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I think that's horrible idea. How do we do that store independent with the linking dependencies?

We should not depend universal use case like this on limited subset of backends, specially non-OpenStack ones. Glance (nor Nova) should never depend having direct access to the actual medium where the images are stored. I think this is school book example for something called database. Well arguable if this should be tracked at Glance or Nova, but definitely not a dirty hack expecting specific backend characteristics.

As mentioned before the protected image property is to ensure that the image does not get deleted, that is also easy to track when the images are queried. Perhaps the record needs to track the original state of protected flag, image id and use count. 3 column table and couple of API calls. Lets not at least make it any more complicated than it needs to be if such functionality is desired.


-          Erno

From: Nikhil Komawar [mailto:nikhil.komawar at RACKSPACE.COM]
Sent: 17 December 2014 20:34
To: OpenStack Development Mailing List (not for usage questions)
Subject: Re: [openstack-dev] [glance] Option to skip deleting images in use?

Guess that's a implementation detail. Depends on the way you go about using what's available now, I suppose.

Thanks,
-Nikhil
________________________________
From: Chris St. Pierre [chris.a.st.pierre at gmail.com]
Sent: Wednesday, December 17, 2014 2:07 PM
To: OpenStack Development Mailing List (not for usage questions)
Subject: Re: [openstack-dev] [glance] Option to skip deleting images in use?
I was assuming atomic increment/decrement operations, in which case I'm not sure I see the race conditions. Or is atomism assuming too much?

On Wed, Dec 17, 2014 at 11:59 AM, Nikhil Komawar <nikhil.komawar at rackspace.com<mailto:nikhil.komawar at rackspace.com>> wrote:
That looks like a decent alternative if it works. However, it would be too racy unless we we implement a test-and-set for such properties or there is a different job which queues up these requests and perform sequentially for each tenant.

Thanks,
-Nikhil
________________________________
From: Chris St. Pierre [chris.a.st.pierre at gmail.com<mailto:chris.a.st.pierre at gmail.com>]
Sent: Wednesday, December 17, 2014 10:23 AM
To: OpenStack Development Mailing List (not for usage questions)
Subject: Re: [openstack-dev] [glance] Option to skip deleting images in use?
That's unfortunately too simple. You run into one of two cases:

1. If the job automatically removes the protected attribute when an image is no longer in use, then you lose the ability to use "protected" on images that are not in use. I.e., there's no way to say, "nothing is currently using this image, but please keep it around." (This seems particularly useful for snapshots, for instance.)

2. If the job does not automatically remove the protected attribute, then an image would be protected if it had ever been in use; to delete an image, you'd have to manually un-protect it, which is a workflow that quite explicitly defeats the whole purpose of flagging images as protected when they're in use.

It seems like flagging an image as *not* in use is actually a fairly difficult problem, since it requires consensus among all components that might be using images.

The only solution that readily occurs to me would be to add something like a filesystem link count to images in Glance. Then when Nova spawns an instance, it increments the usage count; when the instance is destroyed, the usage count is decremented. And similarly with other components that use images. An image could only be deleted when its usage count was zero.

There are ample opportunities to get out of sync there, but it's at least a sketch of something that might work, and isn't *too* horribly hackish. Thoughts?

On Tue, Dec 16, 2014 at 6:11 PM, Vishvananda Ishaya <vishvananda at gmail.com<mailto:vishvananda at gmail.com>> wrote:
A simple solution that wouldn't require modification of glance would be a cron job
that lists images and snapshots and marks them protected while they are in use.

Vish

On Dec 16, 2014, at 3:19 PM, Collins, Sean <Sean_Collins2 at cable.comcast.com<mailto:Sean_Collins2 at cable.comcast.com>> wrote:

> On Tue, Dec 16, 2014 at 05:12:31PM EST, Chris St. Pierre wrote:
>> No, I'm looking to prevent images that are in use from being deleted. "In
>> use" and "protected" are disjoint sets.
>
> I have seen multiple cases of images (and snapshots) being deleted while
> still in use in Nova, which leads to some very, shall we say,
> interesting bugs and support problems.
>
> I do think that we should try and determine a way forward on this, they
> are indeed disjoint sets. Setting an image as protected is a proactive
> measure, we should try and figure out a way to keep tenants from
> shooting themselves in the foot if possible.
>
> --
> Sean M. Collins
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> OpenStack-dev mailing list
> OpenStack-dev at lists.openstack.org<mailto:OpenStack-dev at lists.openstack.org>
> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev


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