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We don't support internal snapshots, but it's not compiled out, this is an image corruption bug and upstream has a fix. So let's fix it anyway.

Reproducible with this script:

qemu-img create -f qcow2 /tmp/sn.qcow2 64M
qemu-img snapshot -c foo /tmp/sn.qcow2
qemu-io -c 'write -b 0 4M' /tmp/sn.qcow2
qemu-img snapshot -a foo /tmp/sn.qcow2
qemu-img check /tmp/sn.qcow2

Reproduced this issue with qemu-kvm-0.12.1.2-2.180.el6 using comment0 steps.

Results:
#qemu-img check /tmp/sn.qcow2
ERROR cluster 1 refcount=0 reference=1
ERROR cluster 7 refcount=0 reference=1
ERROR cluster 8 refcount=0 reference=1
ERROR cluster 9 refcount=0 reference=1
ERROR cluster 10 refcount=0 reference=1
ERROR cluster 11 refcount=0 reference=1
ERROR cluster 12 refcount=0 reference=1
ERROR cluster 13 refcount=0 reference=1
ERROR cluster 14 refcount=0 reference=1
ERROR cluster 15 refcount=0 reference=1
ERROR cluster 16 refcount=0 reference=1

Verified this issue with  qemu-kvm-0.12.1.2-2.184.el6

Results:
#qemu-img check /tmp/sn.qcow2
No errors were found on the image.

This issue has been fixed.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHSA-2011-1531.html