[libvirt] Live Migration with non-shared storage for kvm

Daniel P. Berrange berrange at redhat.com
Tue Jan 26 10:14:47 UTC 2010

On Tue, Jan 26, 2010 at 11:57:25AM +0200, Kenneth Nagin wrote:
> On January 5 I asked for comments on adding libvirt support for live
> migration with non-shared storage on kvm.  I did not get any comments so I
> am repeating my request:

Sorry I missed your original mail - January 5th was unlucky timing, since I had
a backlog of several thousand emails from the xmas break :-)

> Support for live migration between hosts that do not share storage was
> added to qemu-kvm release 0.12.1.
> It supports two flags:
> -b migration without shared storage with full disk copy
> -i  migration without shared storage with incremental copy (same base image
> shared between source and destination).

Interesting, I'd not seen those flags before. How does that work with the
destination side, if the disks do not already exist there ?  Does the 
mgmt app have to just create a zero-sized disk, start QEMU & then let the
data be copied. I'm guessing QEMU can't create the disk itself, since it'll
be trying to open it on the destination before the incoming migration even

> I suggest adding these flags to  virDomainMigrate.

That sounds reasonable.

> If I'm not mistaken qemuMonitorTextMigrate is the function that actually
> invokes the kvm monitor.
> Thus, it would be necessary to pass the flags to qemuMonitorTextMigrate..
> But qemuMonitorTextMigrate does not have a flag input parameter.  I think
> the least disruptive way to support the new flags is use the existing
> "background" parameter
> to pass the flags.   Of course this would require some changes to the
> upstream functions that are invoked for migration.

That is an internal function, so adding extra parameters to it is no
problem at all - no need to reuse/abuse existing parameters.

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