Not sure what that means, but that's not the logging I was referring to.
Did you follow the instructions in my previous email?

No, I am not exactly know what's your mean in that mail. I just get some help from our team to let then open xenstore trace log. So I will try to use your guide to get more trace info.
Another way, how do you think about the ring-ref for a vbd device when save and restore?

Thanks
Wayne