On Mon, Jun 22, 2015 at 09:56:24PM +0800, Wen Congyang wrote:
> >The NBD connect should not be performed during downtime, regardless of
> >whether a monitor command is used or not.
> 
> Why?

NBD connection establishment takes unbounded time.  It can extend the
downtime period and cause users to notice that the guest is unavailable.

The point of COLO is fault tolerance and high availability.  In order to
achieve that, COLO must minimize downtime.  If an operation can be
performed outside the migration downtime phase, then it should be done
outside.

Stefan