[libvirt-users] Create qcow2 v3 volumes via libvirt
Paul O'Rorke
paul at tracker-software.com
Tue May 1 21:54:38 UTC 2018
Thanks for the suggestions Eric,
great stuff and most appreciated.
I must play in my sandbox...
*Paul O'Rorke*
*Tracker Software Products (Canada) Limited *
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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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