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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: Mon, 12 Aug 2019 21:07:17 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 12.08.19 18:50, Vladimir Sementsov-Ogievskiy wrote:
> 12.08.2019 18:56, Max Reitz wrote:
>> 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?
> And if I have raw driver above qcow2, it will not work, like I've described 
> above..

Yep.  That’s why I was wondering.  (This is a more likely case, because
maybe you really want to use raw’s offset capability on top of qcow2.)


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