On 13.08.19 13:04, Kevin Wolf wrote:
> Am 12.08.2019 um 20:11 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> BDRV_BLOCK_RAW makes generic bdrv_co_block_status to fallthrough to
>> returned file. But is it correct behavior at all? If returned file
>> itself has a backing file, we may report as totally unallocated and
>> area which actually has data in bottom backing file.
>>
>> So, mirroring of qcow2 under raw-format is broken. Which is illustrated
>> by following commit with a test. Let's make raw-format behave more
>> correctly returning BDRV_BLOCK_DATA.
>>
>> Suggested-by: Max Reitz <mreitz@redhat.com>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> 
> After some reading, I think I came to the conclusion that RAW is the
> correct thing to do. There is indeed a problem, but this patch is trying
> to fix it in the wrong place.
> 
> In the case where the backing file contains some data, and we have a
> 'raw' node above the qcow2 overlay node, the content of the respective
> block is not defined by the queried backing file layer, so it is
> completely correct that bdrv_is_allocated() returns false,like it would
> if you queried the qcow2 layer directly.

I disagree.  The queried backing file layer is the raw node.  As I said,
in my opinion raw nodes are not filter nodes, neither in behavior (they
have an offset option), nor in how they are generally used (as a format).

The raw format does not support backing files.  Therefore, everything on
a raw node is allocated.

(That is, like, my opinion.)

>                                          If it returned true, we would
> copy everything, which isn't right either (the test cases should may add
> the qemu-img map output of the target so this becomes visible).

It is right.

Max