What does HackerNews think of dokany?
User mode file system library for windows with FUSE Wrapper
What I did to learn the lower-level APIs, and perform initial testing on the driver, was write a "mirror" drive. The user-mode code pointed to a folder on disk, the driver made a virtual disk drive, and all reads and writes in the virtual disk drive went to the mirror folder. All of our (cough) unit tests for virtual drive handling used the mirror drive. ("cough" because the tests fit into that happy area that truly is a unit test but drives people nuts about splitting hairs about the semantics between unit and integration tests.)
On Windows, you can implement something like that using Dokany, Dokan, or Winfsp. On linux, there's the Fuse API. On Mac, there's MacFUSE.
Even if you don't do a "mirror" drive, understanding the callbacks that libraries like Dokany, Dokan, Winfsp, and Fuse do helps you understand how IO happens in the driver. Many IO methods provided in popular languages provide abstractions above what the OS does. (For example, the Windows kernel has no concept of the "Stream" that's in your C# program. The "Stream"'s Position property is purely a construct within the .Net framework.)
https://github.com/dokan-dev/dokany
Another place to start is the OS's documentation itself. For example, you can start with Window's CreateFileA function. This typically is what gets called "under the hood" in most programming languages when you open or create a file: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/...
Keybase.io and sshfs use it, currently. Personally, I love it. I'm sure you're right about performance limitations, but there are very good use cases for it, like Keybase Filesystem and sshfs, where any latency introduced by the driver subsystem is less likely to be felt due to the latency of working with files over the network.
Interestingly, a set of the insider previews of Windows broke Dokany so badly that launching the driver caused everything to hang (hard -- all but the mouse stopped responding to anything, even CTRL+ALT+DELETE). It's tricky stuff, apparently.
I think a standard GPL license would be an improvement that would keep the intention and avoid some of the larger headaches of the Affero version.
Looks like Dokany is already LGPL/MIT-dual licensed though, so probably would just use that if I were building in this space: https://github.com/dokan-dev/dokany