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

[Kevin Wolf]

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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