[libvirt-users] Create qcow2 v3 volumes via libvirt

[Paul O'Rorke]

Thanks for the suggestions Eric, great stuff and most appreciated.

I will have a play in my sandbox...

*Paul O'Rorke*
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On 2018-05-01 02:31 PM, Eric Blake wrote:
> On 05/01/2018 04:17 PM, Paul O'Rorke wrote:
>> I have been using internal snapshots on production qcow2 images for a 
>> couple of years, admittedly as infrequently as possible with one 
>> exception and that exception has had multiple snapshots taken and 
>> removed using virt-manager's GUI.
>>
>> I was unaware of this:
>>> There are some technical downsides to
>>> internal snapshots IIUC, such as inability to free the space used by 
>>> the
>>> internal snapshot when it is deleted,
>
> This is not an insurmountable difficulty, just one that no one has 
> spent time coding up.
>
>>
>> This might explain why this VM recently kept going into a paused 
>> state and I had to extend the volume to get it to stay up.  This VM 
>> is used for testing our software in SharePoint and we make heavy use 
>> of snapshots.  Is there nothing I can to do recover that space?
>
> If you have no internal snapshots, you can do a 'qemu-img convert' to 
> copy just the portion of the image that is actively in use; the copy 
> will use less disk space than the original because it got rid of the 
> now-unused space.  'virt-sparsify' from libguestfs takes this one step 
> further, by also removing unused space within the guest filesystem 
> itself.
>
> In fact, even if you do have internal snapshots, there is probably a 
> sequence of 'qemu-img convert' invocations that can ultimately convert 
> all of your internal snapshots into an external chain of snapshots; 
> but I don't have a ready formula off-hand to point to (experiment on 
> an image you don't care about, before doing it on your production image).
>
>> What would be the best practice then for a VM that needs to be able 
>> to create and remove snapshots on a regular basis?
>
> In a managed environment, external snapshots probably have the most 
> support for creating and later merging portions of a chain of 
> snapshots, although we could still improve libvirt to make this feel 
> like more of a first class citizen.
>

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