Thanks Peter, that’s what I figured that the actual copy is done by the qemu process.
The copy job is setup by openstack volume migration and translate into
<mirror type='file' file='/var/lib/nova/mnt/xxx' format='raw' job='copy'>
<format type='raw'/>
<source file='/var/lib/nova/mnt/yyy' index='4'/>
<backingStore/>
</mirror>
