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

From: Peter Lieven
Subject: Re: [RFC PATCH 0/2] qemu-img convert: Fix sparseness detection
Date: Mon, 19 Apr 2021 11:13:22 +0200

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).


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.


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