History repeats itself. After reporting BZ1262251 and asking for patch backport in BZ1262901 qemu in Fedora is working much better for live merges. Nevertheless mirror operation fails in rare cases at a later stage when VM is doing heavy disk IO. Bugzilla gives BZ1233826 -> BZ1238585 as a possible fix. So I backported http://git.qemu.org/?p=qemu.git;a=commit;h=cae98cb87d269c33d23b2bccd79bb8d99a60d811 to FC21 and the mirror operation is stable now. Could it find its way into FC21/FC22 too?
qemu-2.1.3-11.fc21 has been submitted as an update to Fedora 21. https://bodhi.fedoraproject.org/updates/FEDORA-2015-16368
qemu-2.1.3-11.fc21 has been pushed to the Fedora 21 testing repository. If problems still persist, please make note of it in this bug report. If you want to test the update, you can install it with $ su -c 'dnf --enablerepo=updates-testing update qemu' You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2015-16368
qemu-2.1.3-11.fc21 has been pushed to the Fedora 21 stable repository. If problems still persist, please make note of it in this bug report.