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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: Tue, 20 Apr 2021 08:49:57 +0200

Am 19.04.2021 um 19:12 hat Peter Lieven geschrieben:
> 
> 
> Von meinem iPhone gesendet
> 
> > Am 19.04.2021 um 14:31 schrieb Kevin Wolf <kwolf@redhat.com>:
> > 
> > ´╗┐Am 19.04.2021 um 11:13 hat Peter Lieven geschrieben:
> >> 
> >> 
> >>>> Am 19.04.2021 um 10:36 schrieb Peter Lieven <pl@kamp.de>:
> >>> 
> >>> 
> >>> 
> >>>> 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.
> >> 
> >> Besides from the reproducer described in the ticket, I retried my old
> >> conversion test in our environment:
> >> 
> >> Before commit 8dcd3c9b91: reads 4608 writes 14959
> >> After commit 8dcd3c9b91: reads 0 writes 14924
> >> With Kevins patch: reads 110 writes 14924
> >> 
> >> I think this is a good result if it avoids other issues.
> > 
> > Sounds like a promising way to make the tradeoff. Thanks for testing!
> 
> is this sth for 6.0-rc4?

No, certainly not. It would be for the first 6.1 pull request.

Kevin




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