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Re: Bug? qemu-img convert to preallocated image makes it sparse
From: |
Kevin Wolf |
Subject: |
Re: Bug? qemu-img convert to preallocated image makes it sparse |
Date: |
Thu, 16 Jan 2020 15:50:48 +0100 |
User-agent: |
Mutt/1.12.1 (2019-06-15) |
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.
Kevin
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- Bug? qemu-img convert to preallocated image makes it sparse, Richard W.M. Jones, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Max Reitz, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse,
Kevin Wolf <=
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Max Reitz, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Maxim Levitsky, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Max Reitz, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Richard W.M. Jones, 2020/01/16
- Re: Bug? qemu-img convert to preallocated image makes it sparse, Max Reitz, 2020/01/16
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