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Re: [Qemu-block] [PATCH 0/2] deal with BDRV_BLOCK_RAW

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH 0/2] deal with BDRV_BLOCK_RAW
Date: Tue, 13 Aug 2019 09:33:11 +0000

13.08.2019 12:01, Vladimir Sementsov-Ogievskiy wrote:
> 13.08.2019 11:39, Vladimir Sementsov-Ogievskiy wrote:
>> 12.08.2019 22:50, Max Reitz wrote:
>>> On 12.08.19 21:46, Max Reitz wrote:
>>>> On 12.08.19 20:11, Vladimir Sementsov-Ogievskiy wrote:
>>>>> Hi all!
>>>>> I'm not sure, is it a bug or a feature, but using qcow2 under raw is
>>>>> broken. It should be either fixed like I propose (by Max's suggestion)
>>>>> or somehow forbidden (just forbid backing-file supporting node to be
>>>>> file child of raw-format node).
>>>> I agree, I think only filters should return BDRV_BLOCK_RAW.
>>>> (And not even them, they should just be handled transparently by
>>>> bdrv_co_block_status().  But that’s something for later.)
>>>>> Vladimir Sementsov-Ogievskiy (2):
>>>>>    block/raw-format: switch to BDRV_BLOCK_DATA with BDRV_BLOCK_RECURSE
>>>>>    iotests: test mirroring qcow2 under raw format
>>>>>   block/raw-format.c         |  2 +-
>>>>>   tests/qemu-iotests/263     | 46 ++++++++++++++++++++++++++++++++++++++
>>>>>   tests/qemu-iotests/263.out | 12 ++++++++++
>>>>>   tests/qemu-iotests/group   |  1 +
>>>>>   4 files changed, 60 insertions(+), 1 deletion(-)
>>>>>   create mode 100755 tests/qemu-iotests/263
>>>>>   create mode 100644 tests/qemu-iotests/263.out
>>>> Thanks, applied to my block-next branch:
>>>> https://git.xanclic.moe/XanClic/qemu/commits/branch/block-next
>>> Oops, maybe not.  221 needs to be adjusted.
>> Hmm yes, I forget to run tests.. Areas which were zero becomes data|zero, it
>> don't look good.
>> So, it's not quite right to report DATA | RECURSE, we actually should report
>> DATA_OR_ZERO | RECURSE, which is actually ALLOCATED | RECURSE, as otherwise
>> DATA will be set in final result (generic layer must not drop it, obviously).
>> ALLOCATED never returned by drivers but seems it should be. I'll think a bit 
>> and
>> resend something new.
> Hmmm.. So, we have raw node, and assume backing chain under it. who should 
> loop through it,
> generic code or raw driver?
> Now it all looks like generic code is responsible for looping through 
> filtered chain (backing files
> and filters) and driver is responsible for all it's children except for 
> filtered child.
> Or, driver may return something that says to generic child to handle the 
> whole backing chain of returned
> file at once, as it's another backing chain. And seems even RECURSE don't 
> work correctly as it doesn't handle
> the backing chain in this recursion. Why it works better than RAW - just 
> because we return it together
> with DATA flags and this DATA flag is kept anyway, independently of finding 
> zeros or not.

Hmm, so, is it correct that we return DATA | RECURSE, if we are not really sure 
that it is data?

If we see at

  * BDRV_BLOCK_DATA: allocation for data at offset is tied to this layer

seems like we should report DATA only if there is allocation..

  *  t    t        t       sectors read as zero, returned file is zero at offset
  *  t    f        t       sectors read as valid from file at offset
  *  f    t        t       sectors preallocated, read as zero, returned file not

so, ZERO alone doesn't guarantee that we may safely read?

So, for qcow2 metadata-preallocated images, what about zero-init? We report 
DATA, and probably get ZERO from
file and have finally DAYA | ZERO which guarantees that read will return zeros, 
but is it true?

Finally, what "DATA" mean? That space is allocated and occupies disk space? Or 
it only  means only ALLOCATED i.e.
"read from this layer, not from backing" otherwise, and adds additional meaning 
to ZERO when used together, that
read will return zeros for sure?

Best regards,

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