primarily in Windows and just want to use a Linux container for a specialized task, then it's fine to click "Don't show again". As Microsoft said, "If you're working in a Windows command line, store your files in the Windows file system."

I run with my main development folder in Windows, and I bind mount it to a Linux container that's just used to execute unit tests. So my full build runs in Windows, then I run all my unit tests in Windows, and I finish by running all my unit tests in a Linux container too. Having Linux bind mount to my Windows folder works fast and great for this scenario where the "dotnet test" call in Linux is just loading and executing the required DLLs from my Windows volume.

This setup may sound like heresy to those that believe containers must be used everywhere, but I love containers for application deployment. I'm not convinced that you need to go all in and do all your development inside a container too. I'm happy with Windows (and VS 2019) as my development environment, and then I use Linux containers for application testing and deployment. So the Windows/WSL2 file system performance hit is a minimal impact to me.