Re: [Qemu-devel] [PATCH 1/6 v11] docs: spec for add-cow file format

[Dong Xu Wang]

On Fri, Aug 3, 2012 at 4:26 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Fri, Aug 3, 2012 at 6:56 AM, Dong Xu Wang <address@hidden> wrote:
>> On Thu, Aug 2, 2012 at 6:44 PM, Stefan Hajnoczi <address@hidden> wrote:
>>> On Thu, Aug 2, 2012 at 8:09 AM, Dong Xu Wang <address@hidden> wrote:
>>>> On Wed, Aug 1, 2012 at 9:55 PM, Stefan Hajnoczi <address@hidden> wrote:
>>>>> On Tue, Jul 31, 2012 at 5:51 PM, Dong Xu Wang
>>>>> <address@hidden> wrote:
>>>>>> +test.raw may be larger than ubuntu.img, in that case, the size of 
>>>>>> test.add-cow
>>>>>> +will be calculated by the size of ubuntu.img, test.raw will be used 
>>>>>> from the
>>>>>> +1st byte, the rest part can be used for other purpose.
>>>>>
>>>>> This is not how backing files normally work.  With qcow2 or qed a
>>>>> smaller backing file just means that the guest reads zeroes from the
>>>>> areas beyond the end of the backing file.  Is there a special reason
>>>>> why you want to implement the behavior you described in the spec?
>>>>> Otherwise I suggest implementing the same behavior as qcow2/qed.
>>>>
>>>> The size of add-cow will be caclulated by the size of backing_file, if
>>>> backing_file can be smaller, how can I get the size of add-cow while
>>>> being created?
>>>>
>>>> Do you mean like following steps?
>>>> 1) qemu-img create -f qcow2 source.qcow2 *8G*
>>>> 2) qemu-img create -f t.add-cow -o backing_file=source.qcow2,image_file=t 
>>>> *10G*
>>>> And then reading un-allocated bytes from add-cow after 8G will be 0?
>>>
>>> Yes.  You could also get the virtual disk size from the size of the
>>> image_file during creation.
>>
>> Eric said I shoud make sure  we also support a raw file larger than
>> the backing file. so
>> add-cow will have the same virtual size with image_file, and their
>> virtual size can be larger
>> than backing_file's.
>
> Yes.  Please update the spec because it says the opposite:
>
> "test.raw may be larger than ubuntu.img, in that case, the size of
> test.add-cow will be calculated by the size of ubuntu.img"
Okay. Thanks.
>
> It should use the size of test.raw.
>
> Stefan
>