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

[Stefan Hajnoczi]

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"

It should use the size of test.raw.

Stefan