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Thanks for your attention!

- disk full  ----# I believe there is enough space, because I have tried to boot serveral instances after this error with the same image, they are all booted successfully.
- permission issues    ----# Not this reason, too. explain as above.

I still believe this is a race problem, if we convert to image from qcow2 to raw at a short interval, one of the convertion may fail, 
I have verified this by running two same commands continuously such as:
qemu-img convert -f qcow2 -O raw  test.qcow2 test.part &
and one of them give me the same error:
qemu-img: error while writing sector 1052672: Input/output error