[openstack-dev] [nova][cinder] Nova is now using the Cinder volume attachments API - multiattach comes next

Matt Riedemann mriedemos at gmail.com
Sat Dec 9 19:15:25 UTC 2017

I just wanted to take a minute to recognize that the patch [1] to make 
Nova use the new-style Cinder volume attach flow has merged.

As one can tell from the patch set count alone, this has been in the 
works for a long time now.

We were talking about volume multi-attach support back in Mitaka at the 
midcycle in Bristol. The early talks / patches would have piled more 
technical debt onto Nova and further baked in Nova's need to manage 
volume state, which is something we wanted to avoid, so at the Newton 
summit in Austin we changed course and decided to prioritize working on 
a new data model and API in Cinder, which eventually came out as the 
3.27 volume attachments API in Ocata.

There was then serious work on both sides in Pike to start using the new 
3.27 API and we found out we needed some more changes to Cinder, which 
became the 3.44 Cinder API in Queens. Now Nova has merged the change at 
the end of a very long series of changes to enable the new flow once 
3.44 is available to Nova and computes are all upgraded to understand 
the new flow. One can appreciate the complexity here if you read through 
the Nova spec for the new attach flow [2].

I was really happy to see [1] merge this week because I knew we needed 
to get that in by the queens-2 milestone if we were going to have a shot 
at (1) flushing out stability issues before Queens RC1 and (2) a good 
chance to get multi-attach support into Nova in Queens.

We have a plan for multi-attach support [3] which I think is doable 
before feature freeze. The Cinder 3.48 API is now available too which 
Nova needs to correctly detach a multi-attach volume.

I want to thank everyone that's helped push this along for their 
dedication and patience, especially John Griffith, Ildiko Vancsa, Steve 
Noyes and John Garbutt. And thanks to Sean McGinnis, Jay Bryant, Walter 
Boring and Balazs Gibizer for review support.

We've had weekly meetings between the Nova and Cinder teams for at least 
two years now and I can finally see the end so let's keep up the momentum.

[1] https://review.openstack.org/#/c/330285/




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