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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...)
Max
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Re: Bug? qemu-img convert to preallocated image makes it sparse, David Edmondson, 2020/01/17
Re: Bug? qemu-img convert to preallocated image makes it sparse, Max Reitz, 2020/01/16