Re: [Qemu-devel] [PATCH 2/3] move vm stop/start to migrate_set_state

[malc]

On Sat, 11 Jul 2009, Jamie Lokier wrote:

> Anthony Liguori wrote:
> > Paolo Bonzini wrote:
> > >On 07/09/2009 03:45 PM, Anthony Liguori wrote:
> > >>How does the disk become full during the final stage?  The guest isn't
> > >>running.
> > >
> > >The host disk can become full and cause a "migrate exec" to fail.  Or 
> > >for network migration migration, you could have the connection drop 
> > >exactly during the final stage.  In this case, the VM would be 
> > >unconditionally restarted.
> > 
> > Because migration failed.  Is that not the desired behavior?  It seems 
> > like it is to me.
> > 
> > If I try to do a live migration, it should either succeed and my guest 
> > experiences minimal downtime or it should fail and my guest should 
> > experience minimal downtime.
> 
> What happens if the destination host sends "migration completed", and
> then the connection drops before that message is delivered reliably to
> the sending host?
> 
> The destination host will run the VM,
> and the sending host will restart and run the VM too.
> 
> Two copies of the same VM running together doesn't sound healthy.
> 
> This is a classic handshaking problem and I'm not aware of any perfect
> solution, only ways to ensure eventual recovery, and temporary
> uncertainty errs on the side of caution.  In this case, caution would
> be neither VM running but a notification to the system manager of this
> rare condition, and the possibility to recover when the two hosts are
> able to resume communication.  I don't know how to do better than that.

Sounds like http://en.wikipedia.org/wiki/Two_Generals%27_Problem

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