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Re: Bug? qemu-img convert to preallocated image makes it sparse
From: |
Richard W.M. Jones |
Subject: |
Re: Bug? qemu-img convert to preallocated image makes it sparse |
Date: |
Thu, 16 Jan 2020 14:53:59 +0000 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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 <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.
>
> 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
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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, 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, 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