* What's is postcopy livemigration
It is is yet another live migration mechanism for Qemu/KVM, which
implements the migration technique known as "postcopy" or "lazy"
migration. Just after the "migrate" command is invoked, the execution
host of a VM is instantaneously switched to a destination host.
Sounds like a cool idea.
The benefit is, total migration time is shorter because it transfer
a page only once. On the other hand precopy may repeat sending same
pages
again and again because they can be dirtied.
The switching time from the source to the destination is several
hunderds mili seconds so that it enables quick load balancing.
For details, please refer to the papers.
While these are the obvious benefits, the possible downside (that, as
always, depends on the workload) is the amount of time that the guest
workload runs more slowly than usual, waiting for pages it needs to
continue. There are a whole spectrum between the guest pausing
completely
(which would solve all the problems of migration, but is often
considered
unacceptible) and running at full-speed. Is it acceptable that the guest
runs at 90% speed during the migration? 50%? 10%?
I guess we could have nothing to lose from having both options, and
choosing
the most appropriate technique for each guest!