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Re: [Qemu-block] [PATCH v7 8/9] qcow2: skip writing zero buffers to empt
From: |
Max Reitz |
Subject: |
Re: [Qemu-block] [PATCH v7 8/9] qcow2: skip writing zero buffers to empty COW areas |
Date: |
Wed, 31 Jan 2018 19:35:17 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 |
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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- Re: [Qemu-block] [PATCH v7 9/9] iotest 134: test cluster-misaligned encrypted write, (continued)
[Qemu-block] [PATCH v7 4/9] block: treat BDRV_REQ_ALLOCATE as serialising, Anton Nefedov, 2018/01/18
Re: [Qemu-block] [PATCH v7 4/9] block: treat BDRV_REQ_ALLOCATE as serialising, Alberto Garcia, 2018/01/31