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

From: Richard W.M. Jones
Subject: Bug? qemu-img convert to preallocated image makes it sparse
Date: Thu, 16 Jan 2020 14:13:52 +0000
User-agent: Mutt/1.5.21 (2010-09-15)

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 
  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

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.



Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
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