[libvirt-users] preallocation=full Vs preallocation=metadata

Hi All I was reading through this: http://kashyapc.com/2011/12/02/little-more-disk-io-perf-improvement-with-... I was basically searching for pointers on improving disk I/O. I wanted to know the purpose of preallocation=full & preallocation=metadata , What is the difference between them ? and which one would yield a better disk I/O speed ? Appreciate your responses in this regard. Thanks Jatin