openstack team mailing list archive

[Eoghan Glynn <eglynn@xxxxxxxxxx>]

> > Current warts:
> > ...
> >  - maintaining amazon ec2 ids across regions requires twiddling the
> >  nova database where this mapping is stored
> 
> 
> Hi Mikal,
> 
> We discussed that nova s3_images table earlier in the week on IRC.
> Now  at the time, I wasn't fully clear on the mechanics of the glance
> UUID -> EC2-style image ID mapping.
> 
> I've had a dig into the code since then and it turns out that the EC2
> image ID is effectively determined by an auto_increment column in the
> s3_images table.
> 
> So there is nothing globally significant about that ID, in fact it's
> completely determined by the insertion order into the nova s3_images
> table, which occurs *on demand* the first time the image is being
> referenced in an EC2-related context (for example the first call to
> DescribeImages after the image has been uploaded to glance).
> 
> For example in a fresh openstack deployment, if three images are
> uploaded with types aki, ari & aki respectively, the first
> EC2-related
> reference to the images will result in the following EC2 IDs always
> being generated:
> 
>   aki-00000001
>   ari-00000002
>   ami-00000003
> 
> Now, if the underlying glance images are migrated to another
> long-standing openstack deployment, it's likely that there are
> already
> multiple images referenced in the local s3_images table, so that the
> identifiers a[krm]i-0000000[1-3] are already taken.
> 
> So the way things currently stand, the EC2 image ID isn't really
> capable of
> migration.
> 
> I was thinking however that we should change the EC2 image generation
> logic,
> so that there is a reproducible glance UUID -> EC2 mapping (with a
> small
> chance of collision). This change would allow the same EC2 ID to be
> generated
> in multiple regions for a given glance UUID (modulo collisions).
> 
> Would that be helpful in your migration use-case?


Of course, the other way of looking at this would be to just follow the
lead of EC2 itself and not provide an image ID portability across regions.

Though I think we can do better than that by compressing the UUID in a
predictable way.

Cheers,
Eoghan