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Updated report for 'no image' with deleted '_base' behaviour in juno (my 
previous comment was about havana):

1. If snapshot is removed, original image is used (image that was used 
for 1st instance to produce snapshot). Rather strange and unexpected, 
but nice (minus one headache).
2. If all images in chain are removed, behaviour changed:
* hard reboot works fine (raw disks)
* reinstallation asks for new image, seems no problem
* rescue causes ugly problem, rendering instance completely broken (do 
not work but no ERROR state). https://bugs.launchpad.net/nova/+bug/1418590

I didn't test migrations yet.

On 02/05/2015 03:09 PM, George Shuklin wrote:
> Hello everyone.
>
> We are updating our public images regularly (to provide them to 
> customers in up-to-date state). But there is a problem: If some 
> instance starts from image it becomes 'used'. That means:
> * That image is used as _base for nova
> * If instance is reverted this image is used to recreate instance's disk
> * If instance is rescued this image is used as rescue base
> * It is redownloaded during resize/migration (on a new compute node)
>
> One more (our specific):
> We're using raw disks with _base on slow SATA drives (in comparison to 
> fast SSD for disks), and if that SATA fails, we replace it (and nova 
> redownloads stuff in _base).
>
> If image is deleted, it causes problems with nova (nova can't download 
> _base).
>
> The second part of the problem: glance disallows to update image 
> (upload new image with same ID), so we're forced to upload updated 
> image with new ID and to remove the old one. This causes problems 
> described above. And if tenant boots from own snapshot and removes 
> snapshot without removing instance, it causes same problem even 
> without our activity.
>
> How do you handle public image updates in your case?
>
> Thanks!