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I suppose your instances are based on nova storage and not on cinder, so instances on nova storage are based on two qcow2 files, one is the glance image, and the second one is instances differences from glance image. When you start live migration the destination nova-compute (hypervisor), if haven't glance image on his cache -> for example because there aren't any instances running on it based on your glance image, the nova-compute node first have to download from glance this image and then it could migrate your instances.

But your environment could be different... HTH Amedeo