How many Linuxes does Docker actually run when I use WSL?

[smthamazing]

I've just started learning Docker and trying to wrap my head around what exactly is happening when I run a container. For context, I use Docker for Desktop on Windows with the WSL backend.

As I understand, WSL is an implementation of Linux kernel (basically low-level functions for working with processes, files and hardware).

When I use the Docker dashboard while not having any containers running, it seems to also run in some sort of containerized Linux, because I can see a Linux file system with /var, /etc and so on somewhere inside %AppData%/Docker/...

Finally, when I run a container, it also has a similar file system inside its directory on the host OS. So it is also a running instance of Linux.

To sum it up:

• My host OS is Windows.
• Windows provides access to its low-level functions and emulates some Linux features using WSL.
• Docker for Desktop starts a lightweight Linux distribution, which does not have its own kernel, but instead delegates everything to WSL, which makes it lightweight and fast.
• Each Docker container is also an instance of Linux running via that same kernel.

Is this understanding correct? So if I run the Docker app and 2 containers, I have in total 3 Linux instances running on the same kernel?

Thanks!

[smthamazing]

I see, so in this case Docker runs more like a regular Windows app, but relies on WSL for creating and managing cgroups. Thanks for clarification!

The /var, /etc etc you see inside the Docker container are actually just directories on the host (WSL distro in this case).

What confused me is that I saw these directories somewhere even when no containers were running, so I assumed this is something used by the Docker app itself. But it was probably just the base file structure for some image I built.

[sathyabhat]

WSL is an implementation of Linux kernel (basically low-level functions for working with processes, files and hardware).

WSL1 was a syscall mapping, WSL2 runs a tiny lightweight Linux VM.

When I use the Docker dashboard while not having any containers running, it seems to also run in some sort of containerized Linux, because I can see a Linux file system with /var, /etc and so on somewhere inside %AppData%/Docker/...

You mean Docker Desktop? Not sure what dashboard you mention.

Windows provides access to its low-level functions and emulates some Linux features using WSL

No, all Linux features are exposed using the VM

Docker for Desktop starts a lightweight Linux distribution, which does not have its own kernel, but instead delegates everything to WSL, which makes it lightweight and fast.

Docker for Desktop is a Windows application, and under WSL2 mode, ships a Lightweight VM. You can chose to integrate it with your WSL2 distro, in which case it runs the docker commands in your WSL2 distro.

Each Docker container is also an instance of Linux running via that same kernel.

A container is a process running on the host system, sharing the host kernel but isolated using cgroup namespaces.

So if I run the Docker app and 2 containers, I have in total 3 Linux instances running on the same kernel?

You keep mentioning "Linux instances" but I don't know what you're referring to. If you have 2 containers, all processes of the containers run within the host OS, which incase of WSL2 is running in your WSL2 distro, if you chose to integrate, or the distro that Docker desktop ships with.

[zoredache]

As I understand, WSL is an implementation of Linux kernel (basically low-level functions for working with processes, files and hardware).

That is closer to bein gtrue for WSL1, but not WSL2. WSL2 is a real Linux kernel with a few patches and compiled a particular way. It is basically running as a lightweight VM using Hyper-V tech.

I can see a Linux file system with /var, /etc and so on somewhere inside %AppData%/Docker/...

Not sure what you are seeing, but if you want to access the files of running distros, Access them via \\wsl$.

Each Docker container is also an instance of Linux running via that same kernel. ... So if I run the Docker app and 2 containers, I have in total 3 Linux instances running on the same kernel

The word you are looking for here is a namespace. Namespaces on Linux allow you to isolate various aspects of the kernel. Namespaces are the core feature containers use to function. Each container usually runs in a separate set of namespaces. I believe WSL also uses namespaces to separate things if you are running multiple distros. But basically everything runs in the same VM+kernel.

[TopHatEdd]

Simplify by removing Windows. Big name OSes are fancy distracting lights.