Open Stack

The change Dan refers to merged in Kilo. I assume that if you actually
found this problem then you're using an earlier OpenStack, so it's
worth noting that there's an outstanding backport:
https://review.openstack.org/#/c/176768/

Matthew


On Wed, May 20, 2015 at 3:21 AM, Daniel P. Berrange <berrange at redhat.com> wrote:
> 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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