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Description of problem:

qemu ensures that guest writes and qemu metadata writes hit the disk when necessary to prevent data corruption.  However, if an image was in host pagecache prior to starting qemu, for example after running qemu-img convert, then nothing prevents writes from reaching the disk out of order, potentially causing corruption.  The window is quite small - until the first fdatasync issued by qemu.

Version-Release number of selected component (if applicable):
qemu-kvm-0.12.1.2-2.113.el6

How reproducible:
with great care


Steps to Reproduce:
1. 'cp' an image to a local disk
2. start a guest which doesn't issue flushes
3. write some stuff
4. powerfail
  
Actual results:
The guest's writes are not guaranteed to reach the disk (expected) but the disk itself may be corrupted (unexpected)

Expected results:
No corruption beyond unflushed guest writes.

Additional info:
This is unlikely to bite in practice, but let's not take any risks.