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Re: Bug? qemu-img convert to preallocated image makes it sparse

From: Max Reitz
Subject: Re: Bug? qemu-img convert to preallocated image makes it sparse
Date: Thu, 16 Jan 2020 15:55:24 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1

On 16.01.20 15:50, Kevin Wolf wrote:
> Am 16.01.2020 um 15:37 hat Max Reitz geschrieben:
>> On 16.01.20 15:13, Richard W.M. Jones wrote:
>>> I'm not necessarily saying this is a bug, but a change in behaviour in
>>> qemu has caused virt-v2v to fail.  The reproducer is quite simple.
>>> Create sparse and preallocated qcow2 files of the same size:
>>>   $ qemu-img create -f qcow2 sparse.qcow2 50M
>>>   Formatting 'sparse.qcow2', fmt=qcow2 size=52428800 cluster_size=65536 
>>> lazy_refcounts=off refcount_bits=16
>>>   $ qemu-img create -f qcow2 prealloc.qcow2 50M -o 
>>> preallocation=falloc,compat=1.1
>>>   Formatting 'prealloc.qcow2', fmt=qcow2 size=52428800 compat=1.1 
>>> cluster_size=65536 preallocation=falloc lazy_refcounts=off refcount_bits=16
>>>   $ du -m sparse.qcow2 prealloc.qcow2 
>>>   1 sparse.qcow2
>>>   51        prealloc.qcow2
>>> Now copy the sparse file into the preallocated file using the -n
>>> option so qemu-img doesn't create the target:
>>>   $ qemu-img convert -p -n -f qcow2 -O qcow2 sparse.qcow2 prealloc.qcow2
>>>       (100.00/100%)
>>> In new qemu that makes the target file sparse:
>>>   $ du -m sparse.qcow2 prealloc.qcow2 
>>>   1 sparse.qcow2
>>>   1 prealloc.qcow2         <-- should still be 51
>>> In old qemu the target file remained preallocated, which is what
>>> I and virt-v2v are expecting.
>>> I bisected this to the following commit:
>>> 4d7c487eac1652dfe4498fe84f32900ad461d61b is the first bad commit
>>> commit 4d7c487eac1652dfe4498fe84f32900ad461d61b
>>> Author: Max Reitz <address@hidden>
>>> Date:   Wed Jul 24 19:12:29 2019 +0200
>>>     qemu-img: Fix bdrv_has_zero_init() use in convert
>>>     bdrv_has_zero_init() only has meaning for newly created images or image
>>>     areas.  If qemu-img convert did not create the image itself, it cannot
>>>     rely on bdrv_has_zero_init()'s result to carry any meaning.
>>>     Signed-off-by: Max Reitz <address@hidden>
>>>     Message-id: address@hidden
>>>     Reviewed-by: Maxim Levitsky <address@hidden>
>>>     Reviewed-by: Stefano Garzarella <address@hidden>
>>>     Signed-off-by: Max Reitz <address@hidden>
>>>  qemu-img.c | 11 ++++++++---
>>>  1 file changed, 8 insertions(+), 3 deletions(-)
>>> Reverting this commit on the current master branch restores the
>>> expected behaviour.
>> The commit changed the behavior because now qemu-img realizes that it
>> cannot skip writing to areas that are supposed to be zero when it
>> converts to an existing image (because we have no idea what data that
>> existing image contains).  So that’s a bug fix, and I don’t think we can
>> undo it without being wrong.
>> The problem is that qemu-img will try to be quickthat about making these
>> areas zero, and so it does zero writes (actually, it even zeroes the
>> whole image) and in the process it will of course discard all preallocation.
>> Now, about fixing the problem I’m not so sure.
> Wouldn't just passing -S 0 solve the problem? It should tell qemu-img
> convert that you don't want it to sparsify anything.

But it would also convert the falloc preallocation to a full one.

(I had a section to this effect in my first draft, but then I
accidentally deleted it and forgot it in my second version...)


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