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Re: [Qemu-block] [PATCH 1/2] block/raw-format: switch to BDRV_BLOCK_DATA

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH 1/2] block/raw-format: switch to BDRV_BLOCK_DATA with BDRV_BLOCK_RECURSE
Date: Tue, 13 Aug 2019 15:54:30 +0000

13.08.2019 18:41, Kevin Wolf wrote:
> Am 13.08.2019 um 16:43 hat Max Reitz geschrieben:
>> 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 <address@hidden>
>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>> 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.
> So we don't even agree what mirroring the raw node should even mean.
> I can the see your point when you say that the raw node has no backing
> file, so everything should be copied. But I can also see the point that
> the raw node can really just be used as a filter that limits the data
> exposed from the qcow2 layer, and you want to keep the copy a COW
> overlay over the same backing file.
> Both are valid use cases in principle and there is no single right or
> wrong.
> We don't currently support the latter use case because we have only
> sync=full or sync=top, but if you could specify a base node instead, we
> could probably suport the case without all of the special-casing filter
> nodes and backing file childs.
> You would call bdrv_co_block_status_above() with the right base node and
> it would just recurse whereever the data is stored, be it bs->backing,
> bs->file or even driver-specific children. This would allow you to find
> out whether some block in the top node came from the base node that
> we're going to keep. If yes, skip it; if no, copy it.
> This way we wouldn't have to decide whether raw is a filter or not,
> because it wouldn't make a difference. The behaviour would only depend
> on the base node given by the user. If you specified the top-level qcow2
> file as the base, you get your full copy;

ahm, full-copy = base is NULL..

> if you specified the backing
> qcow2, you get the partial copy where the target still uses the same
> backing file.
> (Hm... It would only actually work if the offsets stay the same in the
> chain, which is true for backing file children, but not necessarily for
> other children.

Don't follow, what you mean by offsets stay the same and what is wrong with it?

> Anyway, even if we don't gain much functionality, I
> really want a more generic model that avoids different types of nodes
> and edges as much as possible.)
> Kevin

Best regards,

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