Comment 3 for bug 1396965

A change in this to allow the detaching of a root volume to an instance makes since when the following happens:

cinder snapshots are being created on a regular interval for a nova instance running Linux
cinder creates a volume from a snapshot
volume needs to be attached to the nova instance that has the original volume on /dev/sda

Linux has a specific issue with /etc/udev/rules.d/persistence-net-rules which stores the MAC addresses

If the restored volume is attached to a new nova instance, the new MAC address adds a second entry to the persistence-net-rules and treats the interface like a second non-configured device; thus, breaking networking.

The legitimate use case for this is local Operational Recovery where a customer wants to go backwards to a set of snapshots, where they are dependent on the disks as they have yet to write portable code or ephemeral applications.