[openstack-dev] [cinder] [nova] Problem of Volume(in-use) Live Migration with ceph backend

Jon Bernard jobernar at redhat.com
Tue Oct 23 14:01:42 UTC 2018

* melanie witt <melwittt at gmail.com> wrote:
> On Mon, 22 Oct 2018 11:45:55 +0800 (GMT+08:00), Boxiang Zhu wrote:
> > I created a new vm and a new volume with type 'ceph'[So that the volume
> > will be created on one of two hosts. I assume that the volume created on
> > host dev at rbd-1#ceph this time]. Next step is to attach the volume to the
> > vm. At last I want to migrate the volume from host dev at rbd-1#ceph to
> > host dev at rbd-2#ceph, but it failed with the exception
> > 'NotImplementedError(_("Swap only supports host devices")'.
> > 
> > So that, my real problem is that is there any work to migrate
> > volume(*in-use*)(*ceph rbd*) from one host(pool) to another host(pool)
> > in the same ceph cluster?
> > The difference between the spec[2] with my scope is only one is
> > *available*(the spec) and another is *in-use*(my scope).
> > 
> > 
> > [1] http://docs.ceph.com/docs/master/rbd/rbd-openstack/
> > [2] https://review.openstack.org/#/c/296150
> Ah, I think I understand now, thank you for providing all of those details.
> And I think you explained it in your first email, that cinder supports
> migration of ceph volumes if they are 'available' but not if they are
> 'in-use'. Apologies that I didn't get your meaning the first time.
> I see now the code you were referring to is this [3]:
> if volume.status not in ('available', 'retyping', 'maintenance'):
>     LOG.debug('Only available volumes can be migrated using backend '
>               'assisted migration. Falling back to generic migration.')
>     return refuse_to_migrate
> So because your volume is not 'available', 'retyping', or 'maintenance',
> it's falling back to generic migration, which will end up with an error in
> nova because the source_path is not set in the volume config.
> Can anyone from the cinder team chime in about whether the ceph volume
> migration could be expanded to allow migration of 'in-use' volumes? Is there
> a reason not to allow migration of 'in-use' volumes?

Generally speaking, Nova must facilitate the migration of a live (or
in-use) volume.  A volume attached to a running instance requires code
in the I/O path to correctly route traffic to the correct location - so
Cinder must refuse (or defer) a migrate operation if the volume is
attached.  Until somewhat recently Qemu and Libvirt did not support the
migration to non-block (RBD) targets which is the reason for lack of
support.  I believe we now have all of the pieces to perform this
operation successfully, but I suspect it will require a setup with
correct versions of all the related software.  I will try to verify this
during the current release cycle and report back.


> [3] https://github.com/openstack/cinder/blob/c42fdc470223d27850627fd4fc9d8cb15f2941f8/cinder/volume/drivers/rbd.py#L1618-L1621
> Cheers,
> -melanie
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