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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 12:43:01 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

On 01/31/2018 12:35 PM, Max Reitz wrote:

>>> 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? ;-)

Yes. As long as I don't mind pointing back to the original file (rather
than the overlay) at the point I attempt to revert to the internal
snapshot, then loading the snapshot doesn't have to worry about a size

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

I guess it boils down to whether the optimization is a low-maintenance
freebie.  There's no reason to reject the optimization if a patch
demonstrates it is easy to support and it has iotests coverage.

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

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