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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 14:56:11 +0000

13.08.2019 17:43, Max Reitz wrote:
> 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.

Could you tell me at least, what means "allocated" ?

It's a term that describing a region somehow.. But how? Allocated where?
In raw node, in its child or both? Am I right that if region allocated in
one of non-cow children it is assumed to be allocated in parent too? Or what?

And it's unrelated to real disk allocation which (IMHO) directly shows that
this a bad term.

Best regards,

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