Comment 3 for bug 1441054

Interestingly, in my testing of block migration over tunneled mode I'm seeing libvirt recognize that it can't do the NBD/drive-mirror type of migration but falls back to the older 'inc = true' approach which causes qemu to do the data transfer over the same channel as the memory. The logs show:

  qemuMigrationRun:4254 : Destination doesn't support NBD server Falling back to previous implementation.

but it works just fine. The QMP migrate command is:

  "migrate","arguments":{"detach":true,"blk":false,"inc":true,"uri":"fd:migrate"}

This is with libvirt-1.2.17-13.el7_2.2.x86_64 on RHEL 7.2 but it looks like this has been the behavior since the drive-mirror approach was added: http://libvirt.org/git/?p=libvirt.git;a=commitdiff;h=7b7600b3e6