[Openstack] [Nova][Glance] Nova imports flat images from base file despite ceph backend

Eugen Block eblock at nde.ag
Fri Oct 12 21:50:27 UTC 2018

Hi Melanie,

thanks for your response.

I would consider this thread as closed. After I learned that nova  
imports flat images if the image's disk-format is not "raw" I tested  
some different scenarios to understand more about this topic. I still  
couldn't explain why nova did that in the specific case as I was sure  
the image's format was raw (the image has been deleted in the meantime  
but I can see in the database that the image's disk-format was indeed  

But a couple of days ago I encountered the same problem. I tried to  
upload an image from volume via Horizon dashboard (where a dropdown  
exists to choose disk-format) and the preselected value was indeed  
"qcow2" instead of "raw". I have no idea *why* it was different from  
the default "raw" but this could have happened last week, too, althoug  
according to the database entry it should not have happened. Anyway,  
now I know the reason why some ephemeral disks are flat, I just can't  
tell how Horizon selects the format, but that's enough research for  
now. If I find out more I'll report back.

Thanks again!

Zitat von melanie witt <melwittt at gmail.com>:

> On Tue, 09 Oct 2018 08:01:01 +0000, Eugen Block wrote:
>> So it's still unclear why nova downloaded a raw glance image to the
>> local filesystem during the previous attempt.
>> I always knew that with Ceph as backend it's recommended to use raw
>> images but I always assumed the "disk-format" was not more than a
>> display option. Well, now I know that, but this still doesn't explain
>> the downloaded base image.
>> If anyone has an idea how to reproduce such behavior or an
>> explanation, I'd love to hear it.
> Right, in order to get the ceph CoW behavior, you must use RAW  
> glance image format [1]. You also need to have created the image as  
> 'raw' before you upload to glance (qemu-img info should show the  
> file format as 'raw').
> If you have done this, then you shouldn't see any image downloaded  
> to the local filesystem. Is it possible that your image did not have  
> file format 'raw' before you uploaded it to glance?
> Cheers,
> -melanie
> [1] http://docs.ceph.com/docs/master/rbd/rbd-openstack
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