In message <1129396578.894858.2882.nullmailer@yamt.dyndns.org>, YAMAMOTO Takash
i writes:
>> I'm trying to do a union mount involving a tmpfs file system.  
>> Unfortunately, I'm getting 'operation not supported'.  Is it supposed 
>> to work?
>
>(assuming you mean unionfs)
>
>no.  tmpfs doesn't have whiteout.
>

That's too bad.

I'm putting together a Soekris system, based on the advice given at
http://www.netbsd.org/~jschauma/netbsd-solidstate.html.  For a number 
of reasons, that document suggests mfs filesystems for /dev and /var, 
and (for /var) extracting a tarball of the real /var into the mfs file 
system.  

I'd like to do things differently if possible.  First, of course, I'm 
using tmpfs instead of mfs; the memory efficiency is very important on 
a machine with only 128M of RAM and no swap disk.  Beyond that, if I 
could do a unionfs mount of a tmpfs on top of /var and /dev, I could 
skip the extraction; it would all just work.

/dev presents other problems.  If I create new entries by running 
MAKEDEV inside the tmpfs or mfs file system, I can't do

	mount -uw /

because it complains "specified device does not match mounted device".
For that matter, it wasn't clear to me why I needed a writable /dev, 
until I learned that it's not possible to open for output a device 
special file on a read-only file system.  I don't see any reason to 
enforce that policy -- a device file isn't a resource, it's a pointer 
to a resource whose read-only status is not coupled to that of the 
containing filesystem -- but it isn't clear to me if that's a 
difficult-to-avoid consequence of other code.  The other problem with a 
non-writable /dev is that various parts of the system can't do chown() 
calls on, say, ttys; for an appliance box, that's a lot less relevant,
as long as it doesn't block console and ssh login.

		--Steven M. Bellovin, http://www.cs.columbia.edu/~smb