Should take another look at this issue from the host-side (xen tools). We should at least try reproducing with F16/rawhide guests to see what happens, and if the result is worse.
We should look at fixing blkback first, possibly with a backport of 6f5986bce558, then we can double check userspace after. Moving to kernel-xen.
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*** This bug has been marked as a duplicate of bug 794806 ***