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

[Kevin Wolf]

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!

Kevin