Comment 30 for bug 1548450

And it seems like the shelve issue is real and not fixed by the propose patch.
After overwritting the root disk of an instance (using vda1 as backing file), here is what the shelve action produce:

 7084 ? Ssl 1:05 /usr/bin/python /usr/bin/nova-compute
11792 ? Sl 2:40 \_ qemu-img convert -f qcow2 -O qcow2 \
  /var/lib/nova/instances/f531dd97-3e5c-4f2c-88fb-740e97c3f0d2/disk \
  /var/lib/nova/instances/snapshots/tmpSLRPxG/f345630013ca4fe0a29b203e2b3afa61

# openstack-config --get /etc/nova/nova.conf libvirt use_cow_images
False

# qemu-img info /var/lib/nova/instances/f531dd97-3e5c-4f2c-88fb-740e97c3f0d2/disk
image: /var/lib/nova/instances/f531dd97-3e5c-4f2c-88fb-740e97c3f0d2/disk
file format: qcow2
virtual size: 100G (107374182400 bytes)
disk size: 5.0G
cluster_size: 65536
backing file: /dev/vda1
backing file format: raw
Format specific information:
    compat: 1.1
    lazy refcounts: false

# qemu-img info /var/lib/nova/instances/snapshots/tmpSLRPxG/f345630013ca4fe0a29b203e2b3afa61
image: /var/lib/nova/instances/snapshots/tmpSLRPxG/f345630013ca4fe0a29b203e2b3afa61
file format: qcow2
virtual size: 100G (107374182400 bytes)
disk size: 7.5G
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false

The snapshot is still growing and likely to contain the controller disk, I'll check again tomorrow if the user can retrieve it.