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Re: [Qemu-block] [PATCH 3/3] vpc: Do not return RAW from block_status

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH 3/3] vpc: Do not return RAW from block_status
Date: Tue, 13 Aug 2019 16:49:22 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

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.


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