Comment 3 for bug 568445

Looks like even upstream suggests disabling cache for best performance when using raw volumes:
http://www.linux-kvm.org/page/Tuning_KVM

From the above page:

QEMU also supports a wide variety of caching modes. Writeback is useful for testing but does not offer storage guarantees. Writethrough (the default) is safer, and relies on the host cache. If you're using raw volumes or partitions, it is best to avoid the cache completely, which reduces data copies and bus traffic:

 qemu -drive file=/dev/mapper/ImagesVolumeGroup-Guest1,cache=none,if=virtio