Comment 6 for bug 1597234

Hi Andrea, let me describe the detailed process:
For non-encrypted volume, Nova calls connect_volume to do attaching. This returns connection_info which includes 'device_path' that shows the device path in computer host.
With encrypted volume, after the above steps, it needs to decrypt the device. It creates a decrpted device with 'cryptset open', and then make device_path in connection_info refer to the new decrypted device.

When rebooting an VM, Nova instance is shut down without any detaching/disconnection operations. That is ok.
But when starting, it calls connect_volume for every block device. For non-encrypted volumes, this calls can be called repeatedly. But for encrypted volumes, as device_path refers to a decrypted device after opening. It can't be run with scsi_id. As a result, to call connect_volume on the decrypted device leads exception.