I have a virtual machine (VMM is Xen 3.3) equipped with two IDE HDD's (/dev/hda and /dev/hdb). The root file system is in /dev/hda1, where Scientific Linux 5.4 is installed. /dev/hdb contains an empty ext2 file system.

I want to protect the root file system from writes by the VM by using aufs (AnotherUnionFS) to layer a writable file system on top of the root file system. The changes to / will be written to the file system located on /dev/hdb. (Furthermore, outside the VM, the file backing the /dev/hda will also be set to read-only permissions, so the VMM should also prevent the VM from modifying at that level.) (The purpose of this setup: be able to corrupt a virtual machine using software-implemented fault injection but preserve the file system image in order to quickly reboot the VM to a fault-free state.)

How do I get an initrd init script to do the necessary mounts to create the union file system?

I've tried 2 approaches:

1. I've tried modifying the nash script that mkinitrd creates, but I don't know what setuproot and switchroot do and how to make them use my aufs as the new root. Apparently, nobody else here knows either. (EDIT: I take that back.)

2. I've tried building a LiveCD (using linux-live-6.3.0) and then modifying the Bash /linuxrc script from the generated initrd, and I got the mounts correct, but the final /sbin/init complains about /dev/initctl.

Specifically, my /linuxrc mounts the aufs at /union. The last few lines of /linuxrc effectively do the following:

cd /union
mkdir -p mnt/live
pivot_root . mnt/live
exec sbin/chroot . sbin/init </dev/console >/dev/console 2>&1

When init starts, it outputs something like init: /dev/initctl: No such file or directory. What is supposed to create this FIFO? I found no such filename in the original linuxrc and liblinuxlive scripts.

I tried creating it via "mkfifo /dev/initctl", but then init complained about a timeout opening or writing to the FIFO.

Would appreciate any help or pointers. Thanks.