About a situation that live migration would never complete
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.Cuestión

    I have a question from customer about following description regarding KVM live migration.
    Chapter 4. KVM live migration in Virtualization Administration Guide

 If the original guest modifies pages faster than KVM can transfer them to the destination host, offline migration must be used, as live migration would never complete.

    Is there any example about case that 'the original guest modifies pages faster than KVM can transfer them to the destination host'?
    Also, it says that live migration would never complete, but what behavior is it concretely?
    Is there any recommended method to cancel migration using virsh or virt-manager except for killing of processes (virsh and virt-manager)?

Medio Ambiente

    Red Hat Enterprise Linux 6.3

Resolución

> Is there any example about case that 'the original guest modifies pages faster than KVM can transfer them to the destination host'?

It is very unusual and I have no real cases where it happened, but it's quite easy to simulate (I've got a little C program to do so). Migration process consists in copy guest RAM to new Host. After the copy, migration code loops back to the beginning and re-copies all pages modified after the copy. The hope is that the list of pages which must be copied shrinks with each pass over the memory range. When the number of changed pages goes below a threshold, guest system is stopped, the remaining pages are copied and the current state is also transmitted. Then, the guest can be started on destination host. Systems with high level of memory writes can become impossible to online migrate, since the loop to re-copy modified pages may never finish.

> Also, it says that live migration would never complete, but what behavior is it concretely?

Nothing. The migration process will keep trying to re-copy changed pages forever. The guest continues to work on the source host. User needs to cancel migration to stop the process.

> Is there any recommended method to cancel migration using virsh or virt-manager except for killing of processes (virsh and virt-manager)?

Using virt-manager, we have a window showing the migration progress. This window has a "cancel" button to cancel the live migration process.
Using virsh, we have to open another shell and execute:

# virsh domjobabort <vm_name>

In both cases, the live migration will be stopped and the VM will be kept on source Host, with zero downtime.