Re: [question] The source cannot recover, if the destination fails in the last round of live migration

[Peter Xu]

On Fri, May 07, 2021 at 05:46:44PM +0800, Kunkun Jiang wrote:
> Hi Dave,
> 
> On 2021/5/6 21:05, Dr. David Alan Gilbert wrote:
> > * Kunkun Jiang (jiangkunkun@huawei.com) wrote:
> > > Hi all,
> > Hi,
> > 
> > > Recently I am learning about the part of live migration.
> > > I have a question about the last round.
> > > 
> > > When the pending_size is less than the threshold, it will enter
> > > the last round and call migration_completion(). It will stop the
> > > source and sent the remaining dirty pages and devices' status
> > > information to the destination. The destination will load these
> > > information and start the VM.
> > > 
> > > If there is an error at the destination at this time, it will exit
> > > directly, and the source will not be able to detect the error
> > > and recover. Because the source will not call
> > > migration_detect_error().
> > > 
> > > Is my understanding correct?
> > > Should the source wait the result of the last round of destination ?
> > Try setting the 'return-path' migration capability on both the source
> > and destination;  I think it's that option will cause the destination to
> > send an OK/error at the end and the source to wait for it.
> Thank you for your reply!
> The 'return-path' migration capability solved my question. 😁
> 
> But why not set it as the default? In my opinion, it is a basic ability
> of live migration. We need it to judge whether the last round of the
> destination is successful in the way of 'precopy'.

I think it should be enabled as long as both sides support it; though may not
be suitable as default (at least in QEMU) so as to consider old binaries.

Thanks,

-- 
Peter Xu