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On 07/15/2015 01:41 PM, Andrew Laski wrote:
> On 07/15/15 at 12:19pm, Matt Riedemann wrote:
<snip>
>> The other part of the discussion is around the API changes, not just
>> for libvirt, but having a microversion that removes the device from
>> the request so it's no longer optional and doesn't provide some false
>> sense that it works properly all of the time.  We talked about this in
>> the nova channel yesterday and I think the thinking was we wanted to
>> get agreement on dropping that with a microversion before moving
>> forward with the libvirt change you have to ignore the requested
>> device name.
>>
>> From what I recall, this was supposed to really only work reliably for
>> xen but now it actually might not, and would need to be tested again.
>> Seems we could start by checking the xen CI to see if it is running
>> the test_minimum_basic scenario test or anything in
>> test_attach_volume.py in Tempest.
> 
> This doesn't really work reliably for xen either, depending on what is
> being done.  For the xenapi driver Nova converts the device name
> provided into an integer based on the trailing letter, so 'vde' becomes
> 4, and asks xen to mount the device based on that int.  Xen does honor
> that integer request so you'll get an 'e' device, but you could be
> asking for hde and get an xvde or vice versa.

So this sounds like it's basically not working today. For Linux guests
it really can't work without custom in guest code anyway, given how
device enumeration works.

That feels to me like we remove it from the API with a microversion, and
when we do that just comment that trying to use this before that
microversion is highly unreliable (possibly dangerous) and may just
cause tears.

...

On a slight tangent, probably a better way to provide mount stability to
the guest is with FS labels. libvirt is already labeling the filesystems
it creates, and xenserver probably could as well. The infra folks ran
into an issue yesterday
http://status.openstack.org//elastic-recheck/#1475012 where using that
info was their fix.

It's not the same thing as deterministic devices, but deterministic
devices really aren't a thing on first boot unless you have guest agent
code, or only boot with one disk and hot plug the rest carefully.
Neither are really fun answers.

	-Sean

-- 
Sean Dague
http://dague.net