Block live migration without pausing VM

[Felix Leimbach <felix.leimbach@gmail.com>]

Hi all,

I want to implement live-migration of a highly available VM but I cannot 
use shared storage.
The -b option to the migrate command already allows for copying the 
block device (locally stored raw file) and that is exactly what I want.
This worked in my experiments but the VM is unreachable (CPU halted?) 
while the block device is copied, which is unacceptable for my use-case.

Is there a way to copy the block device while the VM is running (copy on 
read or such)?
The LiveBlockMigration page in the qemu wiki [1] mentions that some of 
this is already implemented, but I cannot find any of it in the latest 
qemu-kvm.
Any pointers?

Thanks for your suggestions.

[1] http://wiki.qemu.org/Features/LiveBlockMigration