Subject: Re: tmpfs and union mounts
To: YAMAMOTO Takashi <email@example.com>
From: Steven M. Bellovin <firstname.lastname@example.org>
Date: 10/15/2005 14:06:34
In message <email@example.com>, YAMAMOTO Takash
>> 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
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