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On Wed, May 20, 2015 at 06:11:26PM +0800, Luo Gangyi wrote:
> Hi devs,
> 
> 
> I find a serious problem when I try to live migrate a hybrid instance
> (instance based on local disk and cinder volume attached) .
> 
> 
> In current nova, such operation is allowed.
> 
> 
> But I don't think libvirt can deal with such condition correctly.
> 
> 
> When block migration of a hybird intance is triggered, nova will initiate
> a new connection to the volume in destination compute node and then
> call libvirt to do the block migration.
> 
> However, libvirt doesn't distinguish local disk and a network volume,
> it may copy both local disk and cinder volume from source to destination.
> 
> It is dangerous  to write the same volume simutaneously on source and
> destination!! and apparently there is no need to do this!
> 
> I believe we should do something to correct this, maybe patch libvirt?
> or just forbid user do such operation?

Both. Nova now forbids this

commit d667b6a63e80b2f8d6311c2cf224ba32628eed84
Author: Chris St. Pierre <stpierre at metacloud.com>
Date:   Wed Dec 3 16:16:34 2014 -0600

    libvirt: Fail when live block migrating instance with volumes
    
    This raises an exception when attempting to live block migrate (nova
    live-migration --block-migrate) an instance with attached volumes.
    libvirt copies these volumes from themselves to themselves. At a
    minimum, this is horribly slow and de-sparses a sparse volume; at
    worst, this could cause massive data corruption.
    
    Closes-Bug: 1398999
    Change-Id: Ibcd423976bb9fea46e3e1cb23cc8e5cd944d8fc2

And work is being done to allow libvirt to be told to skip cinder
volumes when migrating:

  https://www.redhat.com/archives/libvir-list/2015-May/msg00345.html

Regards,
Daniel
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