On 10/08/2013 04:19 PM, Brian Candler wrote:

 > Anyway, I'm not so worried about having broken this machine, as it 
needed a reinstall anyway, but I do wonder what would have been the 
correct way to get mdraid instead of dmraid at boot time for this root 
volume?

After some more searching, it looks like the udev rules were nobbled to 
disable this in
https://bugs.launchpad.net/ubuntu/+source/mdadm/+bug/1030292

A possible way to re-enable is here:
https://bugs.launchpad.net/ubuntu/+source/mdadm/+bug/1054948/comments/9

I'm a bit concerned about the issues around clean shutdown, and hence 
whether is really production-ready yet.

In general, this works. I have seen it work with CentOS, Fedora, and
various SUSE distributions. It may require some work on Ubuntu's side.

 1 The udev rules for incremental mdadm autoassembly need to be in
place. The upstream rules should be fine. They can normally coexist with
the rules for dmraid.
 2 For shutdown, the distro must take care not to kill mdmon before file
systems are unmounted, and to run mdadm --wait-clean after any write
access to file systems is finished.

Martin