[glance] zeroing image, preserving other parameters

Volodymyr Litovka doka.ua at gmx.com
Thu Jun 6 10:49:37 UTC 2019

Hi Brian,

thanks for the response. I solved my issue from other (client) side -
I'm using Heat and Heat don't look whether uuid of image changed, it
just check for existense of image with specified name. So it's safe to
delete image and then create another one with same name and parameters
and zero size.

But in fact, Glance has a bit contradictory approach:

Documentation on db purge says: "Remember that image identifiers are
used by other OpenStack services that require access to images. These
services expect that when an image is requested by ID, they will receive
the same data every time." but there are no ways to get list of images
including 'deleted' or details of 'deleted' image, e.g.

doka at lagavulin(admin at admin):~$ openstack image show
Could not find resource b179ecee-775d-4ee4-81c0-d3ec3a769d35

so preserving image record in database makes no sense for 3rd party
services, which talk to Glance over public API.

On the other hand, having in DB API ready for use 'image_destroy' call,
it's pretty easy (of course, for those who work with Glance code :-) )
to add public API call kind of images/{image_id}/actions/destroy ,
calling DB API's image_destroy. And, in that case, it makes sense to
allow image uuid to be specified during image create (since client can
purge specified record and recreate it using same characteristics),
otherwise I don't see where, in general, specifying uuid (when creating
image) can be useful.

The good news is that I solved my problem. The bad news is that solution
relies on relaxed requirements of 3rd party products but not on Glance's
API itself :-)


On 6/5/19 5:38 PM, Brian Rosmaita wrote:
> On 6/5/19 8:34 AM, Volodymyr Litovka wrote:
>> Dear colleagues,
>> for some reasons, I need to shrink image size to zero (freeing storage
>> as well), while keeping this record in Glance database.
>> First which come to my mind is to delete image and then create new one
>> with same name/uuid/... and --file /dev/null, but this is impossible
>> because Glance don't really delete records from database, marking them
>> as 'deleted' instead.
> The glance-manage utility program allows you to purge the database.  The
> images table (where the image UUIDs are stored) is not purged by default
> because of OSSN-0075 [0].  See the glance docs [1] for details.
> [0] https://wiki.openstack.org/wiki/OSSN/OSSN-0075
> [1]
> https://docs.openstack.org/glance/latest/admin/db.html#database-maintenance
> (That doesn't really help your issue, I just wanted to point out that
> there is a way to purge the database.)
>> Next try was to use glance image-upload from /dev/null, but this is also
>> prohibited with message "409 Conflict: Image status transition from
>> [activated, deactivated] to saving is not allowed (HTTP 409)"
> That's correct, Glance will not allow you to replace the image data once
> an image has gone to 'active' status.
>> I found
>> https://docs.openstack.org/glance/rocky/contributor/database_architecture.html#glance-database-public-api's
>> "image_destroy" but have no clues on how to access this API. Is it kind
>> of library or kind of REST API, how to access it and whether it's safe
>> to use it in terms of longevity and compatibility between versions?
> The title of that document is misleading.  It describes the interface
> that Glance developers can use when they need to interact with the
> database.  There's no tool that exposes those operations to operators.
>> Or, may be, you can advise any other methods to solve the problem of
>> zeroing glance image data / freeing storage, while keeping in database
>> just a record about this image?
> If you purged the database, you could do your proposal to recreate the
> image with a zero-size file -- but that would give you an image with
> status 'active' that an end user could try to boot an instance with.  I
> don't think that's a good idea.  Additionally, purging the images table
> of all UUIDs, not just the few you want to replace, exposes you to
> OSSN-0075.
> An alternative--and I'm not sure this is a good idea either--would be to
> deactivate the image [2].  This would preserve all the current metadata
> but not allow the image to be downloaded by a non-administrator.  With
> the image not in 'active' status, nova or cinder won't try to use it to
> create instances or volumes.  The image data would still exist, though,
> so you'd need to delete it manually from the backend to really clear out
> the space.  Additionally, the image size would remain, which might be
> useful for record-keeping, although on the other hand, it will still
> count against the user_storage_quota.  And the image locations will
> still exist even though they won't refer to any existing data any more.
>   (Like I said, I'm not sure this is a good idea.)
> [2] https://developer.openstack.org/api-ref/image/v2/#deactivate-image
>> Thank you.
> Not sure I was much help.  Let's see if other operators have a good
> workaround or a need for this kind of functionality.
>> --
>> Volodymyr Litovka
>>    "Vision without Execution is Hallucination." -- Thomas Edison

Volodymyr Litovka
   "Vision without Execution is Hallucination." -- Thomas Edison

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