Re: [Qemu-devel] [PATCH 2/10] Allow the monitor to be suspended during non-blocking op

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Sep 10, 2008 at 02:11:56PM +0300, Avi Kivity wrote:
> Daniel P. Berrange wrote:
> 
> >>This means that migration is no longer transparent.  While migration is 
> >>going on, you can't change the cdrom media, look at cpu registers, or do 
> >>anything that requires the monitor.
> >>    
> >
> >Changing cdrom media while in the middle of migration sounds like a rather
> >troubleprone thing todo - you'd need to change the media in both active
> >QEMU instances at the same time to be safe.
> 
> Or rather, such state should be part of the migration.  There's the 
> question of whether to transform the path on the target, but "which 
> media is in the drive" is part of the hardware state.
> 
> (logically we would copy all of the data of all block devices, but 
> that's not very practical, so we assume shared storage).
> 
> What other commands are unsafe during migration? I exclude host device 
> assignment which is obviously migration unfriendly.

USB + virtio device attach/detach - well at least have the same need as
media change - you'd need to propagate the change to the other side in
some way. Currently migrate just assumes the management tool/admin has
started QEMU on the destinations with the matching hardware config, and
for libvirt to use the monitor to add USB /virtio devices at both ends
has the race condition/synchronization problem .

Daniel
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