[libvirt] [PATCH 0/2] Alter refresh algorithm for volWipe

[John Ferlan]

On 08/25/2017 08:28 AM, Martin Kletzander wrote:
> On Fri, Aug 25, 2017 at 07:52:25AM -0400, John Ferlan wrote:
>>
>>
>> On 08/25/2017 05:44 AM, Martin Kletzander wrote:
>>> On Thu, Aug 24, 2017 at 06:28:27PM -0400, John Ferlan wrote:
>>>> Alter wipeVol to do same refresh operation as pool refresh would do.
>>>>
>>>
>>> I think we should rather keep the format as it is.  Did I miss something
>>> here or isn't the source of the problem just the fact that we wipe the
>>> volume without keeping the format?
>>>
>>
>> Once you "wipe" the file/image the format that was there (such as qcow2
>> as noted in the bz from patch2) is no longer there.
>>
>> Consider the following sequence:
>>
>> virsh vol-create-as default bz 10M --format qcow2
>>
>> virsh vol-dumpxml bz default | grep format
>>    <format type='qcow2'/>
>>
>> qemu-img info /home/vm-images/bz
>> image: /home/vm-images/bz
>> file format: qcow2
>> virtual size: 10M (10485760 bytes)
>> disk size: 196K
>> cluster_size: 65536
>> Format specific information:
>>    compat: 0.10
>>    refcount bits: 16
>>
>> virsh vol-wipe bz default
>>
>> qemu-img info /home/vm-images/bz
>> image: /home/vm-images/bz
>> file format: raw
>> virtual size: 196K (200704 bytes)
>> disk size: 196K
>>
>> virsh vol-dumpxml bz default | grep format
>>    <format type='qcow2'/>
>>
>> (without the patch in 2/2 of course)
>>
>> BTW: I did consider just changing the format to RAW regardless, but
>> figured that was just too simple and may not be the right answer for
>> every case.
>>
> 
> Sure, but that's not my point.  My point is that instead of rewriting
> the data with zeros, we could re-initialize it.  Anyway, from what I see
> in the docs, we "fixed" it by documenting this behaviour already, so my
> point is moo:
> 
> https://libvirt.org/html/libvirt-libvirt-storage.html#virStorageVolWipe
> 

Doh! I should have looked there too...  Interesting though... So should
we instead close this bug as documented to work that way?  Or should the
text there be modified as well since the bz will cause the alteration to
at least RAW for "most" storage backends - I think it's only the disk
backend w/ an extended partition and a sparse logical volume that cannot
be wiped, but those fail the wipe - so we'd never get to the change the
target.format code.

John


> (I guess I should know that when I pushed that documentation into the tree)
> 
> So ACK series.