O_DIRECT in freebsd

I noticed this in the FreeBSD 5.1 release notes:

"A new DIRECTIO kernel option enables support for read operations that
bypass the buffer cache and put data directly into a userland buffer. This
feature requires that the O_DIRECT flag is set on the file descriptor and
that both the offset and length for the read operation are multiples of the
physical media sector size. [MERGED]"

MERGED means that it should also appear in FreeBSD 4.9.

Will PostgreSQL pick this up automatically, or do we need to add extra
checks?

Chris