From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Brown <dmlb2000@gmail.com>
Subject: Some Union Mount questions
Date: Fri, 21 May 2010 15:06:30 -0700
Message-ID: <AANLkTinQI4I3n7dsjkjk6S9fVC98Ib7KDd71bV-xuW8O@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Cc: vaurora@redhat.com
To: linux-fsdevel@vger.kernel.org
Return-path: <linux-fsdevel-owner@vger.kernel.org>
Received: from mail-yw0-f180.google.com ([209.85.211.180]:58569 "EHLO
	mail-yw0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752481Ab0EUWGd (ORCPT
	<rfc822;linux-fsdevel@vger.kernel.org>);
	Fri, 21 May 2010 18:06:33 -0400
Received: by ywh10 with SMTP id 10so866821ywh.1
        for <linux-fsdevel@vger.kernel.org>; Fri, 21 May 2010 15:06:31 -0700 (PDT)
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

Okay so its been a while so I figured I'd better update on things.

It Works!

I've gotten it to work using Val's documentation at
http://valerieaurora.org/union/ with some additions to the initramfs
package to generate the correct initrd

# cat etc/initramfs-tools/scripts/nfs-bottom/union
#!/bin/sh

# init-premount script for lvm2.

PREREQS=""
prereqs()
{
	echo $PREREQS
}

case "$1" in
	prereqs)
	prereqs
	exit 0
	;;
esac

echo "mounting union tmpfs"
mount.union -n -o union -t tmpfs none ${rootmnt}
# cat etc/initramfs-tools/hooks/union
#!/bin/sh

PREREQ=""

prereqs()
{
	echo "$PREREQ"
}

case $1 in
prereqs)
	prereqs
	exit 0
	;;
esac

. /usr/share/initramfs-tools/hook-functions

copy_exec /bin/mount /bin/mount.union
#

Then it works pretty good. (I really like debians initramfs package).

I can stand up an 8 node cluster with an nfs read only root file
system and it works!

However, the output of the kernel is very verbose lots of debug
messages about how you are managing the tmpfs part of the union. Also
lots of duplicate entries, which seems weird.

May 21 21:53:24 x6 kernel: [   56.558749] lib: appending to union

Does this mean you are appending /lib to the union tmpfs over and over?

So df doesn't show any leaking of the processes running on the system
syslog is going over the wire to a central server, there's 7 agetty's
running, udev is kinda freaking out (kernel version difference I
guess), and cron is running. So the image is pretty stripped down.
This is pretty usual for an HPC compute node. I'm currently doing some
long tests trying to determine how fast would tmpfs fill up and what
would be filling it up.

Also, when is it going to be included to mainline stuff? Also, should
I put a redhat bugzilla request in for inclusion into RHEL 6.1/2?

Sadly, this wonderful feature would be more easily adopted here if it
was included into RHEL 6.1 or 6.2 at some point.

Thanks,
- David Brown