Open Stack

I wasn't suggesting that we *actually* use filesystem link count, and make
hard links or whatever for every time the image is used. That's prima facie
absurd, for a lot more reasons that you point out. I was suggesting a new
database field that tracks the number of times an image is in use, by
*analogy* with filesystem link counts. (If I wanted to be unnecessarily
abrasive I might say, "This is a textbook example of something called an
analogy," but I'm not interested in being unnecessarily abrasive.)

Overloading the protected flag is *still* a terrible hack. Even if we
tracked the initial state of "protected" and restored that state when an
image went out of use, that would negate the ability to make an image
protected while it was in use and expect that change to remain in place. So
that just violates the principle of least surprise. Of course, we could
have glance modify the "original_protected_state" flag when that flag is
non-null and the user changes the actual "protected" flag, but this is just
layering hacks upon hacks. By actually tracking the number of times an
image is in use, we can preserve the ability to protect images *and* avoid
deleting images in use.

On Thu, Dec 18, 2014 at 5:27 AM, Kuvaja, Erno <kuvaja at hp.com> wrote:

>  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> 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]
> *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>
> 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> 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
> > _______________________________________________
> > OpenStack-dev mailing list
> > OpenStack-dev at lists.openstack.org
> > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>
>
> _______________________________________________
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> OpenStack-dev at lists.openstack.org
> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>
>
>
>
>
> --
>
> Chris St. Pierre
>
>
> _______________________________________________
> OpenStack-dev mailing list
> OpenStack-dev at lists.openstack.org
> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>
>
>
>
>
> --
>
> Chris St. Pierre
>
> _______________________________________________
> OpenStack-dev mailing list
> OpenStack-dev at lists.openstack.org
> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
>
>


-- 
Chris St. Pierre
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