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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 19:12:15 +0200


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?

Peter






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