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Re: [Qemu-devel] [PATCH] qemu-img: simplify img_convert

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] qemu-img: simplify img_convert
Date: Thu, 20 Apr 2017 09:18:33 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.0

On 04/20/2017 09:11 AM, Eric Blake wrote:
> On 04/20/2017 09:05 AM, Fam Zheng wrote:
>> On Tue, 02/28 14:35, Peter Lieven wrote:
>>> img_convert has been around before there was an ImgConvertState or
>>> a block backend, but it has never been modified to directly use
>>> these structs. Change this by parsing parameters directly into
>>> the ImgConvertState and directly use BlockBackend where possible.
>>> Futhermore variable initalization has been reworked and sorted.
>>> Signed-off-by: Peter Lieven <address@hidden>
>> I see an iotest failure with this patch, in Kevin's block-next tree:
>> 019 1s ... - output mismatch (see 019.out.bad)
>> --- /stor/work/qemu/tests/qemu-iotests/019.out       2017-04-17 
>> 16:19:56.523968474 +0800
>> +++ 019.out.bad      2017-04-20 22:03:29.868216955 +0800
>> @@ -1086,8 +1086,8 @@
>>  Checking if backing clusters are allocated when they shouldn't
>> -0/128 sectors allocated at offset 1 MiB
>> -0/128 sectors allocated at offset 4.001 GiB
>> +128/128 sectors allocated at offset 1 MiB
>> +128/128 sectors allocated at offset 4.001 GiB
> Hmm - I wonder if my patch to make 'zero with unmap' on an image with no
> backing file prefer pure unallocated clusters over a reads-as-zero would
> make a difference.
> https://lists.gnu.org/archive/html/qemu-devel/2017-04/msg01728.html
> Testing now...

Nope, did not make a difference, although it's certainly related; per my
comment in that thread that:

    Note that technically, we _could_ write a cluster as unallocated
    rather than zero if a backing file exists but the backing file
    also reads as zero, but that's more expensive to determine, so
    this optimization is limited to qcow2 without a backing file.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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