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

[Dong Xu Wang]

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.

Am I right?
Thanks.

> Stefan
>