NOW: o_direct -- WAS: Re: WARNING in xfs_lwr.c, xfs_write()

[Christoph Hellwig]

O_DIRECT is not a Posix standard and not very portable.  It originated
on IRIX, and Linux inherited it during the 2.4 kernel series days.
These days FreeBSD/NetBSD and AIX support it as well, but for example
Solaris, HP-UX and OpenBSD don't, nevermind Windows or Mac OS.

I have no idea why the MTAs don't want to use it - it's generally easier
to use then memory mapped I/O, and has much more deterministic
performance.