Re: [Qemu-devel] qcow2 snapshot + resize

[Max Reitz]

On 06.01.2016 16:04, Eric Blake wrote:
> On 01/05/2016 07:50 PM, lihuiba wrote:
>> At 2016-01-05 21:55:56, "Eric Blake" <address@hidden> wrote:
>>> On 01/05/2016 05:10 AM, lihuiba wrote:
>>>
>>>>>> In our production environment, we need to extend a qcow2 image with
>>>>>> snapshots in it.
>>>
>>>>> The thing is that one would need to update all the inactive L1 tables. I
>>>>> don't think it should be too difficult, it's just that apparently so far
>>>>> nobody ever had the need for this feature.
>>>
>>> Is resizing a snapshot really what you want?  Ideally, a snapshot tracks
>>> the data from a point in time, including the metadata of the size being
>>> tracked at that time.  Extending the snapshots then reverting to that
>>> snapshot means your guest would see a larger disk on revert than it did
>>> at the time the snapshot was created, which guests might not handle very
>>> well.
>> I want to make resizing (extending only) and snapshot independent to each 
>> other,otherwise going back and forth in snapshots may cause the disk 
>> shrinked and extended. That would introduce some technical trouble, and 
>> possibly confuse user as well.
> 
> If I take a snapshot while the guest sees a 1G disk, then resize the
> disk to 2G, then roll back to the point in time of the snapshot, I'd
> expect the disk to roll back to 1G in size.  Anything else is likely to
> confuse the guest.  And that's what current resize support already does
> (it only resizes the active image, not the snapshots).

No, the current resize operation just refuses to resize the image if it
has any snapshots. Snapshots currently do not store the size of the
image when they were created.

Max

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