Re: [PATCH] qemu-{img,nbd}: Don't report zeroed cluster as a hole

[Nir Soffer <nirsof@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 3352 bytes --]



> ‫ב-11 ביוני 2021, בשעה 11:14, ‏‏Vladimir Sementsov-Ogievskiy ‏<vsementsov@virtuozzo.com> כתב/ה:‬
> 
> 11.06.2021 11:09, Kevin Wolf wrote:
>> Am 10.06.2021 um 22:46 hat Eric Blake geschrieben:
>>>> On Thu, Jun 10, 2021 at 11:09:05PM +0300, Nir Soffer wrote:
>>>>>> But:
>>>>>> 
>>>>>> $ qemu-img map --output=json -f qcow2 json:'{"driver":"qcow2","backing":null, \
>>>>>>   "file":{"driver":"file","filename":"top.qcow2"}}'
>>>>>> [{ "start": 0, "length": 65536, "depth": 0, "zero": true, "data": false},
>>>>>> { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
>>>>>> { "start": 131072, "length": 131072, "depth": 0, "zero": true, "data": false}]
>>>>>> 
>>>>>> also reports the entire file at "depth":0, which is misleading, since
>>>>>> we have just been arguing from the qemu:allocation-depth perspective
>>>>>> (and also from bdrv_block_status) that the qcow2 image is NOT 100%
>>>>>> allocated (in the sense where allocation == data comes locally).
>>>>>> Perhaps it might be better if we tweaked the above qemu-img map to
>>>>>> produce:
>>>>>> 
>>>>>> [{ "start": 0, "length": 65536, "depth": -1, "zero": true, "data": false},
>>>>>> { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
>>>>>> { "start": 131072, "length": 65536, "depth": 0, "zero": true, "data": false},
>>>>>> { "start": 196608, "length": 65536, "depth": -1, "zero": true, "data": false}]
>>>>> 
>>>>> It will be more consistent with "offset" to drop "depth" from output
>>>>> if we don't have it:
>>>>> 
>>>>>     [{ "start": 0, "length": 65536, "zero": true, "data": false},
>>>>>      { "start": 65536, "length": 65536, "depth": 0, "zero": false,
>>>>> "data": true, "offset": 327680},
>>>>>      { "start": 131072, "length": 65536, "depth": 0, "zero": true,
>>>>> "data": false},
>>>>>      { "start": 196608, "length": 65536, "zero": true, "data": false}]
>>> 
>>> Yes, that might work as well.  But we didn't previously document
>>> depth to be optional.  Removing something from output risks breaking
>>> more downstream tools that expect it to be non-optional, compared to
>>> providing a new value.
>> A negative value isn't any less unexpected than a missing key. I don't
>> think any existing tool would be able to handle it. Encoding different
>> meanings in a single value isn't very QAPI-like either. Usually strings
>> that are parsed are the problem, but negative integers really isn't that
>> much different. I don't really like this solution.
>> Leaving out the depth feels like a better suggestion to me.
>> But anyway, this seems to only happen at the end of the backing chain.
>> So if the backing chain consistents of n images, why not report 'depth':
>> n + 1? So, in the above example, you would get 1. I think this has the
>> best chances of tools actually working correctly with the new output,
>> even though it's still not unlikely to break something.
> 
> Did you consider just add a new field?
> 
> So, "depth" keeps its meaning "which level provides data".
> 
> And we add additional optional field like
> 
> absolutely-completely-absent: bool

hole: bool?

> 
> Which is true if data is nowhere in the backing chain.
> 
> 
> -- 
> Best regards,
> Vladimir

[-- Attachment #2: Type: text/html, Size: 9360 bytes --]