Comment 1 for bug 1946752

Before enabling post-copy, we sometimes had VMs which did not manage to get migrated in half an hour. The memory transfer statistics showed that by that time, often around 2 orders of magnitude more memory had been transferred than the VMs memory size. So the migration had been flooding the network for half an hour at high speed, wasting lots of bandwidth and making it unavailable for other VMs.

So having post-copy available is quite important. That's why I'm disappointed that Ussuri can only use it after a fixed time-out. Queens automatically starts post-copy if the normal memory migration does not make enough progress. Sometimes that already kicks in after 30 seconds. On Ussuri you have to wait (and waste bandwidth) for several minutes (and the default-time out is waaaay too long; we set it to live_migration_completion_timeout = 10, down from 800).