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Re: [Qemu-block] [PATCH v7 8/9] qcow2: skip writing zero buffers to empt

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH v7 8/9] qcow2: skip writing zero buffers to empty COW areas
Date: Wed, 31 Jan 2018 12:32:27 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

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.

>> 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 ;)

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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