[Libguestfs] libguestfs and md devices

[Richard W.M. Jones]

On Mon, Oct 31, 2011 at 02:45:23PM +0000, Matthew Booth wrote:
> We've recently discovered that libguestfs can't handle guests which
> use md. There are (at least) 2 reasons for this: Firstly, the
> appliance doesn't include mdadm. Without this, md devices aren't
> detected during the boot process. Simply adding mdadm to the
> appliance package list fixes this.

I've added the mdadm package since it sounds useful to have:

http://git.annexia.org/?p=libguestfs.git;a=commitdiff;h=6f84ef6768e9c625d67f7eb015277b0ec1b2c2af

> Secondly, md devices referenced in fstab as, e.g. /dev/md0, aren't
> handled correctly by inspection. This looks a bit more complicated.
> In my test guest, the guestfs appliance automatically creates the 2
> md devices as:
> 
> * /dev/md/localhost:localdomain:0
> * /dev/md/localhost:localdomain:1

That's a very strange name.  udev going crazy?

> These are symlinks to ../md126 and ../md127 respectively. I haven't
> yet worked out why they aren't created as md0 and md1 as in the
> guest. I think here we need to either work out how to make the
> appliance detect and use their names (if this is even possible), or
> to map the names appropriately using information from
> /dev/md/md-device-map in the appliance's root, and /etc/mdadm.conf
> in the guest's root.
> 
> I don't currently think we'll need any new apis for this. I'm not
> convinced we need apis for creating and managing md devices, for
> example. We could do with a test for inspection of a guest which
> uses md devices, which would obviously require creating that guest.
> However, we can frig that for the test using debug sh "mdadm ...".

New APIs are cheap ...

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://et.redhat.com/~rjones/virt-df/