The synchronous writes in FFS are not intended to make the *data*
fully robust (in the face of system crashes or power failures);
they are to make the *file system structure* fully robust.  That
is, fsck can always tell whether some operation (such as a file
creation/rename/remove/truncate) was completed and, if not, either
complete it or back it out.

For robust data you have fsync and O_SYNC,

Chris