On Thu, Jan 16, 2020 at 03:47:30PM +0100, Max Reitz wrote: > 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 <mre...@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 <mre...@redhat.com> > > Message-id: 20190724171239.8764-2-mre...@redhat.com > > Reviewed-by: Maxim Levitsky <mlevi...@redhat.com> > > Reviewed-by: Stefano Garzarella <sgarz...@redhat.com> > > Signed-off-by: Max Reitz <mre...@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. > > So what this commit changed was that when you take an existing image as > the destination, you can’t assume anything about its contents. Before > this commit, we assumed it’s zero. That’s clearly wrong, because it can > be anything. > > So when you convert to the target image, you have to make sure all areas > that are zero in the source are zero in the target, too. The way we do > that is to write zeroes to the target. The problem is that this > operation disregards the previous preallocation and discards the > preallocated space. > > As for fixing the bug... Can we fix it in qemu(-img)? > > We could try to detect whether areas that are zero in the source are > zero in the (preallocated) target image, too. But doing so what require > reading the data from those areas and comparing it to zero. That would > take time and it isn’t trivial. So that’s something I’d rather avoid. > > Off the top of my head, the only thing that comes to my mind would be to > add a flag to qemu-img convert with which you can let it know that you > guarantee the target image is zero. I suppose we could document it also > to imply that given this flag, areas that are zero in the source will > then not be changed in the target image; i.e. that preallocation stays > intact in those areas. > > > OTOH, can it be fixed in virt-v2v? Is there already a safe way to call > qemu-img convert -n and keeping the target’s preallocation intact? > Unfortunately, I don’t think so. I don’t think we ever guaranteed it > would, and well, now it broke.
>From the fixing virt-v2v point of view, it's a bit tricky since the code has to deal with all kinds of output targets. (For example we sometimes qemu-img convert into an NBD target.) However we do know when the target contains zeroes - in fact it always contains zeroes, so: > So would you be OK with a --target-is-zero flag? (I think we could let > this flag guarantee that your use case works, so it should be future-safe.) this one should work. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW
