[libvirt] what are the differences among disk cache attribute

hi,all there are "default", "none", "writethrough", "writeback", "directsync", "unsafe" cache options 1. could you explain what is default and none? exactly what it does and not does? 2. what is the difference between unsafe and (default or none)? 3. after a long time of running , it seems that vms will slow down? do you know why? cache hit rate goes down or kernel takes time to flush cache? thanks