I suppose ZODB is such an expert application. It has to cope with systems that do not provide fsync(), but it provides degraded service on such platforms. It is very important for the database to call fsync() when it commits a transaction.
I'm not sure that ZODB *intends* to run on any system w/o fsync anymore. It had to before 2.3 because os.fsync() didn't exist on Windows, and ZODB has to run on Windows. I implemented os.fysnc() for Windows in 2.3 precisely so that ZODB would work better on Windows.
More mundanely, as I recall, we were having a specific problem wherein a backup script wasn't getting the correct size for a ZODB Data.fs file on Windows, and file.flush()'ing Data.fs didn't help enough -- backup kept screwing up until FileStorage's
if fsync is not None: fsync(file.fileno())
actually did something on Windows.
Bizarre. On Unix, fsync()'s only point is to do something that is only ever noticeable if there's an OS or hardware failure. But on Windows, it was needed to get the OS to synchronize the directory contents so that another process or thread would see the right thing.
Anyway, I think that on Unix, that problem doesn't exist, and there the only problem of not having fsync() is the minuscule risk of hardware or OS failure right after you wrote the data but before it actually was written to the media.
--Guido van Rossum (home page: http://www.python.org/%7Eguido/)