sir actually this is my plan  
32 V+ 16A dvr with 30 X 2 mp cameras with 15 fps  
so here present there is no storage more than 4 HDD X maximum 4 slots  
so i need more storage i mean customer asking me to provide above 6 months back up  
If i calculate with disk calculator 4 HDD X 6 TB = 24 TB only ( 4 slot maximum available now with maximun 6 th each HDD) its comes to 32- 35 days backup only  
  
so please suggest me for 6 months backup or at least 3 months  
these are must 32 video + 16 audio with 2 mp cameras