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The downtime of a VM during its live migration is given mostly by the VM RAM activity (change rate). Qemu itself allows to setup the value of the maximum amount you are willing to "sacrifice" as a downtime. Unfortunately this value is not in Openstack. The downtime window is provided to transfer the last chunk of dirtied memory (and CPU states) and directly affects convergency of the pre-copy algorithm. Could you please be more specific about your use case? What kind of VM are you trying to migrate? What kind of load does it carry on? What is the downtime value in time unit (How many packets got lost and what is the interval between following two). How long does it take to migrate the VM, version of Qemu/libvirt, etc, ... Assuming you are trying to live migrate essentially unloaded VM, from my experience the downtime doesn't exceed 1s..