Re: [Qemu-block] [PATCH 0/2] qemu-img: Special post-backing convert handle

[Max Reitz]

On 2018-05-01 18:57, Max Reitz wrote:
> This is the issue:
> 
> $ qemu-img create -f qcow2 base.qcow2 512M
> Formatting 'base.qcow2', fmt=qcow2 size=536870912 cluster_size=65536 
> lazy_refcounts=off refcount_bits=16
> $ qemu-img create -f qcow2 -b base.qcow2 source.qcow2 1G
> Formatting 'source.qcow2', fmt=qcow2 size=1073741824 backing_file=base.qcow2 
> cluster_size=65536 lazy_refcounts=off refcount_bits=16
> $ qemu-img convert -O qcow2 -B base.qcow2 -o compat=0.10 \
>     source.qcow2 target.qcow2
> $ qemu-img info target.qcow2
> image: target.qcow2
> file format: qcow2
> virtual size: 1.0G (1073741824 bytes)
> disk size: 512M    <-------------------- this here
> cluster_size: 65536
> backing file: base.qcow2
> Format specific information:
>     compat: 0.10
>     refcount bits: 16
> $ qemu-img map target.qcow2
> Offset          Length          Mapped to       File
> 0x20000000      0x20000000      0x50000         target.qcow2
> 
> So qemu-img convert sees that source.qcow2 contains only zeroes past the
> end of base.qcow2 -- but that is not the backing file, so it will
> explicitly write zeroes to target.qcow2.  But that file is compat=0.10,
> so it does not support efficient zero writes and will actually fill that
> area with real zeroes.  Hence the mapping, hence the disk size.
> 
> However, we don't need to write zeroes to an image when it is
> initialized to zeroes -- which the current qemu-img code says doesn't
> happen for target images with backing files.  But it does happen when
> the target backing file is shorter than the target image, then the area
> past the end of the backing file may indeed read as zeroes and we don't
> need to write anything there.
> 
> So the first patch in this series makes qemu-img convert detect that
> case and handle zeroes past the end of the backing file as "fall back to
> the backing file" (because that means zeroes, at least if unallocated
> areas read as zeroes (which I presume they have to for formats
> supporting backing files, but better be safe than sorry)), and the
> second adds an iotest.
> 
> 
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1527898
> 
> 
> Max Reitz (2):
>   qemu-img: Special post-backing convert handling
>   iotests: Test post-backing convert target behavior
> 
>  qemu-img.c                 | 26 +++++++++++++++++++++++++-
>  tests/qemu-iotests/122     | 42 ++++++++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/122.out | 18 ++++++++++++++++++
>  3 files changed, 85 insertions(+), 1 deletion(-)

Applied to my block branch.

Max

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