O_DIRECT, or madvise and/or posix_fadvise

I caught this thread about O_DIRECT on kerneltrap.org:
http://kerneltrap.org/node/7563

It sounds like there is much to be gained here in terms of reducing
the number of user/kernel space copies in the operating system. I got
the impression that posix_fadvise in the Linux kernel isn't as good as
it could be. I noticed in xlog.c that the use of posix_fadvise is
disabled. Maybe it's time to do some more experimenting and working
with the Linux kernel developers. Or perhaps there is another OS that
would be better to experiment with?

Not sure where to start but do people think this is worth taking a stab at?

Regards,
Mark