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Re: [RFC PATCH 0/2] qemu-img convert: Fix sparseness detection

From: Kevin Wolf
Subject: Re: [RFC PATCH 0/2] qemu-img convert: Fix sparseness detection
Date: Mon, 19 Apr 2021 13:22:41 +0200

Am 19.04.2021 um 10:36 hat Peter Lieven geschrieben:
> > Am 15.04.2021 um 17:22 schrieb Kevin Wolf <kwolf@redhat.com>:
> > 
> > Peter, three years ago you changed 'qemu-img convert' to sacrifice some
> > sparsification in order to get aligned requests on the target image. At
> > the time, I thought the impact would be small, but it turns out that
> > this can end up wasting gigabytes of storagee (like converting a fully
> > zeroed 10 GB image taking 2.8 GB instead of a few kilobytes).
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=1882917
> > 
> > I'm not entirely sure how to attack this best since this is a tradeoff,
> > but maybe the approach in this series is still good enough for the case
> > that you wanted to fix back then?
> > 
> > Of course, it would be possible to have a more complete fix like looking
> > forward a few blocks more before writing data, but that would probably
> > not be entirely trivial because you would have to merge blocks with ZERO
> > block status with DATA blocks that contain only zeros. I'm not sure if
> > it's worth this complication of the code.
> I will try to look into this asap.
> Is there a hint which FS I need to set the extent hint when creating
> the raw image? I was not able to do that.

Grepping the current kernel source, it seems extent size hints still
work only on XFS. But I don't think it's necessary for reproducing this
bug. In fact, disabling the extent size hint should cause a lot more
fragmentation, which should make the problem more visible.


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