[openstack-dev] [cinder] Dependencies of snapshots on volumes

Chris Friesen chris.friesen at windriver.com
Wed Dec 9 17:03:30 UTC 2015

On 12/09/2015 10:27 AM, John Griffith wrote:
> On Tue, Dec 8, 2015 at 9:10 PM, Li, Xiaoyan <xiaoyan.li at intel.com
> <mailto:xiaoyan.li at intel.com>> wrote:
>     Hi all,
>     Currently when deleting a volume, it checks whether there are snapshots
>     created from it. If yes deletion is prohibited.  But it allows to extend
>     the volume, no check whether there are snapshots from it.
> ​Correct​
>     The two behaviors in Cinder are not consistent from my viewpoint.
> ​Well, your snapshot was taken at a point in time; and if you do a create from
> snapshot the whole point is you want what you HAD when the snapshot command was
> issued and NOT what happened afterwards.  So in my opinion this is not
> inconsistent at all.

If we look at it a different way...suppose that the snapshot is linked in a 
copy-on-write manner with the original volume.  If someone deletes the original 
volume then the snapshot is in trouble.  However, if someone modifies the 
original volume then a new chunk of backing store is allocated for the original 
volume and the snapshot still references the original contents.

If we did allow deletion of the volume we'd have to either keep the volume 
backing store around as long as any snapshots are around, or else flatten any 
snapshots so they're no longer copy-on-write.


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