From just this message its difficult to comment .Hows your configuration ,are you using cinder or ceph ?Have you opted 20G volume size while creating .How are you are you trying to launch VM

maybe now i know what's the problem, i think i don't have to using cinder, but i will try it :D

Incase you need to launch instance with volumes you may need to use cinder,ceph etc depending on your environment and configuration.

oke. i have already install cinder, but there is some error, i will find it first

can i ask you something deepa? where should i install cinder-volume? on controller or compute? because i only have 2 servers