From: Max Reitz <mreitz@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
"qemu-block@nongnu.org" <qemu-block@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH 3/3] vpc: Do not return RAW from block_status
Date: Tue, 13 Aug 2019 16:49:22 +0200 [thread overview]
Message-ID: <b83773d6-ab7c-943d-d4f5-df4512e03510@redhat.com> (raw)
In-Reply-To: <20190813103838.GD4663@localhost.localdomain>
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On 13.08.19 12:38, Kevin Wolf wrote:
> Am 12.08.2019 um 17:56 hat Max Reitz geschrieben:
>> On 12.08.19 17:33, Vladimir Sementsov-Ogievskiy wrote:
>>> 25.07.2019 18:55, Max Reitz wrote:
>>>> vpc is not really a passthrough driver, even when using the fixed
>>>> subformat (where host and guest offsets are equal). It should handle
>>>> preallocation like all other drivers do, namely by returning
>>>> DATA | RECURSE instead of RAW.
>>>>
>>>> There is no tangible difference but the fact that bdrv_is_allocated() no
>>>> longer falls through to the protocol layer.
>>>
>>> Hmm. Isn't a real bug (fixed by this patch) ?
>>>
>>> Assume vpc->file is qcow2 with backing, which have "unallocated" region, which is
>>> backed by actual data in backing file.
>>
>> Come on now.
>>
>>> So, this region will be reported as not allocated and will be skipped by any copying
>>> loop using block-status? Is it a bug of BDRV_BLOCK_RAW itself? Or I don't understand
>>> something..
>>
>> I think what you don’t understand is that if you have a vpc file inside
>> of a qcow2 file, you’re doing basically everything wrong. ;-)
>>
>> But maybe we should drop BDRV_BLOCK_RAW... Does it do anything good for
>> us in the raw driver? Shouldn’t it too just return DATA | RECURSE?
>
> DATA | RECURSE is still DATA, i.e. marks the block as allocated. If you
> do that unconditionally, we will never consider a block unallocated.
Which is correct for formats that do not have backing files.
> RECURSE doesn't undo this, the only thing it might do is settting ZERO
> additionally.
>
> So I would say unconditionally returning DATA | RECURSE is almost always
> wrong.
I would say it’s always right when it is a format driver and there is no
backing file.
Max
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next prev parent reply other threads:[~2019-08-13 14:49 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-25 15:55 [Qemu-devel] [PATCH 0/3] block: Make various formats' block_status recurse again Max Reitz
2019-07-25 15:55 ` [Qemu-devel] [PATCH 1/3] vdi: Make block_status recurse for fixed images Max Reitz
2019-08-12 14:47 ` Vladimir Sementsov-Ogievskiy
2019-07-25 15:55 ` [Qemu-devel] [PATCH 2/3] vmdk: Make block_status recurse for flat extents Max Reitz
2019-08-12 14:59 ` Vladimir Sementsov-Ogievskiy
2019-07-25 15:55 ` [Qemu-devel] [PATCH 3/3] vpc: Do not return RAW from block_status Max Reitz
2019-08-12 15:33 ` Vladimir Sementsov-Ogievskiy
2019-08-12 15:56 ` Max Reitz
2019-08-12 16:50 ` Vladimir Sementsov-Ogievskiy
2019-08-12 19:07 ` Max Reitz
2019-08-13 10:38 ` Kevin Wolf
2019-08-13 14:49 ` Max Reitz [this message]
2019-08-12 18:39 ` [Qemu-devel] [Qemu-block] [PATCH 0/3] block: Make various formats' block_status recurse again John Snow
2019-08-12 19:11 ` Max Reitz
2019-08-12 21:45 ` John Snow
2019-08-13 14:48 ` Max Reitz
2019-08-13 22:35 ` John Snow
2019-08-15 15:49 ` [Qemu-devel] " Max Reitz
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