Description of problem:

As seen in this article, 

http://www.ilsistemista.net/index.php/virtualization/11-kvm-io-slowness-on-rhel-6.html

which has been mentioned in other bug reports, qcow2 suffers from performance problems depending on both caching and preallocation.  Raw however, has almost no difference in speed between preallocated and dynamic disk files.  Is this a function of qcow2 or something that can be fixed?  It is really convenient being able to assign a system 100 GB of space but only having to back up what is actually being used.  But the performance hit when using qcow2 is substantial.  Can this be corrected?