Tryed to add a new hard sdb (not formated) to my virtualbox Centos. Successfully extended an existing vg_localhost to /dev/sdb/

# lvdisplay
  --- Logical volume ---
  LV Path                /dev/vg_localhost/lv_root
  LV Name                lv_root
  VG Name                vg_localhost
  LV UUID                DkYX7D-DMud-vLaI-tfnz-xIJJ-VzHz-bRp3tO
  LV Write Access        read/write
  LV Creation host, time localhost.centos, 2012-12-17 
  LV Status              available
  # open                 1
  LV Size                18,03 GiB
  Current LE             4615
  Segments               2
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:0

lsblk
NAME                          MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sdb                             8:16   0    20G  0 disk
+-vg_localhost-lv_root (dm-0) 253:0    0    18G  0 lvm  /
+-vg_localhost-lv_swap (dm-1) 253:1    0     2G  0 lvm  [SWAP]
sda                             8:0    0     9G  0 disk
+-sda1                          8:1    0   500M  0 part /boot
+-sda2                          8:2    0   8,5G  0 part
sr0                            11:0    1  1024M  0 rom


df -h
  /dev/mapper/vg_localhost-lv_root
                      6,5G  6,2G  256M  97% /
tmpfs                 499M  200K  499M   1% /dev/shm
/dev/sda1             485M   78M  382M  17% /boot

it still old sda in use, what i have to do further?