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Another way to make it transparent is using the RBD image backend in Nova
and thus the instances look like they are running on local disk but is spawned on for
example Ceph, however that assumes you have such a backend.

In the future, which I’ve wanted for a long time, is a images backend in Nova that simply
is a proxy to calling Cinder and getting a volume, that way it would be volumes in both cases
but that’s a lot of work and edge cases that might need to be tuned.

On 4 Apr 2022, at 00:28, Laurent Dumont <laurentfdumont at gmail.com<mailto:laurentfdumont at gmail.com>> wrote:

Bummer :(

We have a couple of use cases where I would like this to be transparent and things to automagically-unicorn and rainbows ;)  Thanks for the insight Radoslaw!

On Sun, Apr 3, 2022 at 8:12 AM Radosław Piliszek <radoslaw.piliszek at gmail.com<mailto:radoslaw.piliszek at gmail.com>> wrote:
On Sun, 3 Apr 2022 at 12:46, Laurent Dumont <laurentfdumont at gmail.com<mailto:laurentfdumont at gmail.com>> wrote:
>
> Got it!
>
> By quota, do you mean reserved_host_disk_mb in nova.conf? I could make the /var/lib/nova/instances RO but I am not sure how that would impact config drive that are created locally (since I dont have ceph)

I meant the filesystem quota.
And yes, this affects config drives.
Unfortunately, the error message from nova might be confusing.

> Just to be clear on the behavior, this means that boot-from-image requests would fail?
>
> Looking at nova.conf, I can disable the number of local-disks supported, but this doesn't act as a behavior change when the requests are made.
>
> I assume, from what I now know, that there is no mechanism to default/transform a request to BFV.
>
> # A negative number means unlimited. Setting max_local_block_devices
> # to 0 means that any request that attempts to create a local disk
> # will fail. This option is meant to limit the number of local discs
> # (so root local disc that is the result of --image being used, and
> # any other ephemeral and swap disks). 0 does not mean that images
> # will be automatically converted to volumes and boot instances from
> # volumes - it just means that all requests that attempt to create a
> # local disk will fail.
> #
> # Possible values:
> #
> # * 0: Creating a local disk is not allowed.
> # * Negative number: Allows unlimited number of local discs.
> # * Positive number: Allows only these many number of local discs.
> #                        (Default value is 3).
> #  (integer value)
> #max_local_block_devices = 3

It seems this is actually the best approach (the error message now
makes sense). Also confirmed by the faq -
https://opendev.org/openstack/nova/src/commit/b0851b0e9c82446aec2ea0317514766fbc53abc0/doc/source/user/block-device-mapping.rst#faqs

-yoctozepto

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