On Sun, 09 Dec 2007, David Young wrote:
> > mount_tmpfs(8) seems to be quite stable, so I'd like to
> > start using it instead of mount_mfs(8) for "MAKEDEV -M".
> 
> I don't know if it is important, but tmpfs does not provide whitespace,
> so mounting it -o union may not always work as one expects.

I didn't know that.  I just tested this, and after the following commands

	mkdir ./tmp
	cp MAKEDEV ./tmp
	mount_tmpfs -o union tmpfs $(pwd)/tmp
	rm ./tmp/MAKEDEV

instead of a whiteout in the upper layer, the MAKEDEV in the lower layer
is removed.  If the lower layer had been read-only, I assume that the rm
would have failed.  Is this bug in tmpfs likely to be fixed soon?

Anyway, I don't think the lack of whiteouts in tmpfs is a serious issue
for the common use case, where the root file system contains a /dev
directory with a MAKEDEV script and nothing or very little else, and
init runs "MAKEDEV -MM init".

--apb (Alan Barrett)