you can visit http://blog.zhaw.ch/icclab/an-analysi... having performance analysis (migration time and downtime).

From my understanding , there will be downtime but the downtime depends the state of VM memory.

hypervisors are using either of the two technique to do the memory state transfer from one host to another host. Pre-copy memory migration or Post-copy memory migration.

Visit wiki link to know more. http://en.wikipedia.org/wiki/Live_mig...

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..