Ubuntu on Windows

I wonder if they implemented a copy-on-write fork syscall in NT. Otherwise it will be slower (and use a lot more memory) in some scenarios.

Edit: And cow fork only makes sense if there is memory over-commit. So to be fully featured it would need a separate memory subsystem with memory over-commit.

No, you don't need overcommit for COW fork. You can fully commit the writable pages of the new process. Yes, that uses a bit of pagefile space, but disk space is cheap, and this approach works fine. You can (and should, if you care about robustness) turn off overcommit on Linux too.

Looks like it wouldn't be the first copy-on-write fork() on NT; Interix had that already many years ago.

Solaris doesn't have VM over-commit either, and few people would claim it's not a fully featured UNIX.