From owner-freebsd-current@FreeBSD.ORG  Sat Oct 10 19:38:17 2009
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: freebsd-current@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 08EDD106566B
	for <freebsd-current@freebsd.org>; Sat, 10 Oct 2009 19:38:17 +0000 (UTC)
	(envelope-from ehrmann@gmail.com)
Received: from mxout-07.mxes.net (mxout-07.mxes.net [216.86.168.182])
	by mx1.freebsd.org (Postfix) with ESMTP id D54AF8FC0C
	for <freebsd-current@freebsd.org>; Sat, 10 Oct 2009 19:38:16 +0000 (UTC)
Received: from [10.0.0.171] (unknown [64.9.236.71])
	(using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits))
	(No client certificate requested)
	by smtp.mxes.net (Postfix) with ESMTPSA id 3198122DD6D
	for <freebsd-current@freebsd.org>; Sat, 10 Oct 2009 15:38:14 -0400 (EDT)
Message-ID: <4AD0E286.9030508@gmail.com>
Date: Sat, 10 Oct 2009 12:37:42 -0700
From: David Ehrmann <ehrmann@gmail.com>
User-Agent: Thunderbird 2.0.0.23 (Windows/20090812)
MIME-Version: 1.0
To: freebsd-current@freebsd.org
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Strange md/unionfs issue
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>, 
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Sat, 10 Oct 2009 19:38:17 -0000

I have a system running on a flash disk.  To prevent it from wearing out 
quickly, I don't run with a swap and mount /tmp and /var from memory.  
Because /var needs some directories and contains files I probably should 
save from time to time, I mount it from flash, mount /var to /var_real 
so I can access the underlying files, and then I mount an md device on 
top of /var as a unionfs.  Initially, it seems to work, but seconds 
later, the unionfs seems to fail.

server# cd /var
server# ls
.snap           crash           heimdal         preserve        yp
account         cron            log             run
at              db              mail            rwho
audit           empty           msgs            spool
backups         games           named           tmp
server# mount
/dev/ad0s1a on / (ufs, NFS exported, local)
devfs on /dev (devfs, local, multilabel)
/dev/ad0s1f on /usr (ufs, local, soft-updates)
/dev/ad0s1e on /var (ufs, local, soft-updates)
/var on /var_real (nullfs, local, noatime)
/dev/md0 on /var (ufs, local, union, soft-updates)
/dev/md1 on /tmp (ufs, local, soft-updates)
server# cd /var_real/
server# ls
.snap           crash           heimdal         preserve        yp
account         cron            log             run
at              db              mail            rwho
audit           empty           msgs            spool
backups         games           named           tmp
server# cd /var
server# ls
.snap
server#

Removing /var_real from the fstab didn't help.  Here's the full version:

# Device                Mountpoint      FStype  Options         Dump    
Pass#
/dev/ad0s1a             /               ufs     rw              1       1
/dev/ad0s1f             /usr            ufs     rw              2       2

/dev/ad0s1e             /var            ufs     rw              2       2
/var                    /var_real       nullfs  rw,noatime
md                      /var            mfs     rw,union,-s128M

md                      /tmp            mfs     rw,-s128M

Ideas?