New disk space invisible after RAID reconfiguration

I have a server originally with RAID 6 setting and 5x4TB hard disks. I installed 3 new 4TB hard disk and reconfigured through OpenManage Server Admin (OMSA). The server's system is ubuntu 16.04.1. After reconfiguration done in OMSA, the size of virtual disk shown to be ~20TB in OMSA, consistent with raid 6 setting with 8x4TB hard disk. The device name is /dev/sda shown in OMSA. Seem like reconfiguration has been conducted successfully through OMSA.  

However, in ubuntu, when I use df -u to check the space of server. The disk space is still ~12TB. The additional 8TB has not been successfully added in. What should I do to make the additional 8TB space available from server side?