Re: RBD, OpenStack Nova, libvirt, qemu-guest-agent, and FIFREEZE: is this working as intended?

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Florian;

Forgive my lack of knowledge of OpenStack, and your environment / use case.

Why would you need / want to snapshot an ephemeral disk?  Isn't the point of ephemeral storage to not be persistent?

Thank you,

Dominic L. Hilsbos, MBA 
Director - Information Technology 
Perform Air International Inc.
DHilsbos@xxxxxxxxxxxxxx 
www.PerformAir.com


-----Original Message-----
From: Florian Haas [mailto:florian@xxxxxxxxxxxxxx] 
Sent: Wednesday, August 21, 2019 8:41 AM
To: dillaman@xxxxxxxxxx
Cc: ceph-users
Subject:  Re: RBD, OpenStack Nova, libvirt, qemu-guest-agent, and FIFREEZE: is this working as intended?

Hi Jason! Thanks for the quick reply.

On 21/08/2019 16:51, Jason Dillaman wrote:>
> It just looks like this was an oversight from the OpenStack developers
> when Nova RBD "direct" ephemeral image snapshot support was added [1].
> I would open a bug ticket against Nova for the issue.>
> [1]
https://opendev.org/openstack/nova/commit/824c3706a3ea691781f4fcc4453881517a9e1c55

OK, wow... that was 4 years ago, does that mean that quiesce/freeze/thaw
for RBD-backed Nova instances has probably been non-functional
throughout that time?

Looking at the reno for that commit I had an idea for a workaround:

> features:
>  - When RBD is used for ephemeral disks and image storage, make
>    snapshot use Ceph directly, and update Glance with the new location.
>    In case of failure, it will gracefully fallback to the "generic"
>    snapshot method.  This requires changing the typical permissions
>    for the Nova Ceph user (if using authx) to allow writing to
>    the pool where vm images are stored, and it also requires
>    configuring Glance to provide a v2 endpoint with direct_url
>    support enabled (there are security implications to doing this).
>    See http://docs.ceph.com/docs/master/rbd/rbd-openstack/ for more
>    information on configuring OpenStack with RBD.

So, suppose that deployers running Nova with ephemeral disks on RBD
prefer snapshot consistency over this shortcut. Until Nova fixes the
direct_snapshot() call, I figured that such deployers could tweak the
caps for the Nova CephX identity such that that user were no longer
allowed to write to the Glance pool.

Under those circumstances, the snapshot creation (in the ephemeral pool)
would work, but then the clone() call in this line should throw
nova.exception.Forbidden from an rbd.PermissionError:

https://opendev.org/openstack/nova/src/commit/7bf75976016aae5d458eca9f6ddac92bfe75dc59/nova/virt/libvirt/imagebackend.py#L1075

Which should then trigger this except block:

https://opendev.org/openstack/nova/src/commit/7bf75976016aae5d458eca9f6ddac92bfe75dc59/nova/virt/libvirt/driver.py#L2065

... and Nova/libvirt should go back to the (arguably more correct) fallback.

Would you agree with that assessment, or am I missing something? (Just
trying to make sure that I don't give the Nova folks the wrong facts.)

Thanks again!

Cheers,
Florian
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