From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
To: Max Reitz <mreitz@redhat.com>, Kevin Wolf <kwolf@redhat.com>
Cc: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
"qemu-block@nongnu.org" <qemu-block@nongnu.org>,
Denis Lunev <den@virtuozzo.com>
Subject: Re: [Qemu-devel] [PATCH 1/2] block/raw-format: switch to BDRV_BLOCK_DATA with BDRV_BLOCK_RECURSE
Date: Tue, 13 Aug 2019 15:22:05 +0000 [thread overview]
Message-ID: <8346455b-b250-bffd-c79b-ea8363e2116d@virtuozzo.com> (raw)
In-Reply-To: <d0b0fc4e-eb2f-796d-3413-366a6bb5aeca@redhat.com>
13.08.2019 18:03, Max Reitz wrote:
> On 13.08.19 16:56, Vladimir Sementsov-Ogievskiy wrote:
>> 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 <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.
>>>
>>
>> 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.
>
> It’s a term for COW backing chains. If something is allocated on a
> given node in a COW backing chain, it means it is either present in
> exactly that node or in one of its storage children (in case the node is
> a format node). If it is not allocated, it is not, and read accesses
> will be forwarded to the COW backing child.
>
And this definition leads exactly to bug in these series:
[raw]
|
|file
V file
[qcow2]--------->[file]
|
|backing
V
[base]
Assume something is actually allocated in [base] but not in [qcow2].
So, [qcow2] node reports it as unallocated. So nobdy of [raw]'s storage
children contains this as allocated, so it's unallocated in [raw].
And if you say that logic is different for raw as it don't have COW child,
we may put qcow2 instead of raw here with same problem [yes, qcow2 on qcow2
is a bit strange, but we try to make something generic]
--
Best regards,
Vladimir
next prev parent reply other threads:[~2019-08-13 15:22 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-12 18:11 [Qemu-devel] [PATCH 0/2] deal with BDRV_BLOCK_RAW Vladimir Sementsov-Ogievskiy
2019-08-12 18:11 ` [Qemu-devel] [PATCH 1/2] block/raw-format: switch to BDRV_BLOCK_DATA with BDRV_BLOCK_RECURSE Vladimir Sementsov-Ogievskiy
2019-08-13 11:04 ` Kevin Wolf
2019-08-13 11:28 ` Vladimir Sementsov-Ogievskiy
2019-08-13 12:01 ` Kevin Wolf
2019-08-13 13:21 ` [Qemu-devel] [Qemu-block] " Kevin Wolf
2019-08-13 14:46 ` Max Reitz
2019-08-13 14:43 ` [Qemu-devel] " Max Reitz
2019-08-13 14:56 ` Vladimir Sementsov-Ogievskiy
2019-08-13 15:03 ` Max Reitz
2019-08-13 15:22 ` Vladimir Sementsov-Ogievskiy [this message]
2019-08-13 16:07 ` Max Reitz
2019-08-13 15:41 ` Kevin Wolf
2019-08-13 15:54 ` Vladimir Sementsov-Ogievskiy
2019-08-13 16:08 ` Kevin Wolf
2019-08-13 16:32 ` Vladimir Sementsov-Ogievskiy
2019-08-14 6:27 ` Vladimir Sementsov-Ogievskiy
2019-08-13 16:21 ` Max Reitz
2019-08-12 18:11 ` [Qemu-devel] [PATCH 2/2] iotests: test mirroring qcow2 under raw format Vladimir Sementsov-Ogievskiy
2019-08-13 9:10 ` Kevin Wolf
2019-08-13 9:22 ` Vladimir Sementsov-Ogievskiy
2019-08-13 9:36 ` Kevin Wolf
2019-08-12 19:46 ` [Qemu-devel] [PATCH 0/2] deal with BDRV_BLOCK_RAW Max Reitz
2019-08-12 19:50 ` Max Reitz
2019-08-13 8:39 ` Vladimir Sementsov-Ogievskiy
2019-08-13 9:01 ` Vladimir Sementsov-Ogievskiy
2019-08-13 9:33 ` Vladimir Sementsov-Ogievskiy
2019-08-13 11:14 ` Vladimir Sementsov-Ogievskiy
2019-08-13 11:51 ` Kevin Wolf
2019-08-13 13:00 ` Vladimir Sementsov-Ogievskiy
2019-08-13 14:31 ` Max Reitz
2019-08-13 14:46 ` Vladimir Sementsov-Ogievskiy
2019-08-13 14:53 ` Max Reitz
2019-08-13 15:03 ` Kevin Wolf
2019-08-13 15:04 ` Max Reitz
2019-08-13 14:50 ` Eric Blake
2019-08-13 9:34 ` Kevin Wolf
2019-08-13 14:38 ` Max Reitz
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