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Great, Jon. Thanks for your reply. I am looking forward to your report.


Cheers,
Boxiang
On 10/23/2018 22:01，Jon Bernard<jobernar at redhat.com> wrote：
* 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.

--
Jon


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

Cheers,
-melanie






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