> I would assume most of them? I'd be surprised if distros like Debian, Ubuntu, Fedora, etc. would ship non-mainline kernel features like that.

It's best not to assume with these things. With my stock Debian Stable kernel, Proton says this:

fsync: up and running.

And when I disable fsync, it says this:

esync: up and running.

> But it sounds like fsync got you performance pretty close to what ntsync can do, but esync was quite a bit behind both?

No, esync and fsync trade blows in performance. Here are some measurements taken by Kron4ek, who maintains somewhat widely used Wine/Proton builds:

https://web.archive.org/web/20250315200334/https://flightles...

https://web.archive.org/web/20250315200424/https://flightles...

https://web.archive.org/web/20250315200419/https://flightles...

> With vanilla being quite a bit behind esync?

Yes, vanilla Wine has historically fallen behind all of them, of course.

> Also, jeez, fsync, what a terrible name. fsync is a syscall that has to do with filesystem data. So confusing.

We can agree on this. :)