live storage migration using blockcopy

Hello, I'm seeking the solution to live storage migration using blockcopy. Previously, the "virsh undefine" is required before blockcopy. https://www.redhat.com/archives/libvirt-users/2015-October/msg00027.html QEMU has "block-dirty-bitmap-*" operations now, are there steps for the live storage migration using blockcopy without undefine? By the way, why's the purpose of "undefine" before blockcopy? Thanks. Sincerely, Jing-Wei