Re: [Qemu-devel] [Qemu-block] [PATCH 0/3] block: Make various formats' block_status recurse again

[John Snow]

On 8/12/19 3:11 PM, Max Reitz wrote:
> On 12.08.19 20:39, John Snow wrote:
>>
>>
>> On 7/25/19 11:55 AM, Max Reitz wrote:
>>> Hi,
>>>
>>> 69f47505ee66afaa513305de0c1895a224e52c45 changed block_status so that it
>>> would only go down to the protocol layer if the format layer returned
>>> BDRV_BLOCK_RECURSE, thus indicating that it has no sufficient
>>> information whether a given range in the image is zero or not.
>>> Generally, this is because the image is preallocated and thus all ranges
>>> appear as zeroes.
>>>
>>> However, it only implemented this preallocation detection for qcow2.
>>> There are more formats that support preallocation, though: vdi, vhdx,
>>> vmdk, vpc.  (Funny how they all start with “v”.)
>>>
>>> For vdi, vmdk, and vpc, the fix is rather simple, because they really
>>> have different subformats depending on whether an image is preallocated
>>> or not.  This makes the check very simple.
>>>
>>> vhdx is more like qcow2, where after the image has been created, it
>>> isn’t clear whether it’s been preallocated or everything is allocated
>>> because everything was already written to.  69f47505ee added a heuristic
>>> to qcow2 to get around this, but I think that’s too much for vhdx.  I
>>> just left it unfixed, because I don’t care that much, honestly (and I
>>> don’t think anyone else does).
>>>
>>
>> What's the practical outcome of that, and is the limitation documented
>> somewhere?
> 
> The outcome is that it if you preallocate a vhdx image
> (subformat=fixed), you’ll see that all sectors contain data, even if
> they may be zero sectors on the filesystem level.
> 
> I don’t think it’s user-visible whatsoever.
> 

But it might mean that doing things with sync=top might over-allocate
data depending on the destination, wouldn't it?

That's not crucial, but it's possibly visible, no?

>> (I'm fine with not fixing it, I just want it documented somehow.)
> 
> I am really not inclined to start any documentation on the
> particularities with which qemu handles vhdx images.
> 
> (Especially so considering we don’t even have any documentation on the
> qcow2 case.  The stress in my paragraph was “heuristic”.  If you
> preallocate a qcow2 image, but then discard enough sectors that the
> heuristic thinks you didn’t, you’ll have the same effect.  Or if you
> grow a preallocated image without preallocating the new area.)
> 
> Max
> 

"But our qcow2 docs are also bad" is the kind of argument I can't
*really* disagree with, but...

(I wish we did have a documentation manual per-format that mentioned
some gotchas and general info about each format, but I can't really ask
you to do that now: I just worry when I see patches like this that the
knowledge or memory that there ever was a quirk will vanish immediately.)

--js