Re: [Qemu-devel] nbd: Possible regression in 2.9 RCs

[Eric Blake <eblake@redhat.com>]

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On 04/03/2017 08:00 AM, Kevin Wolf wrote:
>> The question remains whether it is practical not to make an exception.
>> As far as I know, libvirt is only guaranteed to support older qemu
>> versions, not necessarily future ones. So we should be allowed to break
>> existing use cases here until libvirt is updated (assuming it is
>> possible for libvirt to express "guest device allows shared writes" as
>> an option for its next release).
> 
> If I understand correctly, this is a case of incoming live migration,
> i.e. the virtio-blk device which is blocking the writes to the image
> doesn't really belong to a running guest yet.
> 
> So if we need to make an exception (and actually reading the context
> makes it appear so), I guess it would have to be that devices actually
> can share the write permission during incoming migration, but not any
> more later (unless the share-rw flag is set).

That sounds like the right approach - if nbd-server-add can detect that
it is being invoked during the window between 'qemu -S' and before
'migrate', and allow the shared-rw at that time, then revoke it at the
time the migrate command hits.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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