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

[Max Reitz]

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.

> (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

>>
>> Max Reitz (3):
>>   vdi: Make block_status recurse for fixed images
>>   vmdk: Make block_status recurse for flat extents
>>   vpc: Do not return RAW from block_status
>>
>>  block/vdi.c  | 3 ++-
>>  block/vmdk.c | 3 +++
>>  block/vpc.c  | 2 +-
>>  3 files changed, 6 insertions(+), 2 deletions(-)
>>

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