Re: [Qemu-devel] [PATCH v7 8/9] qcow2: skip writing zero buffers to empty COW areas

[Max Reitz <mreitz@redhat.com>]

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On 2018-01-31 19:32, Eric Blake wrote:
> On 01/31/2018 11:40 AM, Max Reitz wrote:
> 
>>> Maybe it's good enough to cover these cases:
>>>  1. no backing
>>>  2. beyond bdrv_getlength() in backing
>>>  3. unallocated in backing qcow2 (covers 'beyond bdrv_getlength()
>>>                                           in backing->file')
>>>
>>> 1 & 2 are easy to check;
>>
>> Not sure how useful 2 is, though.  (I don't know.  I always hear about
>> people wanting to optimize for such a case where a backing file is
>> shorter than the overlay, but I can't imagine a real use case for that.)
> 
> I can; here's what's happened to me personally.  I created an image, and
> took internal snapshots (yeah, I know those aren't in fashion these
> days, but this was long ago).  Later, I ran out of space.  I wanted to
> resize the image, but am not convinced whether resizing the image will
> play nicely with the internal snapshots (in fact, I don't recall
> off-hand whether this was something we prevented in the past and now
> support, or if it is still unsupported now...) - so the easiest way is
> to create a larger overlay image.

But you were convinced that creating an overlay would play nicely with
the internal snapshots? ;-)

I'm not sure whether that sounds like a use case I'd want to optimize
for, but, well.

>>> 3: if that's not too hacky maybe we can do the bdrv_is_allocated() check
>>> for qcow2 exclusively and if there is raw (or any other format) backing
>>> image - do the COW
>>
>> Yes, well, it seems useful in principle...  But it would require general
>> block layer work indeed.  Maybe a new flag for bdrv_block_status*() that
>> says only to look into the format layer?
> 
> Maybe indeed - but first I want my byte-based block_status stuff to land ;)

It could be done as an optimization in a follow-up to this series
anyway, yes.

Max


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