Comment by Annatar

Solaris, from which came OpenSolaris, which was forked into illumos, from which SmartOS is built.

fsync(2) there does exactly what POSIX says: it will only return success if the write safely made it to disk.

This is the #1 reason why all my infrastructure runs on a combination of Solaris and SmartOS and the primary reason why I don't run GNU/Linux on anything that's mine. #2 reason is that Solaris / illumos and therefore SmartOS will not overcommit memory, whereas in GNU/Linux this must be explicitly disabled, since overcommit is enabled by default.