[vdsm] what is 'qcow2 on LV' and how node communicate

[bigclouds]

hi,all
1.it seems that vdsm has the capability to thin providing lv(logical volume), could you
please explain the theory of it?
 
2.i did not notice any code throught which  NODE can communicate with each other, called
mail-box.  if it varies depends on volume type(block,fs), and what is and how they
communicate via mail-box?
 
thanks so much.