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

[Maxim Levitsky <mlevitsk@redhat.com>]

On Thu, 2020-01-16 at 15:55 +0100, Max Reitz wrote:
> 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 <mreitz@redhat.com>
> > > > 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 <mreitz@redhat.com>
> > > >     Message-id: 20190724171239.8764-2-mreitz@redhat.com
> > > >     Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> > > >     Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> > > >     Signed-off-by: Max Reitz <mreitz@redhat.com>
> > > > 
> > > >  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
> 

How about doing write zeros without discard only in this particular case (convert to existing image)
Basically omitting the BDRV_REQ_MAY_UNMAP flag to blk_co_pwrite_zeroes.
It will be slow, but maybe for this particular case, it is acceptable?

Best regards,
	Maxim Levitsky